Due to its abundance in nature and low cost, starch is one of the most relevant raw materials for replacing synthetic polymers in a number of applications. It is generally regarded as non-toxic, biocompatible, and biodegradable and, therefore, a safe option for biomedical, food, and packaging applications. In this review, we focused on studies that report the use of starch as a matrix for stabilization, incorporation, or release of bioactive compounds, and explore a wide range of applications of starch-based materials. One of the key application areas for bioactive compounds incorporated in starch matrices is the pharmaceutical industry, especially in orally disintegrating films. The packaging industry has also shown great interest in using starch films, especially those with antioxidant activity. Regarding food technology, starch can be used as a stabilizer in nanoemulsions, thus allowing the incorporation of bioactive compounds in a variety of food types. Starch also presents potential in the cosmetic industry as a delivery system. However, there are still several types of industry that could benefit from the incorporation of starch matrices with bioactive compounds, which are described in this review. In addition, the use of microbial bioactive compounds in starch matrices represents an almost unexplored field still to be investigated. In the last decades, increased concern regarding environmental impact related to human activity has been expressed worldwide. Plastics have been extensively used in many applications, from packaging to the medical industry, generating large amounts of non-degradable solid waste, which are considered one of the main environmental problems of today.2 and O2 barrier characteristics, thus displaying the capacity to protect food products [In this context, biopolymers emerge as an alternative to synthetic polymers, and can help to reduce the environmental impact caused by the latter. Moreover, biopolymers are promising candidates for materials used in biotechnology applications because they tend to be compatible with diverse types of cells and tissues. In addition, they can be modified in a wide range of manners, which raises their applicability, in addition to being biodegradable and originating from renewable materials . Among tproducts  that couproducts . Howeverproducts ,7. Moreoproducts .The association of biopolymers and bioactive compounds has been extensively studied and has resulted in the development of bioactive materials with improved properties, such as antioxidant  and antiStarch is one of the most abundant biopolymers in the world and is greatly explored because of its abundance and susceptibility to physical and chemical modifications, including its capacity to form thermoplastics ,14. The In order to select the papers that were analyzed in this review, the results from the Web of Science database pertaining to starch-related research were refined by including the terms “bioactive compounds” or “natural extract”, which resulted in 98 papers. Then, from these papers, the ones that used starch as a matrix for incorporating or releasing extracts or bioactive compounds were selected.Starch is the main carbohydrate that functions as an energy store in plants and is a significant nutritional source for animals. It is a polysaccharide that is composed of two polymers, amylose, a linear polymer, and amylopectin, and has highly branched chains . AmyloseDifferent sources of starches tend to have variations in the proportions of amylose and amylopectin and, as a consequence, the resulting product can present different characteristics. A film, for example, may present more flexibility as a result of a higher amount of amylose; therefore, the use of new starch sources presents advantages due to the versatility of starch, and creates numerous forms for the incorporation of bioactive compounds for different applications ,18.Two main starch components, amylose and amylopectin, have different contributions to the supramolecular structure of starch. Single helical chains of amylose disrupt the structural order and lead to the formation of amorphous domains in starch, while double helices of amylopectin form highly ordered crystalline domains of starch. The combination of amorphous and crystalline domains results in the formation of three main types of semi-crystalline starch granules, such as type A  , type B Starch extraction is based primarily on the cellular rupture of the plant tissue, liberating the starch granules to a solvent  . DependiAn important process in starch applications is gelatinization, which occurs when starch undergoes a structural change after being heated above a certain temperature, normally ranging from 60 °C to 70 °C. This leads to the formation of hydrogen bonds between hydroxyl groups of starch and water in a way that, at a determined concentration, water acts as a plasticizer of the amorphous parts of the starch granule. Another process that must be taken into account in the application of starch in industrial processes is retrogradation, in which the starch molecule recovers parts of its crystalline structure. It is important to note that the amylose retrogradation happens in a much shorter time than amylopectin retrogradation; the former can occur in 48 h, while the latter tends to occur in 30 to 40 days .In its native form, starch may not present interesting properties to the industry since it has high water solubility and swelling power , low gelIn general, starch presents good properties for film formation ,33,34,35Depending on the molecular structure of the bioactive, the bioactive compounds can be physically or chemically incorporated into starch. Considering that starch matrices have high swelling capability, the extracts or bioactive compounds can be physically located in the intermolecular structure of starch, and can also act as a plasticizer in starch films . In addiBioactive compounds have been extensively used around the world and, in recent years, have attracted the attention of scientists since their consumption has been connected with healthier habits, in order to prevent and treat various diseases . InitialThe knowledge and applications of bioactive compounds are well-known and widely applied in different segments such as the pharmaceutical, food, and chemical industries. Bioactive compounds are commonly found in a large number of organisms, from which they can be isolated using extraction and biotechnological methods . SeveralMetabolites produced by plants and microorganisms are classified as either primary or secondary metabolites. Primary metabolites provide essential substances for cellular homeostasis, such as sugars, amino acids, nucleotides, organic acids, and fatty acids . SecondaSecondary metabolites are formed by a wide variety of molecules; some are more restricted and are only present within a certain species or taxonomic group . HoweverA large number of plants and microorganisms have been recognized as valuable sources of natural substances, since they are rich in a variety of molecules that are synthesized in their secondary metabolism, such as vitamins, polyphenols, terpenoids, flavonoids, carotenoids, essential oils, and proteins, among others. The diversity of bioactive compounds in plants varies according to the part of the plant, such as leaves, bark, roots, flowers, seeds ,53, or mSecondary metabolites are reported in the literature as having a wide range of biological properties, such as antimicrobial , antioxiThe interest in bioactive substances of natural origin has grown over the years. The development of bioproducts, thus, depends on natural active substances, and these are often unstable compounds, which can cause reactions that promote a decrease or loss of efficiency and even the degradation of the product. Consequently, new technologies have been proposed to improve the performance of pharmaceutical, food, and cosmetic products produced from natural raw materials, which favors their acceptance by the consumer. An alternative that increases stability and allows the controlled release of natural substances is the encapsulation technique using