With the development of ceramic technology, prosthodontic ceramics are becoming a useful option for improving esthetic outcomes in dentistry. In this paper, various ceramic materials were reviewed and evaluated, and their advantages and disadvantages and indications in oral prosthodontics were analyzed objectively. The properties of resin-based ceramics, polycrystalline ceramics, and silicate ceramics were compared and analyzed. Resin-based ceramics may replace other ceramic materials in the CAD/CAM field. With technological advancement of the ceramic field, application of ceramic materials in the field of dental restoration has increased. Ceramic materials have advantages over alloy materials, such as their outstanding optical properties, biocompatibility, low thermal conductivity, color stability, and excellent mechanical properties. Therefore, dental restoration materials have gradually shifted from using alloys to ceramic materials, such as silicate ceramics, polycrystalline ceramics, and resin-based ceramics , 2.Ceramic materials are suitable for subtractive manufacturing, additive manufacturing, hybrid manufacturing , and other technologies. Moreover, inlays, veneers, dental implants, all-ceramic crowns, and other restorations made of ceramic materials have been used with long-term clinical success .This paper reviews the literature on ceramic materials and divides the ceramic systems used in the field of dental restoration into three categories: silicate ceramics, polycrystalline ceramics, and resin-based ceramics , 5. ReprSilicate ceramics are nonmetallic inorganic ceramic materials containing a glass phase, which have good optical properties, high transparency, and an attractive and natural appearance. The glass phase and crystal phase of silicate ceramics have different properties. The glass phase is associated with low fracture strength, brittleness, and a nondirectional fracture mode, while the crystal phase can provide advantageous mechanical properties, stability, and esthetics , 7.The most commonly used silicate ceramics in the field of dental prosthetics are feldspar porcelain, leucite-enhanced glass ceramics, and lithium disilicate ceramics. Before bonding silicate ceramics to the tooth structure, the standard ceramic treatment technique is to etch the restoration with hydrofluoric acid, followed by the use of silane coupling agents and dental adhesives . It is wFeldspar is a common aluminosilicate ore containing calcium, sodium, and potassium. Feldspar porcelain is a kind of traditional ceramic material. It is made of feldspar as the main raw material and sintered with quartz, kaolin, and small amounts of borax and colorants .The main feldspar used in feldspar porcelain is plagioclase, a type of light gray or white crystal, which can be found in rocks abundant in mica and iron . As the Traditional feldspar porcelain is considered to be the ceramic material with the highest translucency. It has the best optical properties and is the “gold standard” in dental esthetics. Feldspar porcelain is clearly the most suitable type of ceramic for meeting the highest esthetic needs , 13. WitThe service life of feldspar porcelain restorations is considerable. Most of the feldspar porcelain used for inlays can maintain a service life of about 10 years, and most of the integral tooth crowns made of feldspar porcelain or restorations for posterior tooth crowns can maintain a service life of about 12 years , 15. PolHowever, the brittleness and low strength of feldspar porcelain make it more susceptible to damage during clinical operation and processing. For example, when it is used for veneers, it is susceptible to fracture and excessive wear. It is also prone to problems such as fracture restoration and dental crown edge caries when it is used for implants , 15, 18.In terms of added ingredients, leucite and fibers were initially used to strengthen feldspar porcelain. In recent years, the use of nanoparticles to improve the strength of feldspar porcelain has been explored. It was found that adding appropriate concentrations of silver and titanium nanoparticles into the feldspar porcelain can improve the strength of the feldspar porcelain and reduce the fracture and notch of the restoration. However, this processing method adversely may affect the color of the feldspar porcelain, and the concentration of nanoparticles should not be too high. High concentrations of silver and titanium nanoparticles will reduce the fracture resistance of the feldspar porcelain .Generally speaking, although feldspar porcelain has shortcomings, it is esthetically pleasing  and the magnitude of the residual stress. The effect of thermal residual stress is considered to be the key factor for strengthening leucite glass ceramics. This stress is caused by thermal expansion and the elastic mismatch between the crystal and glass phases after glass ceramics are cooled. The state of residual stress in leucite-reinforced glass ceramics is conducive to improving its strength and toughness .In the field of oral prosthodontics, leucite-reinforced ceramics are widely used in veneer , and then, the crystalline lithium metasilicate crystal (Li2SiO3) is embedded in the glass phase. At this time, the formed block is easy to mill and has a bending strength of 130 ± 30 MPa. The milled restorations are then tempered at 850°C to form lithium disilicate crystals (Li2Si2O5). The bending strength of lithium disilicate ceramics can reach 360 ± 60 MPa, and dense lithium disilicate crystals can be seen after hydrofluoric acid is used to dissolve the glass matrix on the surface , enAlthough lithium disilicate ceramics are less attractive in their appearance than feldspar porcelain, they are often used to construct the core of restorations and are an important substitute for metal due to their higher bending strength and fracture toughness. The color stability of lithium disilicate ceramics can be improved through glazing procedures. Additionally, in cases where the use of feldspar porcelain materials in the posterior area of the mouth is not recommended, lithium disilicate ceramics have shown excellent and long-term success in the anterior region and posterior region of the oral cavity, such as extended veneers , utilizing a combination of CAD/CAM and 3D printing, will be an interesting endeavor to make dental restorations , 22, 27.Polycrystalline ceramics are manufactured by sintering, and they are also generally referred to as sintered ceramics.Among chairside digital prosthetic materials, polycrystalline ceramics are generally stronger mechanically than glass ceramics. They are a popular class of materials used in chairside digital prosthetics today. In polycrystalline ceramics, no glass phase is present and all crystals are arranged in a dense conventional matrix. This arrangement limits the extension of cracks and provides them with superior mechanical properties . Additio2O3) is the main component. By chemically treating bauxite, raw alumina can be obtained with extremely high purity.Alumina ceramics are ceramic materials in which aluminum oxide (Al2O3), which has four isomorphs: α-Al2O3, β-Al2O3, γ-Al2O3, and δ-Al2O3. The α-Al2O3 crystalline form has the best thermal and chemical stability [The main crystalline phase of alumina ceramics is corundum . Studies have shown that the Rockwell hardness of alumina ceramics is approximately HRA80-90, while the bending strength of sintered and hot-pressed products can reach 250 MPa and 500 MPa, respectively .As the standard of living around the world improves, more and more people are paying attention to the esthetics of their teeth. Thus, we normally try to ensure attractive esthetics in the restoration of teeth. Because of their similar color and natural luster to real teeth, alumina ceramics are commonly used to make crowns, bridges, and veneers for anterior teeth. The use of alumina all-ceramic crowns can achieve better cosmetic restorative dentistry than zirconia all-ceramic crowns and effectively reduce the likelihood of gingivitis. However, alumina ceramics have a high modulus of elasticity, reaching 380 GPa, which makes them prone to fracture . There iAluminum oxide is currently a widely used restorative material in clinical practice. Choosing the bonding agent is very important for all-porcelain alumina crowns. The bond strength can directly affect the restorative effect of an all-ceramic crown, and the use of different bonding agents will produce different bond strengths . Currentμm, which is within the clinically acceptable range [p > 0.05) compared to other ceramic materials like IPS-Empress II and Top-Ceram , introduced in 1993. Procera™ AllCeram has a transparency between Empress™ 1 and Empress™ 2 and has been tested to have a consistent marginal fit of 60-80 le range . In a sile range .  FigureNormally, the microstructure of alumina ceramics consists of isometric particles, with a polycrystalline structure consisting of ionic or covalent bonds. Therefore, the fracture toughness of alumina ceramics is low. Under the action of external forces, the stress will cause fine cracks on the surface of the ceramic, and the rapid expansion of cracks makes alumina ceramics undergo brittle fracture. Toughening research is a central topic in the study of alumina ceramic materials. Currently, there are a few main methods used to improve the fracture toughness of alumina ceramics: particle dispersion toughening, fiber and whisker toughening, zirconia phase change toughening, composite toughening, and self-toughening. Today, alumina ceramics are often toughened by zirconia phase change toughening, and ceramics toughened by this method are called zirconia-toughened alumina ceramics (ZTA). The process involves adding Y-TZP (yttrium-oxide-stabilized zirconium oxide) to the alumina matrix and distributing it evenly throughout the alumina matrix to achieve a good toughening effect. A study of the wear and corrosion phenomena on the toughened alumina ceramics showed that the wear and corrosion resistance of ZTA was superior to that of yttrium-oxide-stabilized zirconium oxide (YSZ) . The cliIn recent years, due to the advent of zirconia ceramics and their superior physical and chemical properties, alumina ceramics have been gradually replaced by zirconia ceramics in the field of restorative dentistry. However, compared to zirconia ceramics, alumina ceramics still have advantageous characteristics, in that they are very stable at high temperatures and in physiological fluids. In conclusion, alumina ceramics may still have a promising future in the field of prosthodontics.In the early 1990s, zirconium oxide was introduced into the field of denture processing, and today, it is a popular material in the field of dentistry. In general, zirconia is yellow or gray in color, although high-purity zirconia is white.2), tetragonal zirconia (t-ZrO2), and cubic zirconia (c-ZrO2). Today, zirconia ceramics are a popular type of dental restorative ceramic on the market. At atmospheric pressure, pure zirconia has three crystalline forms: monoclinic zirconia , which must be polished after grinding to ensure adequate mechanical properties in the Y-TZP ceramics . Y-TZP cOne of the biggest drawbacks of zirconia ceramics is their low-temperature ageing. Kobayashi et al.  first suMany discussions have been made about the mechanism of low-temperature ageing. Currently, one of the widely accepted ageing mechanisms is the point defect reaction ageing mechanism based on oxygen vacancies and water molecules . This agReaction  describeμm [2O3 is added to it at 0.25 wt% [α-Al2O3) to zirconia as a matrix. The α-Al2O3 dispersive phase particles contained in the composite can, to a certain extent, hinder the t-m phase transition of 3Y-TZP [It has been found that Y-TZP has excellent ageing resistance when the zirconia grain size is controlled to be 0.3-0.4 μm , but the0.25 wt% . Aluminaf 3Y-TZP . The phyf 3Y-TZP  evaluateFully anatomic zirconia is a hot topic of research in recent years. Fully desorbed zirconia refers to a restoration with a fully desorbed morphology that is designed and manufactured directly from zirconia by CAD/CAM technology. This process eliminates the need for veneering porcelain and reduces the possibility of restoration failure. Fully desquamated zirconia is extremely strong mechanically and does not cause excessive wear on natural teeth because it is less abrasive than feldspathic ceramics . As thisGraded zirconia was also created as a new material in response to the chipping of veneered porcelain. Graded zirconia materials are manufactured by infiltrating glass into 3Y-TZP while sintering it. Due to the penetration of the glass, the graded zirconia ceramics have a better esthetic effect. A study of the wear properties of the new graded zirconia material showed that polished graded zirconia has better wear properties compared to zirconia. It can be inferred that it has adequate clinical wear properties and has a smooth wear surface that can be used to reduce wear on the tooth .VITA In-Ceram® YZ (VITA Zahnfabrik) and Lava™ Frame Zirconia (3M ESPE) are two of the better-known products in the world today. Certain other specific data for the two zirconia ceramic products are shown in It is undeniable that zirconia ceramics offer excellent physicochemical features compared to many other dental restorative materials . Therefore, they are easier to mill and easier to use when repairing composite resins. The new material has a lower modulus of elasticity and higher resistance to damage than traditional dental ceramics .2 (58-63%), Al2O3 (20-23%), Na2O (9-11%), K2O (4-6%), B2O3 (0.5-2%), CaO (<1%), and TiO2 (<1%) [The composition analysis of the main ceramic network shows that the main ceramic phases consist of SiOO2 (<1%) . PICN haAs is expected in composites, the properties fall somewhere between the ceramic and particle-filled resin. Compared with traditional veneer ceramics, the elastic moduli of materials that constitute PICN are in the range of 30 GPa, which is closer to common dentin (within the 30 GPa range) . CompareClinical simulation shows that PICN is of great practical value in antifatigue applications. Five years of chewing simulation results showed that all-ceramic crowns made from ENAMIC did not crack. ENAMIC behaves just like lithium disilicate glass ceramics over 500,000 fatigue tests .Dental wear also results in damage to hard tissues from causes other than caries or trauma, and it is a normal physiological process throughout a person's life. Rapid wear of teeth can lead to problems such as dentin hypersensitivity, pulp exposure, and even periapical disease . In manyPrevious studies have shown that traditional ceramics may cause excessive wear on teeth, resulting in damage to tooth anatomy and morphology, abnormal occlusion curves, and even temporomandibular joint disorder. ENAMIC, as a new type of ceramic material for oral repair, addresses the problems of poor wear resistance, poor aging sensitivity, small leakage, and long-term stability of composite materials. Mechanical properties show that ENAMIC is a better repair material than glass ceramics or resin composites.Nevertheless, the desired restorative material should have excellent mechanical properties, as well as a wear rate similar to that of tooth enamel .Enamel is more wear-resistant than tooth enamel. The wear pattern of ENAMIC is similar to that of tooth enamel. The polymer phase wears preferentially, similar to the interrod area in enamel rods, and then, the ceramic phase is removed from the wear surface, similar to what happens in enamel rods. Although its wear resistance is lower than that of tooth enamel, it is still superior to other many materials .The development of intraoral scanning, quicker and more powerful milling machines, and stronger CAD/CAM materials has led to the innovation and development of indirect prosthetics. As a result, CAD/CAM technology is becoming more common . But it Therefore, resin nanoceramics came into being. The material is a mixture of resin and composite materials, which has the advantages of both, but is mainly composed of ceramics. Not only is it not brittle and fracture resistant like composite materials, it also has the advantages of glass ceramics, including excellent gloss retention and long-lasting beauty . This ne3M ESPE's Lava Ultimate is recognized as a commonly used resin-based nanoceramic (RNC), which consists of nanoceramic particles combined at a high level in a cross-linked polymer matrix .μm. Nano-sized particles allow a high proportion of ceramic fillers (accounting for about 80% of the weight) to be incorporated into the resin [Lava Ultimate contains two monodispersed and nonaggregate nanopolymers: a 20 nm diameter silica nanopolymer and a 4-11 nm diameter zirconia nanopolymer, which synthesize nanoclusters composed of 20 nm silica and 4-11 nm zirconia. The average particle size was 0.6-10.0 he resin .This procedure is faster than procedures using other CAD/CAM materials because it requires no firing, and it allows for grinding, polishing, and adjustment for improved durability and shock absorption characteristics . CompareIt also has better fracture performance and wear resistance, and it significantly improves polishability and optical properties . In the Flexible nanoceramics are a ceramic-like resin material with a nanofiller, as introduced by GC Company of Japan. They have with a flexural strength of 231 MPa . This ceμm, pressure: 0.2 MPa). If there is no sandblasting, it can also be etched with 5% hydronitacid for 60 s and then coated with a silicon coupling agent to enhance the bonding strength [Due to its dynamic, proprietary nanoceramic matrix and complete homogeneity, CERASMART is a truly self-polishing material. It not only stays polished longer but also is proven to retain its luster even after wear and tear . Comparestrength .Indications for this material include veneers, inlays, high inlays, single crowns, implant superstructures, and other repair forms. With the continued development of resin-based ceramics technology, we believe that resin-based ceramics like this product will become increasingly popular in the field of oral repair in the near future due to the various excellent properties.Ceramic materials have become a mainstream material in restorative dentistry. Owing to their outstanding chemical and mechanical characteristics, these can be used in a wide range of clinical situations. The esthetic characteristics of ceramic materials are superior to those of other dental restorative materials.With the increasing needs of people, ceramic materials have been continuously developed. In addition to traditional silicate ceramics and popular polycrystalline ceramics of recent years, resin-based ceramics are becoming more widely used in dental prosthetics. Silicate ceramics are impeccable in terms of their esthetics, but they are obviously weaker than polycrystalline ceramics and resin-based ceramics in terms of the mechanical properties, so they are currently used to make veneer ceramics. Resin-based ceramics are easier to grind and adjust in the oral cavity than other ceramics due to their elastic modulus (relative to dentin). Polycrystalline ceramics are less translucent, but their mechanical properties are superior, and their esthetic properties compare favorably with other nonceramic restorative materials.Although ceramic materials can now handle a variety of clinical situations, research into ceramic materials has not stopped, and researchers continue to look for ways to further refine the various characteristics of ceramic materials to better meet the needs of patients. It should be noted that all-ceramic prostheses are not recommended when the patient has poor oral hygiene or periodontal disease. Based on our review of ceramic materials and their current state of development, the future of ceramic materials is bright.