polymeric matrices. The compartmentalization of bioactive substances in polymeric carriers with specific physicochemical properties such as starch is, therefore, an interesting alternative for increasing the use of natural bioactive compounds in different applications, as can be observed through the wide range of applications presented in this review and summarized here in Controlled-release devices appear as an alternative to traditional drug administration methods, which usually present low efficacy and the necessity for large dosages . They maSince 2017, starch has been used in various forms as a carrier for the release of different drugs. Panyoyai, Shanks, and Kasapis  showed tOne strong trend in delivery systems using starch is the encapsulation of curcumin. For this purpose, Athira, Jyothi, and Vishnu  developeIncorporation of turmeric in films based on starch and chitosan was performed by Schaefer et al. . AlthougOther studies also should be mentioned. Oliyaei et al.  producedStarch has also been used in the formulation of orally disintegrating films, which are alternatives to traditional methods of drug administration, especially because they have the potential for fast release and are absorbed directly into the oral mucosa without the need for ingestion of water. In this context, special attention has been given to biopolymers due to their hydrophilicity, non-toxicity, and mucoadhesion properties .Malpighia emarginata) extract, which contains high levels of vitamin C. The authors observed that in films with higher concentrations of starch, vitamin C presented greater stability when compared to those with higher concentrations of gelatin, thus demonstrating the potential of starch as a film matrix for natural compounds. A similar study was performed with camu-camu (Myrciaria dubia) extract [Starch-based orally disintegrating films have a remarkable ability to deliver molecules with pharmaceutical potential. Garcia et al.  produced extract , in whicCordia verbenacea, which successfully retained anti-inflammatory and antioxidant activity. In addition, the films presented a higher concentration of flavonoids.Guerra et al.  includedAs can be seen, by being non-toxic and biocompatible with various natural compounds, starch-based materials are potential carriers of bioactive compounds, especially in the form of orally disintegrating films.Synthetic polymers have been the main component used in the packaging industry in the last decades. Despite their remarkable characteristics, such as good mechanical properties and relatively low production cost, they originate from fossil resources and generate a large volume of solid residues . In thisStaphylococcus aureus and Escherichia coli.To improve the properties of a film intended to be used in the packaging industry, a wide variety of natural compounds can be used, such as adding antimicrobial or antioxidant properties to the films. One of the first studies in this area was done by Pelissari et al. . They prLippia sidoides cham. and pomegranate peel extracted at different concentrations and tested its effects on the quality of tomatoes. The results showed that starch is a potential material for food packaging by lowering the weight loss when compared to the control group of uncoated tomatoes.Araújo et al.  producedCaetano et al.  developeCampomanesia xanthocarpa) pulp, a natural antioxidant, and analyzed its application in the storage of extra virgin olive oil. They noted that the stored product was within the Brazilian parameters for the studied period (15 days). Cheng et al. [Malherbi et al.  produced period 1 days. Chg et al. , films cPeralta et al.  used an Araucaria angustifolia extract, as demonstrated by Da Silva et al. [A. angustifolia starch, which was extracted from its seeds. The authors also successfully incorporated rosemary and green tea aqueous extract in the films, and the extract directly influenced the physicochemical properties of the produced films.Another natural extract with the potential to be incorporated into biopolymeric films is the a et al. , which wa et al.  demonstrListeria monocytogenes.An interesting approach to the development of active packaging materials is the incorporation of antibacterial molecules into the biopolymer. An example of this case can be seen in the research performed by De Oliveira et al.  who deveCunha et al.  investigTherefore, the natural properties of starch films, allied to the potential of the active compounds’ biodiversity, show that the packaging industry can take advantage of these products while reducing the amount of non-biodegradable residues generated, and improving the functions of the food-packaging.Starch can also be used as a raw material to encapsulate compounds with interesting properties for food applications, especially molecules that are sensitive to oxidation or degradation under different conditions, and which diminishes the product’s final quality. Encapsulation techniques, such as spray drying, extrusion, and emulsification can be used with starch to encapsulate the desired compound .Souza et al.  used staStarch has also been used as a stabilizer in nanoemulsions, as shown by Abbas et al. . A curcuAstrocaryum aculeatum) pulp with various biopolymers and observed that the modified starch was the one with best bioactive compound retention. Similar work was performed by Ravichai and Muangrat [Another method that can be used to encapsulate compounds in biopolymers is freeze-drying, as shown in Silva et al. . The autMuangrat , who useZheng et al.  producedNatural extracts can also be used to supplement starch-based foods, as is the case of Rodrigues et al. , who useConsidering the studies in the literature regarding the use of starch with natural extracts, it becomes clear that there is great potential for developing functional foods that could be used as raw material and enhance the quality of a wide range of products. Moreover, these functional foods would bring several health benefits to consumers.Biopolymers are already broadly used in cosmetic formulations, especially as moisturizers and thickeners; starch can form hydrogels with cross-linked networks that tend to have high water absorption capacity . TherefoSthaphylococcus aureus and the yeast Candida albicans, thus showing that the biopolymers present potential as a matrix for the release of natural compounds, and resulting in a product with a diverse range of applications. An interesting study was developed by Wu et al. [Silveira et al.  developeu et al. , who useHornung et al.  producedTrichilia catigua) extract in starch films, and the films loaded with the highest concentration of catuaba extract showed promising results for use as a delivery system in cosmetic formulations.Another study that examined the use of starch as a matrix in cosmetic formulations was developed by Pereira, Lonny, and Mali , who incTherefore, as the need for substitution of plastic material increases, being a biopolymer with good availability, non-toxicity, and biodegradability, starch appears to be an alternative and is a good candidate for use as a matrix in a wide range of applications, especially when incorporated with bioactive compounds. The incorporation of bioactive compounds in biopolymeric matrices can enhance the bioactive stability, one of the main points of concern in the use of certain types of bioactive compounds, allowing these devices to be used in applications in pharmaceutical products, packaging, food technology, and others. As an example, it is possible to cite the incorporation of curcumin in starch matrices ,62,63. OIn the packaging industry, bioactive compounds that add antioxidant activity to the films and coatings are the main targets . In addiStarch has also been well studied as an adjuvant for spray drying and as a stabilizer for nanoemulsions ,86, demoJust a few studies in the literature have explored starch-based matrices for cosmetic applications , making In this review, we highlighted the potential use of starch as matrix for incorporation and release of a wide variety of bioactive compounds, which enhances its stability and increases its range of application.