The additional reaction of 9,10-dihydro-oxa-10-phosphophenanthrene-10-oxide (DOPO) to the carbonyl group of benzaldehyde yielded PFR, which was then used to prepare epoxy composites having a phosphorus content ranging from 1.5 to 4 wt%, while the boron content was 2 wt%. The structure, morphology, thermal stability and flammability of resulted epoxy composites were investigated by FTIR spectroscopy, scanning electron microscopy (SEM), thermogravimetric analysis, differential scanning calorimetry, and microscale combustion calorimetry (MCC). Thermogravimetric analysis indicated that the simultaneous incorporation of PFR and H3BO3 improved the thermal stability of the char residue at high temperatures. The surface morphology of the char residues, studied by SEM measurements, showed improved characteristics in the case of the samples containing both phosphorus and boron atoms. The MCC tests revealed a significant reduction in flammability as well as a significant decrease in heat release capacity for samples containing both PFR and H3BO3 compared to the neat epoxy thermoset.While plastics are regarded as the most resourceful materials nowadays, ranging from countless utilities including protective or decorating coatings, to adhesives, packaging materials, electronic components, paintings, furniture, insulating composites, foams, building blocks and so on, their critical limitation is their advanced flammability, which in fire incidents can result in dramatic human fatalities and irreversible environmental damage. Herein, epoxy-based composites with improved flame-resistant characteristics have been prepared by incorporating two flame retardant additives into epoxy resin, namely 6-(hydroxy(phenyl)methyl)-6H-dibenzooxaphosphinine-6-oxide (PFR) and boric acid (H Epoxy resins represent one of the most developed classes of thermosetting polymers due to their numerous applications in the construction, transportation, aerospace industry, biomedical systems, electrical and electronic fields. They have excellent mechanical properties, good thermal and chemical resistance, good electrical insulating properties, and low shrinkage after curing ,2,3. EpoPhosphorus-containing flame retardants have received extensive attention because they are environmentally friendly, highly efficient in reducing the flammability of polymers, and do not produce very toxic compounds by burning ,16,17,183BO3) particles into epoxy resin leads to a significant modification of the structure and properties of epoxy thermosets, and to a reduction in the flammability of the composites. H3BO3 is colorless, inexpensive, and exhibits low volatility and relatively low toxicity. This compound eliminates water at a temperature higher than 100 ᵒC ,2oxaphosYield: 93%. (30.97% P).−1): 935 (P–O–Ph stretching vibrations), 1203 (P=O stretching vibrations), 1474 (P–Ph aromatic ring in-plane stretching vibrations), 3245 (O–H stretching vibrations).FTIR : 3.15 , 3.58 , 5.26 , 5.31 , 7.19–7.08 , 7.33–7.23 , 7.47–7.40 , 7.72–7.63 , 7.97–7.81 .13C NMR (CDCl3-d1): 73.55 , 73.73 , 119.93–119.86 (C-16 both isomers), 121.82–121.10 , 122.9 , 123.3 , 124.2 , 124.8 , 127.02–126.97 , 128.31–127.91 , 130.4 , 132.10–131.88 , 133.7 , 134.9 , 136.92–136.79 , 150.35–150.18 .3BO3 was introduced. They were ultrasonicated for 30 min and cooled to 25 °C. Then HA hardener was added, and the stirring was continued for 30 min. The product was cured at room temperature for 24 h, and at 60 °C for 4 h. The formulation of the epoxy resin, PFR and H3BO3 pre-curing mixtures is listed in Various amounts of PFR were mixed with Epoxy, under stirring at 130 °C for 30 min. The mixtures were cooled to 50 °C, and H1H and 13C NMR chemical shifts (δ) in ppm are calibrated to residual solvent peaks .The structure of PFR was investigated by using FTIR and NMR spectroscopy. The FTIR spectrum was recorded on a FTIR Bruker Vertex 70 Spectrophotometer. The proton and carbon NMR experiments were recorded on Bruker Avance NEO 400 MHz operating at 400.1 and 100.6 MHz, equipped with a 5 mm direct detection four nuclei probe . The phosphorus NMR spectrum was recorded on a Bruker Avance NEO 600 MHz operating at 600.1 MHz and 242.9 MHz, equipped with a 5 mm inverse detection multinuclear probe. −1 at a resolution of 4 cm−1.The structure of the thermosets was determined by FTIR spectroscopy using a BioRad ‘FTS 135′ FTIR spectrometer equipped with a Specac “Golden Gate” ATR accessory. A LUMOS Microscope Fourier Transform Infrared (FTIR) spectrophotometer , equipped with an attenuated total reflection (ATR) device, was used to record the scans between 4000 and 600 cmMicroscopic investigations of epoxy thermosets and of their corresponding chars were performed on an Environmental Scanning Electron Microscope Type Quanta 200, operating at 10 kV with secondary electrons in a low vacuum mode (LFD detector). The Quanta 200 microscope is equipped with an Energy Dispersive X-Ray (EDX) system for qualitative and quantitative analysis and elemental mapping.−1, in the temperature range of 25–800 °C. Differential scanning calorimetry (DSC) measurements of PFR and epoxy thermosets were carried out using a Mettler Toledo DSC1 type device in an inert atmosphere, with a heating rate of 10 °C min−1 and nitrogen purge at 100 mL min−1.Thermogravimetric (TG) curves and thermogravimetric derivative (DTG) curves of PFR and epoxy thermosets were recorded with Mettler Toledo TGA-SDTA851e equipment, in a nitrogen atmosphere, and a heating rate of 10 °C minThe flammability behavior of PFR and epoxy thermosets was tested using an FTT Micro Calorimeter .1H NMR, 13C NMR and 31P NMR spectroscopy. The NMR analysis of PFR showed the existence of two diastereomers in molar ratio of 1:0.7. This phenomenon has been reported in the literature [1H NMR spectrum of PFR displays some characteristic signals: the hydroxyl group has a broad resonance signal in the region 3.15–3.58 ppm, the CH group proton was associated with the two doublets (one for each isomer) from 5.31 ppm and 5.26 ppm, and all aromatic protons have complex resonance signals were in the region 7.00–8.00 ppm, mainly due to the proton–phosphorus couplings, and also due to the presence of the two isomers causing more overlap of the signals , 3053 cm−1 (C–H tension of the methylene group of the epoxy ring (stretching vibrations)), 2926 , 2865 cm−1 , 1610 and 1510 cm−1 (aromatic –C=C– stretching vibration), 1240 and 1036 cm−1 . The EP containing phosphorus and boron additives (EP1-EP5) exhibited characteristic absorption peaks at 3340 (O–H and N–H stretching vibrations), 2960 and 2865 cm−1 , 1607 and 1510 cm−1 (aromatic C=C stretching vibrations), 933 cm−1 (aromatic P–O–C stretching vibrations), 754 cm−1  and 699 cm−1 (deformation vibrations of the aromatic rings).The resulting thermoset structures were investigated by FTIR spectroscopy . The nea3BO3 were observed.Scanning electron microscopy (SEM) characterization was used to investigate the morphology in the fracture surfaces of cured epoxy resin and its composites. Certain amounts of each sample were immersed under liquid nitrogen, then they were broken by applying mechanical stress. The SEM micrographs taken upon the fractured area of each sample are shown in onsetT , maxT (the temperature at which the mass loss rate is the highest), and char yield  are summarized in 3BO3, and second thermal dehydration of the latter [onsetT of the second step of thermal decomposition was in the interval 295–342 °C. The introduction of PFR and H3BO3 slightly decreased onsetT of the samples due to the lower thermal stability of PFR and thermal dehydration process of H3BO3.The thermal stability of epoxy composites was evaluated by TGA analysis. The main parameters e latter ,44. The e latter . The TonThe maximum weight loss temperature in the second step of decomposition was in the interval 330–370 °C. This temperature decreased slightly as the PFR content increased. As can be seen from 3BO3 in increasing the char yield values [3BO3 was used together with a phosphorus-containing compound to improve the flame-resistant characteristics of epoxy resins [The char yield values at 800 °C were in the interval 12.9–25.6 wt%. The chars of the samples were relatively stable in the temperature interval of 500–800 °C. The lowest values of the char yield were obtained for the EP-0, EP-1 and EP-2 having lower phosphorus content. An increase in the char yield value was observed for the EP-3 containing 4 wt% phosphorus. The highest values of the char yield were obtained for EP-4 and EP-5 containing 2 wt% boron and 1.5 and 2 wt% phosphorus, respectively. This could be attributed to a synergistic effect of PFR containing phosphorus and Hd values . It is ay resins .gT) of the samples was in the interval 52.5–71.0 °C (gT appeared for the samples containing both H3BO3 and PFR (EP-4 and EP-5), probably because of increased co-interaction between H3BO3 and PFR with the polymer matrix that resulted in an increase in the rigidity of the macromolecular chains thus increasing the gT values.The glass transition temperature  tests were used to evaluate the flammability of the samples. MCC is an important method for evaluating the combustion behavior of polymer materials ,46. The  172 W/g . It can 3BO3 together with compound PFR, in the case of EP-4 and EP-5, substantially decreased THR values, the lowest values being obtained for EP-4 containing 1.5 wt% P and 2 wt% boron (THR = 18.9 kJ/g).EP-0 exhibited the highest value of THR (26.5 kJ/g). A decrease in THR values can be observed in the case of epoxy thermosets containing phosphorus EP-1, EP-2 and EP-3. The lowest value of THR was obtained for EP-3 containing 4 wt% phosphorus (21.9 kJ/mol). The incorporation of H3BO3 improved considerably the flame resistance of epoxy thermosets.The HRC can be calculated from the MCC test and can be used to classify the flammability of materials, where a low HRC value indicates low flammability of the sample. The HRC of EP-0 was 513 J/(g·K). A substantial decrease in HRC was obtained by incorporating PFR. Thus, sample EP-3 containing 4 wt% phosphorus had an HRC value of 287 J/(g·K). The lowest value of HRC was obtained in the case of EP-5 containing 2 wt% phosphorus and 2 wt% boron (HRC = 213 J/(g·K); this value is lower than those of the EP-2 and EP-3 containing 3 and 4 wt% phosphorus, respectively. It can be concluded that simultaneous incorporation of PFR and HThe char yield (CY) resulting from MCC analysis was in the interval 5.94–20.08 wt%. As it can be seen from 3BO3 into the epoxy resin. Pyrolysis of epoxy composites containing phosphorus and boron atoms produced a char residue with a compact structure that could act as a barrier to inhibit gaseous products and to insulate the polymer from heat and air. The higher char yield of the samples containing H3BO3 was beneficial for improving the flame retardance. The SEM micrograph of EP-4 and EP-5 char residues showed the presence of phosphorus and boron atoms distributed relatively uniformly on the char surface. The MCC analyzer revealed that the introduction of 2 wt% boron and 2 wt% phosphorus considerably decreased the flammability of epoxy thermoset. For this sample the THR and HRC greatly decreased, whereas the residue amount increased considerably. In comparison to pure EP-0, the MCC results displayed that the PHRR, THR and HRC values were decreased by 55.03, 22.93 and 58.365%, respectively. In conclusion, the flame resistance of epoxy resin was significantly improved by simultaneous incorporation of a DOPO derivative and H3BO3. It was demonstrated that these two components did possess a positive role in the condensed phase. In respect of environmental protection, the use of PFR and H3BO3 as flame retardants for epoxy resins is promising.New environmentally friendly flame-resistant epoxy composites were prepared by the incorporation of PFR and H


Consumer trends towards environmentally friendly products are driving plastics industries to investigate more benign alternatives to petroleum-based polymers. In the case of adhesives, one possibility to achieve sustainable production is to use non-toxic, low-cost starches as biodegradable raw materials for adhesive production. While native starch contains only hydroxyl groups and has limited scope, chemically modified starch shows superior water resistance properties for adhesive applications. Esterified starches, starches with ester substituents, can be feasibly produced and utilized to prepare bio-based adhesives with improved water resistance. Syntheses of esterified starch materials can involve esterification, transesterification, alkylation, acetylation, succinylation, or enzymatic reactions. The main focus of this review is on the production of esterified starches and their utilization in adhesive applications . The latter part of this review discusses other processes  to prepare modified starches that can be further applied in adhesive production. Further discussion on the characteristics of modified starch materials and required processing methods for adhesive production is also included. The global adhesives and sealants market is growing rapidly and is expected to be worth USD 85.8 billion by 2026 . IndustrNatural resources are the basis of ‘green’ raw materials for producing quality adhesives. Lignin, a renewable bio-based aromatic polymer derived from wood, and its derivatives have been utilized as substrates in polyurethanes, which are major adhesive constituents . FunctioStarch is one of the most abundant polysaccharides in nature, consisting of amylose and amylopectin in various ratios depending on the botanical source. Starch has been used extensively in food, medicine, and agricultural products. Due to its low cost, renewable nature and biodegradable quality, it is attracting the attention of plastic industries as a potential building block for petroleum-free adhesives, especially for paper and wood surfaces. However, the hydrophilicity of native starch makes formulating waterproof adhesives from it challenging as the starch hydroxyl groups easily form hydrogen bonds with water . The intEsterified starch is one of the important modified starches used in the adhesive and food industries. They are typically synthesized through esterification or transesterification reactions, in which the hydroxyl groups of starch are replaced with more bulky functional groups of free fatty acids or their derivatives .p-toluene sulfuric acid, hydrochloric acid, and sulfuric acid. The mechanism of esterification depends on the mode of catalysis, although studies by Sakamura et al. using isotopically labelled (18O) alcohol unequivically show that in both cases the 18O label is incorporated into the product at the end of the process, indicating bonding of this atom to the carbonyl carbon  is the concentration of the HCl solution, while V1H NMR spectra. The DS can then be calculated using Equations (2) and (3) [1H NMR peaks corresponding to acetate or starch H-atoms.This method requires identification of specific protons related to the ester units in the product, and integration of their relative areas in  and (3) . This meThe morphology of starch granules (native and modified) can be studied using scanning electron microscopy (SEM), and this technique provides information about the granule shape, and surface features. Starch granules are the vehicle of carbohydrate storage in plant cells, and the morphology of these depends on the biochemistry of the chloroplast/amyloplasts and the physiology of the plant. Normally, native starch granules are elliptical, have a smooth surface, and range from 1 to 100 μm in size . ModificIn addition to the source of starch and conditions used to make the paste, functionalization of the starch can also markedly affect the viscosity of the paste. A study on the paste viscosity of OSA-esterified starch ,49 indicAppending larger functional groups onto the starch units results in increased hydrophobicity and lower water solubility. On the other hand, solubility in lower polarity solvents increases  and thisg), the temperature at which a glassy state of amorphous materials converts into a rubbery state, is affected by structure. In the case of functionalized starch, steric hindrance between starch chains increases as the functional groups become larger, resulting in higher Tg. On the other hand, the extension of free volume of chain mobility can reduce Tg [The thermal properties of starch are dependent on the amylose content or amylose/amylopectin ratio ,52, and educe Tg . 2HPO4, K2CO3, and sodium acetate), with base strength correlating with DS [The improved thermal stability of starch can be reported based on their gelatinization, retrogradation, and glass temperature. In  with DS . When co with DS . Other works ,58 also Starch-based adhesives are widely used in the production of plywood ,73 and pFigure 3a illustrates general starch modification processes that have been used in production of starch-based adhesives, while the structure of selected modified starches is given in Figure 3b. Apart from esterified starches, biobased adhesives can be obtained by using other chemically modified starches such as carboxymethyl-, chitosan modified-, and crosslinked starches as raw materials. Details some starch medication methods are given in Table 4 with additional discussion as follows.The application of native starches in adhesives is limited due to their relatively poor shear strength, low thermal stability, and hydrophilicity . ChemicaEtherification can be used to modify the water resistance properties of starch through the introduction of lipophilic functional groups. This is generally accomplished by treating starch with epoxides, such as propylene or ethylene oxides. These epoxides are highly reactive, with the starch OH groups attacking the ring which results in cleavage of the C-O bonds of the epoxide, affording diol products. The lipophilic alkyl groups produced are subsequently linked to the starch chains, thus increasing the hydrophobicity of the starch and improving its water resistance. . Hydroxy3) are the most utilized [Crosslinking, the process of forming nonpolar covalent bonds between the hydroxyl groups of starch, is also a strategy for improving the utility of starches in adhesives. Crosslinked starch exhibits superior mechanical (tensile strength), thermal, and water stability compared to native starch. ,89,90. Futilized . Detduanutilized .Another convenient method to modify starch is by grafting synthetic monomers or polymers with desirable properties onto the natural starch backbone. In these instances, it is desirable if the crystallinity and biodegradability properties of the starch are unchanged. Grafting typically occurs at the Cl–C2 end groups, and C2–C3 glycol groups on the starch glucose units. Graft copolymers produced from addition of vinyl- or other monomeric acrylates show improved water resistance and shear strength over natural starch . The use2, nano-SiO2, and montmorillonite (MMT) nanoparticles have been used as crosslinking or grafting agents to improve the characteristics of biopolymer-based adhesives in recent years [2, [The unique properties of nanoparticles such as their small size, high surface energy, and the ability to functionalize their surfaces render them attractive for the development of high-performance composite materials. Nano-TiOnt years ,96. Signears [2, ,98 salinears [2, , vinyl aears [2, , acrylatears [2, ,93 and pears [2, ,100 wereNatural polymers derived from biomass , and chitosan have the potential to be used as bridging agents in adhesive formulations. These may be useful in systems containing inorganic materials and organic components such as starch, and their use can improve the adhesive properties of the formulation ,101,102.Prior to crosslinking or grafting, starch is often pretreated using acid hydrolysis or oxidation . Acid hyUnlike acid hydrolysis, oxidation causes partial depolymerization of native starch, creating free carboxylic or aldehyde groups from the oxidation of primary and secondary hydroxyl groups. The oxidation of native starch involves loss of crystallinity due to a reduction in the number of network hydrogen bonds. The remaining carboxyl and hydroxyl groups subsequently react with other polymers or monomers to form covalent bonds, which result in changes in adhesive properties and starch stability. While periodate, chromic acid, potassium permanganate, nitrogen dioxide, and sodium hypochlorite are often employed, sodium hypochlorite is the widely used oxidant in industry as it results in adhesives having increased tensile strengths . Moreno 2SO4, HSO3CL, and SO3) could result in the modified starches having biological activity  [In addition to the results of synthetic studies, the properties of native starches may be predicted computationally. Computational and mathematical modeling at the molecular level may allow for selection and optimization of reaction conditions for producing adhesives tailored to specific applications. For instance, sulfation of potato starch using formic acid/urea is a simple and environmentally friendly method to obtain starch sulfates . Furtherctivity) . Sulfatectivity)  rather tUtilization of natural resources or bio-based materials as adhesive raw materials could help future societies to become less reliant on hazardous chemicals, volatile organic compounds, and finite petroleum-based chemicals . Formald


Here we map daily marine coastal algal blooms between 2003 and 2020 using global satellite observations at 1-km spatial resolution. We found that algal blooms occurred in 126 out of the 153 coastal countries examined. Globally, the spatial extent (+13.2%) and frequency (+59.2%) of blooms increased significantly (P < 0.05) over the study period, whereas blooms weakened in tropical and subtropical areas of the Northern Hemisphere. We documented the relationship between the bloom trends and ocean circulation, and identified the stimulatory effects of recent increases in sea surface temperature. Our compilation of daily mapped coastal phytoplankton blooms provides the basis for global assessments of bloom risks and benefits, and for the formulation or evaluation of management or policy actions.Phytoplankton blooms in coastal oceans can be beneficial to coastal fisheries production and ecosystem function, but can also cause major environmental problems Satellite observations reveal global increases in the extent and frequency of phytoplankton blooms between 2003 and 2020 and provide insights into the relationship between blooms, ocean circulation and sea surface temperature. Many algal blooms are beneficial, fixing carbon at the base of the food chain and supporting fisheries and ecosystems worldwide. However, proliferations of algae that cause harm ) have become a major environmental problem worldwide7. For instance, the toxins produced by some algal species can accumulate in the food web, causing closures of fisheries as well as illness or mortality of marine species and humans10. In other cases, the decay of a dense algal bloom can deplete oxygen in bottom waters, forming anoxic ‘dead zones’ that can cause fish and invertebrate die-offs and ecosystem restructuring, with serious consequences for the well-being of coastal communities11. Unfortunately, algal bloom frequency and distribution are projected to increase with future climate change13, with some changes causing adverse effects on aquatic ecosystems, fisheries and coastal resources.Phytoplankton blooms are accumulations of microscopic algae in the surface layer of fresh and marine water bodies. Although many blooms can occur naturally, nutrients linked to anthropogenic eutrophication are expected to intensify their frequency globally14, and thus present knowledge does not allow us to answer one of the most fundamental questions: whether algal blooms have changed in recent decades on a global basis16. For example, although HAB events have been compiled into the UNESCO  Intergovernmental Oceanographic Commission Harmful Algae Event Database (HAEDAT) globally since 1985, bloom trends are difficult to resolve, owing to inconsistent sampling efforts and the diversity of the eco-environmental or socio-economic effects6. Alternatively, satellite data have been used to monitor the ocean surface continuously since 1997 and have enabled bloom detection in many coastal regions19. However, the technical difficulties in dealing with complex optical features across different types of coastal waters have so far prohibited their application globally20. To fill this knowledge gap, we developed a method to map global coastal algal blooms and used this tool to examine satellite images between 2003 and 2020, addressing three fundamental questions: (1) where and how frequently global coastal oceans have been affected by phytoplankton blooms; (2) whether the blooms have expanded or intensified over the past two decades, both globally and regionally; and (3) the identity of the potential drivers.Owing