In conclusion, starch is a promising material for bioplastic production, with great potential for further improvements. This review summarizes the recent advances in starch-based blends and composites and highlights the potential strategies for overcoming the major drawbacks of using starch in bioplastics applications.Environmental pollution by synthetic polymers is a global problem and investigating substitutes for synthetic polymers is a major research area. Starch can be used in formulating bioplastic materials, mainly as blends or composites with other polymers. The major drawbacks of using starch in such applications are water sensitivity and poor mechanical properties. Attempts have been made to improve the mechanical properties of starch-based blends and composites, by e.g., starch modification or plasticization, matrix reinforcement, and polymer blending. Polymer blending can bring synergetic benefits to blends and composites, but necessary precautions must be taken to ensure the compatibility of hydrophobic polymers and hydrophilic starch. Genetic engineering offers new possibilities to modify starch  Polymers comprise a wide group of materials in which many small molecules (monomers) are linked together to form long chains. Polymers are used in a wide range of applications. Natural polymers have been used for thousands of years, although to a very limited extent in early times, e.g., the use of plant gums as a wood adhesive in house construction or as waterproof material in boat making. The first synthetic polymer, a thermosetting phenol-formaldehyde resin called Bakelite, was invented in 1907 by Leo Hendrik Baekeland . Major pPetroleum-based plastics are not environmentally friendly, since it takes hundreds of years for them to degrade into harmless end-products , and petBioplastics must comply with the requirement of either being produced from bio-based raw material or exhibiting biodegradability or possessing both properties. As displayed in The use of bio-based polymers instead of synthetic polymers has several advantages, e.g., they are biodegradable, non-toxic, and renewable, and are therefore a good substitute for commodity plastics. Starch has proven to be a promising bio-based polymer that can be used in bioplastics applications and compared to other biopolymers the starch market has a long history . AccordiAmylopectin has around 5% branch points, which means that its properties differ from those of amylose .Starch is a major source of energy in the human diet and has various non-food applications, e.g., in the textile industry as a sizing agent and in the paper industry as an adhesive and binder. There has been a considerable amount of research and development on starch-based materials in recent years, with the aim of utilizing starch for bioplastics applications, due to environmental concerns regarding the use of fossil fuel-based plastics. Starch plays a key role in the field of bioplastics, mainly due to its abundance, renewability, biodegradability, and low permeability to oxygen, in addition to being an inexpensive raw material . In 2021Composite materials with unique properties can also be created using starch, by incorporating various petroleum-based polymers and other biopolymers into blends with starch. In early development in the 1970s, starch was used as an additive to synthetic polymers to improve biodegradability and exploit the potential of this abundantly available natural resource . HoweverStarch is a suitable filler material because of its thermal stability and limited interference with the melt flow properties of most synthetic plastic materials . BlendinThe properties of filled polymer materials depend on the size and shape of the filler and its compatibility with the polymer matrix . When stSince the degree of compatibility between the filler and the polymer matrix plays a major role in determining the properties of filled polymer materials, much research focus should be directed toward investigating ways to improve the compatibility of starch with synthetic polymers. The hydrophilic nature of starch and the hydrophobic nature of synthetic polymers make them incompatible, by hindering the formation of strong interfacial hydrogen bonds between synthetic polymer and starch. Hence, modification of either the starch or the polymer is needed to improve the compatibility. Otherwise, lack of compatibility results in poor adhesion, making the matrix incapable of distributing loading forces equally, leading to a “pull-off” of starch from the polymer matrix upon application of force . This seThe use of starch as a filler for LDPE has received much attention, but the immiscibility of starch with LDPE limits its applications. A study characterizing LDPE-corn starch blend films with various concentrations of starch ranging from 2.5% to 50% reported weakening of tensile properties, such as tensile strength and elongation at break, with increasing starch content, an effect attributed to incompatibility of starch and LDPE . This waAnother limitation of using starch as a filler material is the poor barrier properties of the resulting polymer-starch blends. The incorporation of starch causes a reduction in intermolecular attractive forces between the synthetic polymer layers, which leads to a large amount of void content in the polymer-starch blend, compromising the barrier properties. Poor barrier properties for water vapor are a common problem associated with starch blends and are primarily due to the hydrophilic nature of the starch. One study reported an increase in water vapor permeability with increasing starch content in an LDPE/starch composite, with a more significant increase in water vapor permeability when the starch content was over 30% . An incrPVOH has been identified as a more compatible material with starch than LDPE, due to the polar nature of PVOH. Starch and PVOH are compatible since the functional hydroxyl groups of these two materials form hydrogen bonds that keep the materials tightly linked to each other . IncorpoThe use of starch as a filler in biodegradable polymers has received much research attention in recent years since it can help in producing fully biodegradable polymer/starch blends and composites. Particularly, starch-filled PHA materials have been studied extensively, because although PHA is a fully biodegradable polymer, its usage is limited due to the high cost. Hence, blending PHA with starch filler is an economically feasible solution. Other than the economic advantage, starch serves as a reinforcing filler, contributing to improved mechanical properties while producing a fully biodegradable blend . It has Various attempts have been made to improve the performance of starch-filled polymer blends and composites, with a focus on improving the compatibility of starch and other polymers. This has been attempted through the modification of starch or the other polymer. Various approaches to improve the miscibility between starch and other polymer components have been extensively reviewed . IntroduModifying the starch component to impart hydrophobic characteristics is another approach used to improve the compatibility of starch and synthetic polymers. In conjunction with this approach, converting starch into a hydrophobic derivative by phthalation is common practice. For example, starch-phthalate-filled LDPE blends are reported to exhibit better adhesion, resulting in better mechanical properties compared with unmodified LDPE/starch blends, and also better degradation in soil . In an aA recent study reported using acid hydrolysis of rice and potato starch to produce starch nanocrystals to add to the LDPE matrix. LDPE/nano starch blend displayed better compatibility and promising properties in films. The incorporation of 1% rise starch nanocrystals could reduce the oxygen permeability of films while increasing the thermal stability. The incorporation of 1% potato starch displayed an improved modulus of elasticity than LDPE films. The addition of nanocrystals of both types increases the hydrophobicity of the films, and elongation in the Longitudinal direction but made the films opaquer than LDPE films . Physical and/or chemical cross-linking of starch and/or polymer blends has also been proven to be effective in improving the properties of starch-filled polymer blends. For example, a study examining the influence of trimethylolpropane triacrylate as a cross-linking agent and electron beam irradiation of sago starch before incorporation into an LDPE polymer found that these modifications effectively altered the mechanical, thermal, and degradation properties of the polymer . In thatEpichlorohydrin (ECH) has been used as a crosslinking agent in fossil fuel based-polymer/starch blends. A study reported improvement in tensile strength, percent elongation and strain energy of the ECH crosslinked starch-filled LDPE films compared to the native starch-filled LDPE films . ECH croOther than the chemical methods of starch modification, physical and enzymatic methods of starch modification exist, and enzymatic methods are minimally in use due to their complexity and time-consuming nature. Physical modification methods are preferred as environmentally friendly and safe methods of starch modification. Gamma irradiation is one of the methods to physically modify the starch . Gamma iPlasticized or destructured starch, called TPS, is produced by the formation of hydrogen bonds between the hydroxyl groups of starch and molecules such as water, glycerol, and sorbitol, which are generally referred to as “plasticizers”. In the presence of plasticizers and at high temperatures (90–180 °C) and shear, starch readily melts and flows, providing the possibility for its use in injection, extrusion, or blowing production processes, similar to those used for synthetic thermoplastic polymers . There aThere are two methods of producing TPS, the casting solution method, and the extrusion process. The casting solution method is mostly used for experimental purposes in laboratories, while in large-scale industrial production the extrusion process is the preferred method . The casHowever, TPS has some disadvantages, such as a tendency to retrograde with time, poor water resistance, and unsatisfactory mechanical properties, particularly in wet or dry environments. To address those drawbacks of simple TPS, several approaches are used in practice, e.g., selecting different sources of starch, using different kinds of plasticizers, blending TPS with other polymers , and using fillers or reinforcing materials. Recent advances in various approaches to improve the properties of TPS systems are discussed in this section.The plasticizers commonly used to produce TPS include polyols such as glycol, sorbitol, ethylene glycol, glycerol, and sugars. However, simple TPS made by only plasticizing starch tends to retrograde with time and this retrogradation embrittles the material . To achiPlasticizers containing amide groups, such as formamide, acetamide, and urea, have been used as another way of preparing retrogradation-resistant TPS. A study testing formamide as a novel plasticizer found an ability to form more stable hydrogen bonds with hydroxyl groups of starch . FormamiAnother study found that formamide (10 wt%) and urea (20 wt%) together make a more effective novel plasticizer mix to produce TPS with good mechanical properties , high thermal stability, and better retrogradation resistance than TPS plasticized with glycerol (30 wt%) . It alsoA study comparing TPS made using a combination of ethylene bisformamide and sorbitol with TPS made using single plasticizers  observed superior performance of the TPS made using a combination of ethylene bisformamide and sorbitol, which exhibited better tensile stress, elongation at break, water resistance, and thermal stability . The use of urea and ethanolamine as plasticizers for TPS was examined in another study . It founAliphatic amidediol as a candidate plasticizer has been used in combination with glycerol to produce TPS from corn starch . That stHence, amide groups containing plasticizers used in combination with other plasticizers have been proven to be promising for TPS production.Reinforcing the TPS matrix is another approach to improving the properties of TPS. Reinforcing helps the TPS to overcome limitations related to low tensile strength, severe deformation, and high hygroscopicity . DiffereThe chemical composition and morphology of the fibers, interfacial adhesion, fiber distribution, and orientation between the fiber and the polymeric matrix result in the varying performance of different types of fibers when incorporated into TPS .Fiber size plays an important role in determining the final properties of TPS . In geneCorn starch-based biocomposites plasticized with glycerol and reinforced with nano-sized bacterial cellulose have been found to be superior to biocomposites made using micro-sized jute fiber in terms of mechanical properties, thermal stability, water resistance, and optical transparency, improved properties attributed to better interfacial bonding between the matrix and the nano-sized fibers . Some other modifications of fiber are reported to have positive impacts on the properties of the TPS matrix, e.g., surface modification of cellulose fiber using air plasma treatment improves the adhesion of fiber to the TPS matrix, resulting in improved mechanical properties of the fiber-modified TPS compared with pure TPS . A study2) to reinforce the TPS matrix has been found to yield better results in improving the properties of the matrix [2 plays an important role in determining the properties of TPS composites [2 with potato starch molecules compared with other sizes  in forming potato starch films. The 100 nm nano-SiO2 formed strong hydrogen bonds with potato starch, so the water vapor transmission rate was reduced, and film tensile strength increased with the incorporation of nano-SiO2. The incorporation of nano-SiO2 was also effective in imparting antibacterial activity to the films, particularly against Escherichia coli, because nano-SiO2 was able to destroy the structure of cell membranes by adsorption to bacterial cell walls [Using nano silicon dioxide . However, there are several limitations associated with the use of PLA, such as the high cost of production and its brittle nature . Thus, bOne common approach to improve the interfacial adhesion and reduce the phase separation