to substantial heterogeneity in space and time, algal blooms are challenging to characterize on a large scale21. However, whether a bloom produces toxins or is harmful to humans or the marine environment is not distinguishable from satellite data. We delineated bloom-affected areas , and enumerated the bloom count at the 1-km pixel level   and North America  contributed the largest bloom areas. By contrast, the most frequent blooms were found around Africa and South America (median bloom counts of more than 6.3 per year). Australia experienced the lowest frequency (2.4 per year) and affected area  of blooms.We generated a satellite-based dataset of phytoplankton bloom occurrence to characterize the spatial and temporal patterns of algal blooms in coastal oceans globally. The dataset was derived using global, 1-km resolution daily observations from the Moderate Resolution Imaging Spectroradiometer (MODIS) onboard NASA’s Aqua satellite, and all 0.76 million images acquired by this satellite mission between 2003 and 2020 were used. We developed an automated method to detect phytoplankton blooms using MODIS images  Fig. , and thides Fig. . Europe 26. European LMEs showed mostly large proportions of bloom-affected areas, and some also showed frequent bloom occurrences. By contrast, Asian LMEs exhibited mainly infrequent blooms, given their large affected areas. We identified more bloom events in estuarine regions than along coasts in regions without major river discharge , northeastern USA, Latin America, the Baltic Sea, Northern Black Sea and the Arabian Sea Fig. . Five LM2 (13.2%) between 2003 and 2020, equivalent to 0.14 million km2 yr−1  over the observed period. Spatially, areas showing significant increasing trends (P < 0.05) in bloom frequency were 77.6% larger than those with the opposite trends , Alaska (+5.22% yr−1), Canary (+4.28% yr−1), Malvinas (+3.02% yr−1), Gulf Stream (+2.42% yr−1) and Benguela (+2.30% yr−1)  coasts were mainly featured with strong bloom weakening Fig. , primari1) Figs.  and 3.Fi0° N coasP < 0.05) between the annual mean bloom frequency and the coincident SST , the Oyashio Current (r = 0.48) and the Baltic Sea (r = 0.41)  can stimulate bloom occurrence. We found significant positive correlations (n a year ) in seve41) Fig. . These fas Figs.  and  3b.32. We used the spatial SST gradient (in °C m−1) as a proxy for the magnitude of oceanic mesoscale currents )) by following the methods of a previous study33, and examined its effects on algal blooms (P < 0.05) between the SST gradient and bloom frequency for various coastal current systems, including the Canary (r = 0.84), Malvinas (r = 0.83), California (r = 0.81), Benguela (r = 0.73), Gulf Stream (r = 0.61) and Oyashio (r = 0.58) currents.Changes in climate can also affect ocean circulation, altering ocean mixing and the transport of nutrients that drive the growth of marine phytoplankton and bloom formationl blooms . The tre25. Conversely, the Canary and Benguela currents were characterized by strengthened upwelling and increased bloom frequency. The two western boundary current systems at high latitudes —although characterized by less pronounced upwelling34—exhibited a similar mechanism to the subtropical eastern boundary regions. For the subtropical western boundary Gulf Stream current, the strengthened current jets (manifested as a larger SST gradient) brought more nutrients from the continental shelf35, triggering more algal blooms. Nevertheless, whether these changes in oceanic mesoscale activities were responses to wind, stratification, the shear of ocean currents or other factors33 requires region-based investigations.Trends in bloom frequency in subtropical eastern boundary upwelling regions  followed the changes in mesoscale currents Fig. . In the 36 (MEI), also showed connections with coastal bloom frequency. The minimum MEI in 2010 (a strong La Niña year) was followed by a low bloom frequency in the following year, and the largest MEI in 2015 (a strong El Niño year) was followed by the strongest bloom frequency in 2016   and bloom frequency and found high positive correlations in China, Iran, Vietnam and the Philippines. Paradoxically, decreased fertilizer uses and increased bloom frequency were identified in some countries, suggesting that nutrient control efforts might have been counterbalanced by the stimulatory effects of climate warming or other factors. Furthermore, the intensified aquaculture industry in Finland, China, Algeria, Guinea, Vietnam, Argentina, Russia and Uruguay may also be linked to their increased bloom incidence, as suggested by the significant positive correlations  between their aquaculture production and bloom frequency. A similar relationship between aquaculture expansion and positive trends in HAB incidence was reported from an analysis of HAEDAT data6. However, analogous positive feedbacks for fertilizer or aquaculture were not found in many other countries. Thus, an ecosystem model incorporating terrestrial and oceanic nutrient transport and nutrient–plankton relationships of different species39 is required to quantify the contributions of natural and anthropogenic factors to algal blooms14.Changes in anthropogenic nutrient enrichment may have also contributed to the trends in blooms6 report a dozen or more events per year for each of nine regions over a 33-year study period, compared to the global median bloom count of 4.3 in this study. There are several possible explanations for this discrepancy, such as the many low-cell-concentration HABs that are not detectable from space but that can still cause harm, as well as sensor sensitivities and algorithm thresholds. Furthermore, our bloom count was averaged over all 1-km pixels within the EEZs, whereas HAEDAT entries are based on discrete sampling regions. This underestimation does not, however, alter the trends and other conclusions of this study, as the metrics used here were constant across time and space. Underestimates would have been uniform across regions globally. In this regard, it is of note that the study of Hallegraeff et al.6 found a significant link between the number of HAEDAT events over time and the global expansion of aquaculture production, similar to findings in our study.We acknowledge that our criteria for a detectable bloom event is operationally defined by sensor sensitivities and other factors, and that the bloom count metric used here may underestimate algal bloom incidence, particularly compared to harmful events entered in HAEDAT. For example, in a recent global analysis of the HAEDAT events, Hallegraeff et al.40. This could aid in developing forecasting models  that can help minimize the consequences of harmful blooms, and can also help in policy decisions relating to the control of nutrient discharges and other HAB-stimulatory factors. Noting again that many blooms are beneficial, particularly in terms of their positive effects on ecosystems as well as on wild and farmed fisheries, the results here can also contribute toward policies and management actions that sustain those beneficial blooms.The major contribution of our study is to provide a spatially and temporally consistent characterization of global coastal algal blooms between 2003 and 2020. Globally, increasing trends in algal bloom area and frequency are apparent. Regionally, however, trends were non-uniform owing to the compounded effects of changes in climate (such as changes in SST and SST gradient and climate extremes), anthropogenic eutrophication and aquaculture development. Our daily mapping of bloom events offers valuable baseline information to understand the mechanisms underlying the formation, maintenance, and dissipation of algal bloomsRrc (dimensionless), which was converted using the rhos (in sr−1) product (rhos × π) from SeaDAS)41, remote sensing reflectance (Rrs (sr−1)) and quality control flags (l2_flags). If a pixel was flagged by any of the following, it was then removed from phytoplankton bloom detection: straylight, cloud, land, high sunglint, high solar zenith angle and high sensor zenith angle (https://oceancolor.gsfc.nasa.gov/atbd/ocl2flags/). MODIS level-3 product for aerosol optical thicknesses (AOT) at 869 nm was also obtained from OB.DAAC NASA GSFC (version R2018.0), which was used to examine the impacts of aerosols on bloom trends.MODIS on the Aqua satellite provides a global coverage within 1–2 days. All images acquired by this satellite mission from January 2003 to December 2020 were used in our study to detect global coastal phytoplankton blooms, with a total of 0.76 million images. MODIS Level-1A images were downloaded from the Ocean Biology Distributed Active Archive Center (OB.DAAC) at NASA Goddard Space Flight Center (GSFC), and were subsequently processed with SeaDAS software (version 7.5) to obtain Rayleigh-corrected reflectance  away from coastlines, which include all continental shelf areas and offer the majority of marine resources available for human use. Regional statistics of algal blooms were also performed for LMEs. LMEs encompass global coastal oceans and outer edges of coastal currents areas, which are defined by various distinct features of the oceans, including hydrology, productivity, bathymetry and trophically dependent populations42. Of the 66 LMEs identified globally, we excluded the Arctic and Antarctic regions and examined 54 LMEs. The boundaries of LMEs were obtained from https://www.sciencebase.gov/catalog/item/55c77722e4b08400b1fd8244.We examined the algal blooms in the EEZs of 153 ocean-bordering countries (excluding the EEZs in the Caspian Sea or around the Antarctic), 126 of which were found with at least one bloom in the past two decades. The EEZ dataset is available at http://haedat.iode.org) is a collection of records of HAB events, maintained under the UNESCO Intergovernmental Oceanographic Commission and with data archives since 1985. For each HAB event, the HAEDAT records its bloom period (ranging from days to months) and geolocation. We merged duplicate entries when both the recorded locations and times of the HAEDAT events were very similar to one another, and a total number of 2,609 HAEDAT events were ultimately selected between 2003 and 2020.We used HAEDAT to validate our satellite-detected phytoplankton blooms in terms of presence or absence. The HAEDAT dataset . We used the SST gradient to explore the effects of ocean circulation dynamics on algal blooms.We used the ¼° resolution National Oceanic and Atmospheric Administration Optimum Interpolated SST (v. 2.1) data to examine the potential simulating effects of warming on the global phytoplankton trends. We also estimated the SST gradients following the method of Martínez-Morenohttps://ourworldindata.org/fertilizers. Annual aquaculture production includes cultivated fish and crustaceans in marine and inland waters, and sea tanks, and the data between 2003 and 2018 are available from https://ourworldindata.org/grapher/aquaculture-farmed-fish-production.Fertilizer uses and aquaculture production for different countries was used to examine the potential effects of nutrient enrichment from humans on global phytoplankton bloom trends. Annual data between 2003 and 2019 on synthetic fertilizer use, including nitrogen and phosphorus, are available from 36, was used to examine the connections between El Niño–Southern Oscillation activities and marine phytoplankton blooms. The dataset is available from https://psl.noaa.gov/enso/mei/.The MEI, which combines various oceanic and atmospheric variables6. However, such an overall increasing trend was found to be highly correlated with recently intensified sampling efforts6. Once this potential bias was accounted for by examining the ratio between HAB events to the number of samplings5, there was no significant global trend in HAB incidence, though there were increases in certain regions. With synoptic, frequent, and large-scale observations, satellite remote sensing has been extensively used to monitor algal blooms in oceanic environments19. For example, chlorophyll a (Chla) concentrations, a proxy for phytoplankton biomass, has been provided as a standard product by NASA since the proof-of-concept Coastal Zone Color Scanner (1978–1986) era44. The current default algorithm used to retrieve Chla products is based on the high absorption of Chla at the blue band46, which often shows high accuracy in the clear open oceans but high uncertainties in coastal waters. This is because, in productive and dynamic coastal oceans, the absorption