of starch and PLA is to use compatibilizers. Greater interfacial adhesion improves the desired material properties of the blends. The compatibilizers, which act as cross-linking agents that chemically bind PLA and starch molecules together, are grafted onto PLA chains  to starcIncorporating the third component as a softening agent to impart ductility and flexibility to the matrix has been proven to be effective in starch/bio-based polymer blends. Examples of such components include PBS  and PCL Incorporating an elastomer is another approach for toughening PLA-starch blends. Incorporating elastomers toughens a material by absorbing energy upon stress, due to the rubbery dispersed phase that forms within the brittle polymer matrix . HoweverAnother way of improving the compatibility of starch and PLA is by starch modification, which is a more economical approach than modification of PLA since starch is usually the minor component in a PLA/starch blend. A common approach for starch modification is to make the starch component hydrophobic by substituting the hydrophilic hydroxyl groups with hydrophobic groups, which improves the interfacial adhesion with the hydrophobic PLA component. Incorporating plant oil derivatives via compatible agents is one of the most promising approaches to impart hydrophobicity to starch. Various attempts have been made to couple starch with different types of plant oils, such as soybean oil  cardanolA recent study reported the effectiveness of a sandwich-architecture film of pea starch and PLA for fruit preservation. The adhesion between pea starch and PLA was improved by the incorporation of an octenyl succinic anhydride-modified pea starch (OMPS) interlayer. Octenyl succinic anhydride esterification of the pea starch made the starch more hydrophobic. The modified starch contained both hydrophilic and hydrophobic functional groups which made it effective in using it as an interlayer for improving the interfacial incompatibility between starch and PLA. The films with sandwich architecture had improved barrier properties for water vapor and oxygen .Some studies report good effectiveness of using reinforcing agents to improve the compatibility between TPS and PLA, e.g., the use of coir fiber to produce a stiff, hard composite with improved compatibility between the TPS and PLA phases was reported . The impA novel method to improve the compatibility of starch and PLA without using external components such as compatibilizers or plasticizers is co-grinding during the production of PLA/starch composites . In thatHevea brasiliensis However, phase separation limited using a higher amount of rubber in starch/rubber blends and composites. Phase separation depends on the plasticizer content (glycerol), where glycerol acts as both the plasticizer and starch rubber compatibilizer [A few studies have tested the blending of TPS with natural rubber, e.g., natural rubber from ibilizer . Various approaches to improve TPS/rubber blends have been reported. One study showed the good effectiveness of rubber modification in formulating thermoplastic starch/natural rubber/clay nanocomposites . ScanninPHAs are a group of natural biodegradable polyesters of microbial origin. Their applications as a biomaterial have been limited to date due to poor mechanical properties, high cost of production, limited functionality, incompatibility with conventional thermal processing techniques, and susceptibility to thermal degradation. Those poor characteristics of PHAs require various modifications, e.g., blending PHAs with other types of polymers . BlendinPoly-3-hydroxybutyrate (PHB) is the most common category of PHA used to date in formulating blends with starch. PHB has physical properties similar to those of PP, but the high cost of PHB limits its usage. Hence, blending PHB with starch, which is an inexpensive and completely biodegradable polymer, is an interesting approach. The feasibility of blending thermoplastic starch with PHB has been reported . Blending) for all the PHB/modified corn starch blends. Additionally, the inclusion of modified corn starch improved the thermal stability of PHB, as demonstrated by higher thermal degradation temperature in the PHA/modified starch blends than in pure PHB [Although blending PHAs with starch is a feasible approach to reduce the cost and improve the mechanical properties, the incompatibility between starch and PHAs prevents the formation of intact films, making the films more brittle. Various attempts have been made to improve the compatibility between the two polymers. A recent study reported the use of poly(vinyl acetate) (PVAc)-modified corn starch to improve the compatibility and flexibility of PHA/starch blends . Graftinpure PHB . A recent study tested the use of various cross-linking agents  to improve the mechanical and barrier properties of starch/PHA composite films produced by extrusion blowing . In thatA recent study reported in situ grafting of PHAs onto starch as an appropriate method to improve the interfacial adhesion of starch and PHA in an economical and environmentally friendly way, with limited usage of chemicals . In thatStarch-based composites with antimicrobial and/or antioxidant properties, for food packaging applications, have attracted research interest in recent years. This type of packaging ensures food safety as an additional advantage, apart from biodegradability. It can also eliminate the use of food additives such as food preservatives and antioxidants, making it a viable alternative in terms of production economics, sensory properties of the food, and consumer health. This section focuses on some recent advances in producing novel, functionalized starch-based composites.Successful incorporation of antimicrobial agents into food packaging materials has been reported in recent years. Several types of antimicrobial agents have been incorporated into starch-based composites, as presented in Another recent advance in the development of starch-based food packaging materials is the incorporation of antioxidants into starch-based packaging materials that have the capability for delaying food spoilage due to oxidation. In this approach, antioxidants are added to the packaging material, as opposed to adding them directly to the food in high doses. Several studies have examined the use of antioxidants in food packaging . Some stAraucaria angustifolia) starch and found a visual color change in the anthocyanin incorporated biofilms when immersed in solutions with different pH values, indicating good potential for using the material as a pH indicator in starch-based food packaging [The development of intelligent packaging is another recent advance in producing functionalized starch-based composites in the food packaging sector. Intelligent packaging carries out special functions such as tracking, sensing, and communicating, which can influence consumer decision-making, and this novel approach can be used to enhance food safety and quality . For exaackaging . Inplanta modification of starch opens new possibilities to enhance the positive attributes and eliminate the shortcomings of native starch in industrial applications. The native starch from many plants lacks the