of Chla in the blue band can be obscured by the presence of suspended sediments and/or coloured dissolved organic matter (CDOM)47. To address this problem, various regionalized Chla algorithms have been developed48. Unfortunately, the concentrations of the water constituents  can vary substantially across different coastal oceans. As a result, a universal Chla algorithm that can accurately estimate Chla concentrations in global coastal oceans is not currently available.A recent study by the UNESCO Intergovernmental Oceanographic Commission revealed that globally reported HAB events have increased21 (nFLH), red tide index49 (RI), algal bloom index47 (ABI), red–blue difference (RBD)50, Karenia brevis bloom index50 (KBBI) and red tide detection index51 (RDI). In practice, the most important task for these index-based algorithms is to determine their optimal thresholds for bloom classification. However, such optimal thresholds can be regional-or image-specific20, due to the complexity of optical features in coastal waters and/or the contamination of unfavourable observational conditions , making it difficult to apply spectral-index-based algorithms at a global scale.Alternatively, many spectral indices have been developed to identify phytoplankton blooms instead of quantifying their bloom biomass, including the normalized fluorescence line height52 was adopted and modified here. In that study, the remotely sensed reflectance data in three visible bands  were converted into two-dimensional colour space created by the Commission Internationale del’éclairage (CIE), in which the position on the CIE chromaticity diagram represented the colour perceived by human eyes  and the concentrations of different water constituents. Furthermore, the Chla concentrations of the coastal blooms are typically lower than those in inland waters, thus demanding more accurate classification algorithms. Thus, the algorithm proposed by Hou et al.52 was modified when using the CIE chromaticity space for bloom detection in marine environments. Specifically, we used the following coordinate conversion formulas to obtain the xy coordinate values in the CIE colour space:R, G and B represent the Rrc at 748 nm, 678 nm (fluorescence band) and 667 nm in the MODIS Aqua data, respectively. By contrast, the R, G and B channels used in Hou et al.52 were the red, green and blue bands. We used the fluorescence band for the G channel because, for a given region, the 678 nm signal increases monotonically with the Chla concentration for blooms of moderate intensity21, which is similar to the response of greenness to freshwater algal blooms. Thus, the converted y value in the CIE coordinate system represents the strength of the fluorescence. In practice, for pixels with phytoplankton blooms, the converted colours in the chromaticity diagram will be located within the green, yellow or orange–red gamut . By contrast, for bloom-free pixels without a fluorescence signal, their converted xy coordinates will be located in the blue or purple gamut. Therefore, we can determine a lower boundary in the CIE two-dimensional coordinate system to separate bloom and non-bloom pixels, similar to the method proposed by Hou et al.52.Here, the colour of phytoplankton blooms in the coastal oceans can be greenish, yellowish, brownish, or even reddishRrc data as training samples to determine the boundary of the CIE colour space. These sample points were selected from nearshore waters worldwide where frequent phytoplankton blooms have been reported  and enhanced red–green–blue composite (ERGB) to delineate bloom pixels manually. The FLHRrc image was calculated as:Rrc667, Rrc678 and Rrc748 are the Rrc at 667, 678 and 748 nm, respectively, and F667, F678 and F748 are the corresponding extraterrestrial solar irradiance. ERGB composite images were generated using Rrc of three bands at 555 (R), 488 (G) and 443 nm (B). Although phytoplankton-rich and sediment-rich waters have high FLHRrc values, they appear as darkish and bright features in the ERGB images  instead of Rrc . However, when using the NASA standard algorithm to further perform aerosol scattering correction over Rrc, 20.7% of our selected bloom-containing pixels failed to obtain valid Rrs , especially for those with strong blooms  of the corresponding Y for each bin; then, we fit the Q1% using two-order polynomial regression. Sensitivity analysis with Q0.3%  resulted in minor changes (<1%) in the resulting bloom areas for single images. Notably, sample points were rarely located near white points  in the diagram, and we used two polynomial regressions to determine the boundaries for x values greater and less than 1/3, which can be expressed as:We converted the xy coordinate (converted from Rrc spectrum) satisfies the conditions of (x < 1/3 AND y > y1) or (x > 1/3 AND y > y2), it is classified as a ‘bloom’ pixel.Based on the above, if a pixel’s Rrc in the 667-, 678- and 754-nm bands derived from visual interpretation of the 80 pairs of FLHRrc and ERGB imagery. Instead of a simple threshold, we used a lower boundary of the sample points in the chromaticity diagram to define a bloom. In simple words, a pixel is classified as a bloom if its fluorescence signal is detectable (the associated xy coordinate in the CIE colour space located above the lower boundary). Histogram of the nFLH values from the 53,820 training pixels demonstrated the minimum value of ~0.02 mW cm−2 μm−1  between Rrc869 < 0.027) and associated straylight flags can be used to exclude most clouds, we found that residual errors from thin clouds or cloud shadows could affect the spectral shape and cause misclassification for bloom detections. Thus, we designed two spectral indices to remove such effects:Rrc443, nRrc488 and nRrc555 in index1 are the normalized Rrc, obtained by normalizing Rrc488. Similar calculations were performed for index2. The purpose of normalizations is to eliminate the effect of the absolute magnitude of the reflectance, so that the thresholds of these two indices are influenced by only the relative magnitude . We determined thresholds for Index1 (>0.12) and Index2 (<0.012) through trial-and-error and ensured that the misclassifications caused by residual errors from clouds and cloud shadows could be effectively removed. After applying the cloud/cloud shadow and various other masks that are associated with l2_flags, we obtained an annual mean valid pixel observation (Nvobs) of ~2.0 × 105 for global 1° × 1° grid cells, and the fluctuation patterns and trends of Nvobs, either annually or seasonally, are different from that of the global bloom frequency and affected areas  frequently occur in many coastal oceans63. To verify whether our CIE-fluorescence algorithm could eliminate such impacts, we compared the spectra between micro-and macroalgal blooms  within the same images as bloom-containing images. We applied our algorithm to all these pixels, and compared the algorithm-identified and manually delineated results. Our CIE-fluorescence algorithm showed high values in both producer and user accuracies (92.04% and 98.63%) , and then estimated the ratio between Ns and the total HAB events for each year. Results showed that substantial amounts (averaged at 51.2%) of satellite observations acquired during the HAB event periods were found with phytoplankton blooms by our algorithm. Overall, 79.3% of the global HAB events were successfully identified by satellite. The discrepancies between satellite and in situ observations could be explained by the following reasons: first, our study focused only on the phytoplankton blooms that are resolvable by satellite fluorescence signals; other types of HAB events in the HAEDAT dataset may not have been detectable by satellite observations, such as events with lower phytoplankton biomass but high toxicity, occurrences at the subsurface layers, or fluorescence signals overwhelmed by suspended sediments67. Second, although the HAEDAT recorded HAB events could be sustained for long periods, high biomass of surface algae may not have occurred every day within this period due to the changes in stratification, precipitation, wind, vertical migration of cells, and many other factors68. Third, the spatial scale of certain HAB events may have been too small to be identified using the 1-km resolution MODIS observations. Fourth, a reduced MODIS satellite observation frequency by the contaminations of clouds and land adjacency effects69. Therefore, we believe the underestimations of satellite-detected blooms compared to the in situ reported HAB events were mainly due to inconsistencies between the two observations rather than uncertainties in our algorithm.Third, we validated the satellite-detected phytoplankton blooms using in situ reported HAB events from the HAEDAT dataset. For each HAB event in the HAEDAT dataset, we obtained all MODIS images over the reported bloom period (from days to months). Within each year, we estimated the ratio between the number of satellite images with ‘bloom detected’ against the number of valid images (see definition above) during the bloom periods across the entire globe  and solar and viewing geometry, a sensitivity analysis was used to determine whether such variables could impact bloom detection. Aerosol reflectance (ρa) with different AOTs at 869 nm was simulated using the NASA-recommended maritime aerosol model . Then, ρa of each MODIS band was added to Rrc images, and the resulting bloom areas with and without added ρa were compared. Results showed that even with a change of 0.02 in AOT at 869 nm, the bloom areas showed minor changes (<2%) in the tested images; minor changes were also found when we used different aerosol models to conduct ρa simulations70. Note that 0.02 represents the high end of the AOT intra-annual variability in coastal oceans  mission by NASA in 2024, where the hyperspectral measurements over a broad spectrum (350–885 nm) will make species-level classifications possibleRrc images, and a total number of 0.76 million images between 2003 and 2020 were processed. We mapped all detected blooms into 1-km daily scale level-3 composites. The number of bloom counts within a year for each location can be easily enumerated, and the long-term annual mean values were then estimated , and N is the total number of 1-km MODIS pixels in this grid cell. We estimated the bloom frequency for each year between 2003 and 2020, and determined the long-term trend over global EEZs through a linear least-squares regression  to represent the density of phytoplankton blooms for a year by integrating the bloom count and bloom-affected areas within 1°×1° grid cells within that year, which is expressed as:see Fig. .Continental and country-level statistics were performed for bloom count, bloom-affected areas, and bloom frequency Fig. , using b32.To assess the impacts of climate change on long-term trends in coastal phytoplankton blooms, we correlated the annual mean bloom frequency and the associated SST and SST gradient in various coastal current systems for grid cells with significant changes in bloom frequency Fig. . The SST72 was used to determine the shape of the phenological curves  was then determined by identifying the inflection point on the bloom growth curve , where the maximum bloom-affected areas were expected in the early period of the year. Cluster II was mostly found in higher latitudes, with bloom developments (quasi-) synchronized with increases in SST. Cluster III was detected along the coastlines, where the bloom-affected areas increase throughout the entire year. In practice, the growth window for clusters I and III was set as the entire year, and that for cluster II was set from day 150 to day 270 within the year. We further found that the TMBAA for cluster II showed small changes over the entire period (Extended Data Fig. 77.We determined the growth window of phytoplankton blooms for each 1° × 1° grid cell (Extended Data Fig. Further information on research design is available in the Any methods, additional references, Nature Portfolio reporting summaries, source data, extended data, supplementary information, acknowledgements, peer review information; details of author contributions and competing interests; and statements of data and code availability are available at 10.1038/s41586-023-05760-y.Supplementary InformationThis file contains Supplementary Fig 1, and Supplementary Tables 1-3.Reporting SummaryPeer Review FileSupplementary DataThis file contains source data for Supplementary Fig. 1.