necessary properties for direct industrial applications. Therefore, modification of starch by physical or/and chemical approaches is a common practice. In recent years, producing tailor-made starches within the growing plant has attracted much attention, as it would eliminate or reduce the need for post-harvest physical and chemical modifications of starch, saving money and the environment. inplanta modification of starch composition and molecular structure has become one of the major focuses since starch composition is crucial in determining subsequent applications of starch. For example, in packaging applications, high-amylose starch is preferred over high-amylopectin or normal starch. Amylose films have better mechanical properties and barrier properties than amylopectin films [In this research area, in films , with onin films . Becausein films ,131 or ain films  for use in films . © plastic [inplanta modified starches as a functional alternative to commercial plastics [Amylose-only barley starch (99% amylose-like α-glucan), produced by silencing all three genes of the starch branching enzymes using the RNAi technique, has been demonstrated to be a useful raw material for bioplastics production . The ext plastic . The amy plastic . A higheplastics .inplanta modified starches, with favorable properties for bioplastics applications [High-amylose potato starches produced by RNAi technology possess superior tensile and oxygen barrier properties compared to wild-type starch . This isications . inplanta modification of granule size to produce a new type of starch has been attempted, through the expression of a tandem starch-binding domain derived from Bacillus cyclodextrin glycosyltransferase in potato plants [Starch granule size influences film properties such as film thickness, tensile strength, and elongation rate, with starch granules of smaller size being preferable . Hence, o plants . A novelo plants .Inplanta modification of potato starch phosphate content for bioplastics applications has recently been reported in a study where a novel type of potato starch with low phosphate content obtained from transgenic potato was found to display high robustness, transparency, mechanical strength, and extensibility of films, even in wet conditions [Phosphorylation of starch has significant importance in various technical applications and potato starch has a relatively high content of phosphate esters compared to cereal starches. nditions . These bnditions . This canditions . Even though the research in the field of starch-based food packaging is at its early stage, there is a growing interest in using starch as a “green” natural resource for packaging . The reaStarch-based packaging materials are currently in use in both rigid packaging and flexible packaging. Rigid packaging applications accounts for around 45.6% of the global bioplastic market share in 2020 . The tecinplanta modification of starch using novel genetic engineering approaches might prove an important area for future research, to eliminate the need for post-harvest physical and chemical modification of starch for bioplastics applications, particularly for the packaging sector. Future research should examine the potential for using starch in higher amounts for starch-based blends and composites for bioplastics applications, to produce highly biodegradable materials from this abundantly available natural resource. It could also examine the possibility of using starch as the sole polymer in the formulation of bioplastic materials. Importantly, future research should also investigate possible approaches to address the major drawbacks of starch-based bioplastics, such as high-water sensitivity and poor mechanical properties. Apart from looking for chemical methods to compatibilize starch and other polymers in blends and composites, efforts should be made to develop methods that are more sustainable for both the environment and the economy. In addition, Applications of starch as a bioplastics material, particularly for use in the packaging sector, have been extensively researched. Starch has been shown to be a promising biopolymer with great potential in bioplastics applications. Starch is used in blends with both fossil fuel and other bio-based polymers to facilitate the degradability of the synthetic polymers, reduce the cost, achieve better mechanical properties for subsequent applications, and minimize the problems associated with starch . However, incompatibilities between starch and other polymers need to be addressed by an appropriate selection of approaches to improve the compatibility between the polymers. Modification of starch and/or other polymer components, using plasticizers, reinforcing agents, and compatibilizers have shown positive results in minimizing the problems associated with the incompatibility of starch and other polymers when used as a bioplastics packaging material. Apart from the post-harvest modifications of starch, in-planta modification of starch has resulted in the generation of tailored starches that are better suited for bioplastics applications. These approaches are also more sustainable, since they potentially reduce or minimize the need for post-harvest modification of starch or minimize the requirement of a second polymer, compatibilizers, reinforcing agents, etc. in bioplastics applications. Apart from the biodegradability and other mentioned advantages of starch-based materials, starch-based composites with antimicrobial and/or antioxidant properties, for food packaging applications, have attracted research interest in recent years as packaging materials with added advantages to ensure food safety.


The rice quality and starch functional properties, as well as the storability of three YY-IJHR cultivars, which included YY12 (biased japonica type YY-IJHR), YY1540 (intermedius type YY-IJHR) and YY15 (biased indica type YY-IJHR), were studied and compared to N84 . The study results suggested that the three YY-IJHR varieties all had greater cooking and eating quality than N84, as they had lower amylose and protein content. The starch of YY-IJHR has a higher pasting viscosity and digestibility, and there was a significant difference among the three YY-IJHR cultivars. Rice aroma components were revealed by GC-IMS, which indicated that the content of alcohols vola-tile components of YY-IJHR were generally lower, whereas the content of some aldehydes and esters were higher than N84. In addition, YY-IJHR cultivars’ FFA and MDA contents were lower, which demonstrated that YY-IJHR had a higher palatability and storability than those of N84 in fresh rice and rice stored for 12 months. In conclusion, this study suggested that YY-IJHR had better rice quality and storability than N84. PCA indicated that the grain quality and storability of YY12 and YY15 were similar and performed better than YY1540, while the aroma components and starch functional properties of YY-IJHR cultivars all had significant differences. Oryza sativa) with abundant starch, is a globally important starch-based food that provides calories to over 60% of the world’s population [Rice  have an unpleasant pungent odor, such as butyraldehyde, glutaraldehyde and furfural. With the increase in molecular weight, C8–C10 aldehydes have an obvious floral or fruit aroma, such as nonanal, with a rose and citrus aroma; Octanal had a strong orange aroma. The other detected organic compounds were mainly 2-pentylfuran, which were oxygenated heterocyclic compounds with fruit and grass aromas .As shown in There were also significant differences in volatile flavor components among different varieties of YY-IJHR. The volatile flavor components of YY15 were mainly aldehydes and esters, whereas the YY12 were mainly aldehyde and ketone. Differential aroma compounds are landmark substances, which distinguish them from other samples . CombineThe relative concentration of volatile flavor components was an important factor in distinguishing rice aroma. The contribution of each compound to the fresh rice flavor was evaluated by calculating the relative odor activity value (ROAV). An ROAV compound of greater than 1 indicated that compounds made a significant contribution to the flavor, and a value < 1 meant a small contribution to the flavor. As shown in The vital compounds that formed the flavor of fresh rice comprised differential volatile compounds and main volatile compounds. Therefore, the key aroma compounds were defined by these two major components . HexanalA PCA performed on the rice aroma compounds and samples of different rice varieties showed that PC1 and PC2 accounted for 78.6% of the total variation. As shown in The volatile flavor components of YY15 were mainly aldehydes and esters, while those of YY12 were mainly aldehydes and ketones A. As shoRice starch is used in various processed foods as an adhesive, thickener, extending agent, and inflating agent. The functional properties of rice starch are influenced by the crystalline structure, the amylose–amylopectin ratio and the fine structure of amylopectin . This isThe digestive properties of rice starch have important potential implications for human health. Starch can be grouped into rapidly digestible starch (RDS), slowly digestible starch (SDS) and resistant starch (RS) . RapidlyRS was positively correlated with amylose content in rice starch, while SDS and RDS decreased as amylose content increased . The stuThe characteristic pasting parameter values of rice have a great impact on the properties  of many rice-starch-based food matrices . The pasDuring storage, the changes in pasting property parameters occurred due to the variation in granule size distribution, amylose content, and crystallinity. As shown in Interestingly, a decreasing trend of PV was shown in YY1540, whereas an increasing trend was shown in the other three varieties during storage. PV represents the water-binding capacity of starch granules  and the To comprehensively analyze the pasting properties, data including PV, TV, BD, FV, SB, PT of fresh rice and rice stored for 12 months among the four cultivars were loaded on PCA and are summarized in the scores plot B. The poThe cooking quality of rice was an important indicator of consumer acceptance. To evaluate the cooking quality of fresh rice in YY-IJHR, the cooking expansion rate, cooking elongation rate, soaked water absorption rate and heating water absorption rate were measured and compared with N84. As shown in The swelling ability of starch granules when heated with water to cook rice could be reflected by the PV . The YY1Ageing also enhances the volume expansion and water absorption of rice upon cooking, resulting in a product with a firmer texture and less stickiness . In thisThe free fatty acid (FFA) content, a sensitivity indicator of the quality changes, is often employed as a measure of the deterioration of stored grain. As shown in Malondialdehyde (MDA) is an end-product of seed lipid peroxidation and its content may represent the degree of rice seed cell damage . To explIn conclusion, both YY-IJHR cultivars’ FFA and MDA contents were lower, which indicated that YY-IJHR had a lower deterioration in palatability and higher storability than N84 over the same storage time.Catalase (CAT) and peroxide (POD) are rice-seed-viability-protecting enzymes that scavenge hydrogen peroxide in rice seeds. To explore the rice-seed-protecting enzymes’ activity, we determined the CAT and POD activity of fresh rice and rice stored for a year. As shown in The relationship between rice cultivars with different aging indexes and antioxidant enzymes was summarized in the biplot E. The PCRice seeds were obtained from Shipu Town, Xiangshan County, Ningbo City, Zhejiang Province, China. N84 seeds were used as the high-quality rice control. YY15  and YY1540 (intermedius-type indica–japonica hybrid rice) were harvested from the same field at the end of October. YY12 (biased japonica-type indica–japonica hybrid rice) was harvested around 15 days later due, to its late-maturing character. Fresh rice in the current season were dried to a moisture content of 14–15% after threshing. The rice was packaged into a universal transparent preparation bag and stored in the dark at 4 °C before the formal start of the experiment. The samples were stored at room temperature (26 °C) at the formal start of the experiment. The harvested mature rice grains were dehulled with a rice-huller  and brown rice was milled into fine rice with a rice mill (Osaka MB-RC52 Japan). Finally, they were ground into powder by a cyclone grinder  Co., Ltd., Suzhou, China) and passed through an 80-mesh standard examination sieve to produce rice powder. On a dry basis, the chemical composition of rice flour consists of carbohydrates , moisture (about 10–15 g/100 g) proteins (about 7–10 g/100 g), ash (about 3 g/100 g), fats (about 1.5 g/100 g), and pigments .Protein contents (46–11.02) were measured in triplicate according to AACC-approved methods. The amylose content (AACC Method 76–31.01) was determined three times by commercial assay kit .A rice-grain taste analyzer  was used to evaluate the taste value of cooked rice and every sample was determined three times. Freshness score was determined in triplicate by the fresh tester . Ten rice grains were randomly picked and measured with a right-angle measurement , and the average was subsequently calculated. Head rice rate was determined in triplicate according to the Chinese National Standard method (GB/T 21719-2008). Pictures of brown rice and milled rice from four varieties were taken by a stereomicroscope ; Three parallel runs were performed for each sample.The hardness and stickiness of N84 and YY-IJHR varieties were measured by a hardness and viscosity tester . The values of hardness (kgf) and stickness (kgf) were derived from the instrument software . The hardness and stickiness unit was kgf, where 1 kgf unit is equal to 9.8 Newtons (N).