There are many variations in valve-sparing aortic root replacement techniques. Our aim is to determine the impact of the graft on mid-term outcomes: Valsalva graft vs. two straight tubular grafts. n = 270) or two straight tubular grafts (two-graft group: n = 62). Data were obtained through chart review and the National Death Index. Primary outcomes were mid-term survival and freedom from reoperation.  From 2004 to 2020, 332 patients underwent valve-sparing aortic root replacement with either a Valsalva graft . Five-year freedom from reoperation in the two-graft group was 100% compared to 93% in the Valsalva group (p=0.29).  The preoperative characteristics of the groups were similar, but the two-graft group had more type A dissections (32% vs. 19%) and emergent operations (26% vs. 15%) and was younger (45 vs. 50 years). Intraoperatively, the groups were similar, but the two-graft group had longer cross-clamp (245 vs. 215 minutes) and cardiopulmonary bypass times (284 vs. 255 minutes). Postoperative complications including reoperation for bleeding, stroke, pacemaker implantation, and renal failure were slightly more frequent in the Valsalva group, but the differences were not significant. Operative mortality was similar between the Valsalva and two-graft groups (0.7% vs. 0%). Five-year survival in the two-graft group was 100% compared to 96% in the Valsalva group ( The Valsalva and two-graft techniques both have excellent short- and mid-term outcomes. The two-graft technique might have slightly better survival and freedom from reoperation, but a larger sample size and longer follow-up are needed to determine if these advantages are significant. For just over 30 years, valve-sparing aortic root replacement (VSARR) has been performed to treat aortic root aneurysms and aortic root dissection . VSARR iThe original VSARR can be generalized into two procedures—Yacoub remodeling and David reimplantation. The remodeling techniques orient the root prosthesis on top of the annulus, while the reimplantation techniques  encapsulate the valve . It is uThere is currently no literature directly comparing the short and mid-term clinical outcomes of patients undergoing VSARR using the David reimplantation Valsalva graft or two-graft techniques. We therefore present the perioperative and mid-term outcomes of patients undergoing David reimplantation with the Valsalva graft and two-graft techniques to treat aortic root aneurysm or acute type A aortic dissection. We hypothesize that the two-straight tubular grafts technique will provide similar short- and mid-term outcomes to the Valsalva graft technique.The Institutional Review Board (IRB) or equivalent ethics committee of the University of Michigan, Michigan Medicine  , approved the study protocol and publication of data. Patient written consent for the publication of the study data was waived by the IRB due to minimal risk to patients.n = 270) or two straight tubular grafts (n = 62). Data were obtained through the Society of Thoracic Surgery Data Warehouse to identify the relevant cohort and determine the perioperative, operative, and postoperative variables. These data were supplemented through medical chart review including echocardiographic details for specified variables. All derived data collected that support the findings of this study are available by request from the corresponding author. Survival and reoperation data were collected by medical record review and National Death Index data through June 30th, 2021 [Between September 2004 and September 2020, 332 consecutive patients with aortic root aneurysm or acute type A aortic dissection underwent valve-sparing aortic root replacement with a Valsalva graft . Univariable comparisons between groups were performed using chi-square tests for categorical data. Wilcoxon rank-sum tests were performed for continuous data. Survival curves and long-term freedom from aortic insufficiency were estimated using the Kaplan–Meier method with the log-rank test. Cumulative incidence function curves were adjusted for death as a competing risk using the Fine and Gray subdistribution method to assess the incidence of reoperation due to severe aortic insufficiency over time. The Gray test was used to test the difference in the cumulative incidence function curves between groups. Cox proportional hazards model was used to evaluate risk factors for late mortality adjusting for VSARR technique, age, sex, bicuspid aortic valve, prior cerebrovascular accident, prior cardiac surgery, and primary indication. Follow-up rates were determined by using Clark's completeness index. p values less than 0.05 were considered statistically significant. Statistical calculations were performed with SAS .Continuous data were presented as median  and categorical data as Patient demographics were similar between the two-graft and Valsalva groups, except the two-graft group was younger (45 vs. 50 years). Both groups did not differ in diabetes mellitus (9.7% vs. 6.3%), coronary artery disease (18% vs. 19%), chronic lung disease (6.5% vs. 4.9%), hypertension (66% vs. 61%), congestive heart failure (8.1% vs. 3.3%), and previous cardiac surgery. The two-graft group had less aortic stenosis (0% vs. 6.3%) and more acute type A aortic dissections (32% vs. 19%) .Intraoperatively, the two-graft group had fewer concomitant aortic valve procedures (4.8% vs. 15%), but more zone one (11% vs. 1.1%) and zone two (9.7% vs. 1.5%) arch replacement. The two-graft group had longer cardiopulmonary bypass (284 vs. 255 minutes) and cross-clamp (245 vs. 215 minutes) times. The average size of the implanted Valsalva graft was 30 mm. The mean size of the large and small grafts for the two-graft group was 34 and 24 mm, respectively, with a median large graft plication diameter of 29 mm .There were no differences in perioperative outcomes between the two-graft and Valsalva groups, including operative mortality (0% vs. 0.7%). The incidence of complications such as reoperation due to bleeding or tamponade (0% vs. 1.9%), hemorrhagic stroke (0% vs. 2.2%), transient ischemic attack (0% vs. 0.8%), atrial fibrillation (27% vs. 31%), pneumonia (3.2% vs. 4.8%), and length of postoperative hospital stay (6 days vs. 6 days) were similar between the groups .The median follow-up time for mid-term survival was 6.4 years , and the follow-up rate for survival was 100% in both groups. The 5-year survival rate of the whole cohort was 97% . The 5-year survival rate in the two-graft group was 100%, and in the Valsalva group was 96% . Cox proThe median follow-up time for mid-term reoperation data was 3.3 years , and the follow-up rate for reoperation was 65% in the two-graft group and 79% in the Valsalva group for the study period. The Valsalva group had a total of 13 reoperations due to severe aortic insufficiency, one reoperation due to acute type A aortic dissection, and one reoperation due to aortic root prosthetic graft infection while the two-graft group had no reoperations. The 5-year freedom from reoperation rate due to severe aortic insufficiency, adjusting for death as a competing factor, was 100% in the two-graft group and 93% in the Valsalva group .The median follow-up time for echocardiographic data was 2.5 years , and the follow-up rate for echocardiographic data was 65% in the two-graft group and 79% in the Valsalva group for the study period. The 5-year freedom from aortic insufficiency grade moderate or higher, which was either detected clinically and confirmed by follow-up echocardiography, or found on routine yearly echocardiographic follow-up, was 96% in the two-graft group and was 91% in the Valsalva group .Our study is the first to directly compare the early and mid-term clinical outcomes of VSARR with David reimplantation using either the Valsalva graft or two-graft techniques. We report excellent outcomes for both techniques; there were no differences in postoperative complications in addition to no differences in operative, in-hospital, and 30-day mortalities between the two-graft and Valsalva groups . There w1In 2000, De Paulis and colleagues introduced a new Dacron graft—the Valsalva graft—for the David reimplantation technique. The Valsalva graft skirt stretches in the horizontal plane to better recreate the sinotubular junction for a more natural aortic root anatomy. De Paulis et al. and colleagues reported exceptional early postoperative results and no operative deaths , and ourAn additional David reimplantation technique, although far less commonly used due to its steep learning curve and unforgiving nature, is the David-V-Smod. The Stanford group reasoned that this technique allows the aortic valve to be more easily reimplanted within the larger graft, which recreates the neosinuses. Subsequently, the distal end of the smaller graft, which makes up the sinotubular junction and recreates the proximal ascending aorta, can be better matched to the diameter of the aorta than in the Valsalva reimplantation technique . The advA common determinant for reoperation of VSARR procedures is the postoperative progression of aortic insufficiency after the initial VSARR surgery. Thus, we found that the David-V may offer better hemodynamics as we report a 5-year freedom from moderate or severe aortic insufficiency of 96% and 91% in the two-graft and Valsalva groups, respectively, although this difference was not significant . The ValReoperation between both techniques was also not different in our study, confirming previous reports. Similar to our results for the Valsalva group, studies have reported the 5-year freedom from reoperation due to aortic insufficiency ranges from 89–91%, and De Paulis et al. reported an overall 10-year freedom from reoperation of 91% –7 Figur. It seemOn average, patients undergoing the David-V-Smod were on cardiopulmonary bypass and cross-clamped 29 and 30 minutes longer, respectively, than those who received the Valsalva reimplantation. This was due to more complex operations in the two-graft group, including more acute type A dissection repair and more aggressive arch replacement. Although short- and mid-term results are excellent for both the Valsalva reimplantation and David-V-Smod, there is far less experimental and clinical evidence for the two-graft technique; however, our study demonstrated that the two-graft technique may have exhibited mildly better survival, reoperation, and lower incidence of moderate or severe aortic insufficiency. Thus, a longer follow-up is required to determine whether this difference is significant in the long term.This study is limited as a retrospective cohort study—the choice of Valsalva graft versus two straight tubular grafts was by surgeon preference and not randomized, creating some differences between the groups. Because of the relatively low sample size of the two-graft group, it was difficult to control for some important preoperative factors including age and incidence of acute type A aortic dissection as the primary indication. That being said, the fact that the two-graft group had significantly more aortic dissection patients than the Valsalva group, while still having similar midterm outcomes, shows that at the very least, the two-graft technique can be safely used in a more high-risk set of patients with comparable results to the Valsalva graft. The National Death Index database was combined with a manual chart review to assess for long-term mortality, but this method may not catch 100% of deaths. Our institution aims to conduct follow-up yearly echocardiograms in aortic valve replacement patients, but we are not able to get full echocardiographic follow-up data for all patients. Lastly, although our institution is one of the few tertiary health centers in its area and we therefore expect most of our patients requiring reoperation would return to our institution, some patients may have had reoperations at outside hospitals, which could have caused us to underestimate reoperation rates.The mid-term clinical outcomes of both techniques of valve-sparing aortic root replacement with David reimplantation are excellent, including survival and freedom from reoperation. Long-term results are needed to determine how the newer two-graft technique compares to the Valsalva graft technique.


Live fish in nature exhibit various stiffness characteristics. The anguilliform swimmer, like eels, has a relatively flexible body, while the thunniform swimmer, like the swordfishes, has a much stiffer body. Correspondingly, in the design of biomimetic robotic fish, how to balance the non-uniform stiffness to achieve better propulsion performance is an essential question needed to be answered. In this paper, we conduct an experimental study on this question. First, a customized experimental platform is built, which eases the adjustment of the non-uniform stiffness ratio, the stiffness of the flexible part, the flapping frequency, and the flapping amplitude. Second, extensive experiments are carried out, finding that to maximize the propulsion performance of the biomimetic robotic fish, the non-uniform stiffness ratio is required to adapt to different locomotor parameters. Specifically, the non-uniform stiffness ratio needs to be reduced when the robotic fish works at low frequency, and it needs to be increased when the robotic fish works at high frequency. Finally, detailed discussions are given to further analyze the experimental results. Overall, this study can shed light on the design of a non-uniform biomimetic robotic fish, which helps to increase its propulsion performance. Through millions of years of evolution, fish have possessed remarkable swimming performance. They can cross miles of ocean in search of habitat or maneuver quickly to prey. The high efficiency, low energy consumption, and high mobility of their movements are mainly due to their streamlined shapes, strong muscle, and ability to modulate the stiffness of the body ,2,3. In In this paper, we perform an experimental study on how non-uniform stiffness affects the propulsion performance of a biomimetic robotic fish. A customized experimental platform is built and extensive experiments are carried out. The contributions of this paper are twofold. On one hand, both the non-uniform stiffness ratio and the stiffness of the flexible part are investigated in the nonuniform flapping model for the first time. On the other hand, it is interesting to find that to maximize the propulsion performance of the biomimetic robotic fish, the non-uniform stiffness ratio needs to be reduced when the robotic fish works at low frequency, and it needs to be increased when the robotic fish works at high frequency.The rest of this paper is organized as follows. It is known that the eel has a soft body, as shown in 2, 2, and 2, respectively.The mechanism model is shown in Please note that the effect of the mass distribution of the beam on the fluid-structure interactions can be neglected for two main reasons. On one hand, the servo motor in this paper works in position control mode. It can eliminate the influence of some disturbing factors, such as beam mass distribution and vibration. For example, when the foil weighs down or weighs up, the servo motor can decrease or increase its torque corresponding, which overcomes this factor and ensures that the foil can bend to the target position. On the other hand, the combination of rigid foil and servo motor is considered as a rigid base, which is capable to output rotational motion. Therefore, if we want to know the effect of the fluid-structure interactions on the propulsion performance, we need to focus more on the flexible foil part, while the mass distribution of the flexible foil is uniform.f represents the pitch frequency, and The experimental platform is shown in The experiments are divided into three parts based on different flapping frequencies. As a whole, the movement frequency of the foil is set at 0.5 Hz in the first group, the second and third groups are set at 1.0 Hz and 1.5 Hz, respectively. The flapping amplitude of the foil in each group of the experiments is set to the range of 20° to 50° with an interval of 10°. In addition, the foil is assembled by matching five kinds of non-uniform stiffness ratios and three different thicknesses of flexible plates in the experiment. There are rigid plates with a thickness of 3 mm and the thicknesses of flexible plates are 0.5 mm, 0.7 mm, and 0.9 mm respectively. To minimize the experimental error, each experiment is repeated three times, and it is conducted when the water surface is close to stationary.From In this part of the experiment, the flapping frequency of the foil is set to 1.0 Hz, and the rest of the experimental parameters are kept the same as in the above experiments. In From The flapping frequency of the foil is set to 1.5 Hz in this experiment, and the rest of the experimental parameters are kept consistent with To demonstrate the experimental results more clearly, we take the mean values of the experimental results for each model in The main reason for the poor propulsion performance of the traditional “rigid” biomimetic robotic fish is that the body cannot yet adjust its stiffness like a real fish. In nature, the stiffness of a real fish decreases along its axial direction from the head to the tip of the caudal fin, and its non-unitary stiffness plays a crucial role in the modulatory of propulsive force. Numerous experiments by previous researchers have proven that if the body of the biomimetic robotic fish uses pure rigid material, the propulsion performance is partially improved, but not significantly. Excessive rigidity of the material will severely limit the movement flexibility of the robotic fish. In addition, although the purely flexible material is helpful to enhance the flexibility of movement, its propulsion performance will not be able to meet the working requirements when the robotic fish is under high-frequency movement. Currently, a large number of experimental studies on uniform and non-uniform stiffness have been conducted, but the optimal stiffness required to maximize propulsive force has not been found. In this paper, the research idea of variable non-uniform stiffness material is applied to investigate the method of improving the propulsive force.Experimental results show that a lower non-uniform stiffness ratio will provide a higher propulsive force of motion when the robotic fish is at a low-frequency swimming speed. Specifically, the non-uniform stiffness material labeled 1-3 (flexible plate’s thickness is 0.9 mm) produces the largest average propulsive force with a value of 1.11 N when the flapping frequency of the foil is set to 0.5 Hz. Moreover, as the thickness of the flexible material increases, the propulsive force will also become larger. When in a high-frequency motion, the foil needs a high non-uniform stiffness ratio to achieve the largest propulsive force. Specifically, the 2-2 foil (the flexible plate’s thickness is set at 0.9 mm) produced the largest average propulsive force of 8.21 N when the foil flapping frequency is 1.5 Hz. Why is there such a big difference in the propulsive force generated by different non-uniform stiffness models when switching between low and high frequencies?As shown in It is worth noting that no matter how the thickness of the flexible material of the foil varies, the propulsive force generated by the 3-1 (stiff anterior three quarters) foil will be at a high level, and its propulsive force output is more stable at the same motion frequency. The emergence of this experimental result may be related to the active and passive flexural stiffness of the creatures doing propulsive motions. The literature ,33 exploCurrently, many efforts have been made to find the optimal stiffness used to drive the biomimetic robotic fish, but neither uniform stiffness nor non-uniform stiffness nor variable stiffness studies with “artificial muscles” have found the optimal stiffness. What has been demonstrated in the concept of variable non-uniform stiffness of the foil material presented in this paper is that a different movement frequency requires adjustments to the non-uniform coupling ratio of the foil to achieve the relative maximization of the propulsive force of the robotic fish. In the future, biomimetic robotic fish’s material stiffness should be researched along the direction of “dynamic” and “non-uniform stiffness”, which has highly bionic significance.In this paper, we conduct an experimental study on how non-uniform stiffness affects the propulsion performance of a biomimetic robotic fish. A customized experimental platform, which eases the adjustment of the non-uniform stiffness ratio, the stiffness of the flexible part, the flapping frequency, and the flapping amplitude, is built, and extensive experiments are carried out. It is found that to maximize the propulsion performance of the biomimetic robotic fish, the non-uniform stiffness ratio needs to be reduced when the robotic fish works at low frequency, and it needs to be increased when the robotic fish works at high frequency. Specifically, when the flapping frequency is 0.5 Hz, the largest average propulsive force of 1.11 N is generated by the foil with a 1-3 non-uniform stiffness ratio. When the flapping frequency is 1.0 Hz, the largest average propulsive force of 4.45 N is generated by the foil with a 2-2 non-uniform stiffness ratio. When the flapping frequency is 1.5 Hz, the largest average propulsive force of 8.21 N is generated by the foil with a 2-2 non-uniform stiffness ratio.In the future, more factors, such as the shape, the size, and the flapping pattern, will be taken into consideration. Moreover, the stiffness adjustment mechanism to change the non-uniform coupling ratio online, and its influence on propulsion efficiency, speed, and maneuverability, will also be investigated.