®, the flavor analyzer developed GC-IMS analysis, and each sample was assayed 3 times in parallel. The test conditions of GC-IMS were in accordance with the method of Zhao and Shen [Rice samples  from each cultivar were collected in 20-mL headspace bottles and incubated at 80 °C for 15 min before headspace injection, with an injection volume of 500 μL. The injection needle temperature was 85 °C and incubation speed was 500 R/min. The carrier gas was high-purity nitrogen. With flavourspecand Shen  and eachmax and given a value of 100, with the other volatile components being calculated according to Equation (1):i represented relative concentration , and Ti indicated the odor threshold of the target volatile components. Cmax and Tmax referred to compounds with the maximum odor activity value [The contribution of each compound to the fresh rice flavor was evaluated by calculating the relative odor activity value (ROAV). The present study adopted the ROAV method, where the ROAV compound greater than 1 indicated that compounds made a significant contribution to the flavor, and a value of <1 meant a small contribution to the flavor. The component with the greatest contribution to the odor of all samples was defined as ROAVty value .The pasting properties of rice flour were determined by a Rapid Visco Analyzer . The rice flour samples  were stirred with distilled water (25 mL) in an aluminum vessel. The test parameters were set according to the descriptions of Geng, et al. . The cha1 was the volume measured immediately after injecting 5 mL of water, V2 (V2 = V1 − 5) was the rice volume; V3 (V3 = V2 × (W/5)) was the volume of total rice. The volume of the raw rice V0 was measured according to the same method. Rice kernels  were accurately weighed and cooked in a small aluminum vessel with 20 mL of water, from which 5 g of cooked rice was loaded into a 10-mL measuring cylinder to determine the volume. The volume of raw rice V0 was measured according to the same method. The cooking expansion rate (%) was calculated according to Equation (2):0) was determined. Ten rice grains were placed into a 20-mL test tube, and allowed to soak for 30 min after the addition of 10 mL distilled water. These soaked rice were subsequently boiled in boiling water for 10 min and the rice grains were removed and placed on filter paper for 60 min to determine the average length (L1). Cooking elongation rate was measured according to Equation (3)1 represented the average length of cooked rice kernels and L0 represented the average length of raw rice kernels [Actual elongation was measured by dividing the average length of 10 cooked kernels from that of 10 raw rice kernels. Intact rice grains (10 grains) were randomly taken from each sample, and the average length  were accurately weighed, and the samples were cooked for 30 min in a small aluminum vessel with 20 mL of water in a water bath. The cooked rice kernels were collected and placed on filter paper to drain the surface water. The heating water absorption rate (%) was calculated according to Equation (4)ked rice .0 represented the weight of 3-g unsoaked rice kernels and W1 represented the weight of soaked rice.To measure the soaked water absorption rate, 3 g of rice were accurately weighed and the samples were soaked for 60 min in a small aluminum vessel with 20 mL of water in a water bath and the aluminum vessel was kept in a constant-temperature water bath of 25 °C. The soaked rice kernels were collected and placed on filter paper to drain the surface water. The soaked water absorption rate (%) was calculated according to Equation (5)The FFA content of the samples was measured in triplicate according to the Chinese National Standard method (GB/T 29405-2012). In brief, 5 g of rice was added to a conical flask and shaken with 30 mL of ethanol for 1h, the extract was centrifuged, and the supernatant was added to a solution of 1% phenolphthalein ethanol (95%) and titrated with potassium hydroxide ethanol solution to the endpoint. The FFA content was calculated according to the volume of potassium hydroxide ethanol that was consumed by rice grains. The MDA content was determined three times by the kit .2O2 every second was defined as a unit of catalase activity (U/mg protein). The POD activity was measured in triplicate by commercial assay kit , and this method used the change in absorbance at 420 nm using the principle that POD catalyzes the reaction of hydrogen peroxide. The amount of enzyme catalyzing 1 mg of substrate per minute per milligram of tissue protein was defined as one unit of enzyme activity.The CAT activity was determined in triplicate by a commercial assay kit . Every milligram of tissue protein that breaks down 1 μmol HData processing was conducted with Microsoft Excel software  and reported as mean ± standard deviation (SD). One-way ANOVA with Ducan’s tests were performed by IBM SPSS statistical software (version 26.0) at 0.05 significance level. The principal component analysis (PCA) was performed with Origin software (version Pro 2021).As an important global starch-based food, rice contributes to nearly 40% of the world’s total caloric intake. YY-IJHR, as a new variety with a high yield and wide planting area, along with its rice quality, starch functionality and storage characteristics, have been attracting increasing attention. The previous study mainly focused on cultivation techniques and yield. However, the rice quality and starch’s functional properties, as well as the storability of YY-IJHR, are important for consumers and industrial applications. Here, three typical YY-IJHR varieties, which included YY12 (biased japonica type YY-IJHR), YY1540 (intermedius type YY-IJHR) and YY15 (biased indica type YY-IJHR), were studied and comparatively analyzed. They were compared with N84  in terms of comprehensive rice quality and the functional properties of starch, as well as storability.The results of this study suggested that the three YY-IJHR varieties all had a greater cooking and eating quality than N84, as they had lower protein and amylose contents. Rice aroma components were revealed by GC-IMS. The alcohol contents in the volatile flavor components of YY-IJHR were generally lower, whereas the content of some aldehydes and esters were higher compared with N84. The volatile aroma components of YY15 were mainly aldehydes and esters, whereas YY12 were mainly aldehydes and ketones. The data from the current study provided the knowledge that YY-IJHR starch has a higher pasting viscosity and digestibility, which may be helpful to common consumers. In addition, both YY-IJHR cultivars’ FFA and MDA contents were lower, which demonstrated that YY-IJHR had higher palatability and storability than N84 in fresh rice and rice stored for 12 months. Therefore, YY-IJHR not only has a high yield, but also exhibited a superior grain quality and storability compared with the N84. PCA indicated that grain quality and storability of YY12 and YY15 were similar and performed better than YY1540, while the aroma components and starch functional properties of the YY-IJHR cultivars all showed significant differences. This study will provide valuable information for common consumers when selecting suitable rice varieties and necessary knowledge for physiologists to maximize the yield potential and optimize the application of rice.


Background: Lower extremity stiffness simulates the response of the lower extremity to landing in running. However, its relationship with running economy (RE) remains unclear. This study aims to explore the relationship between lower extremity stiffness and RE.Methods: This study utilized articles from the Web of Science, PubMed, and Scopus discussing the relationships between RE and indicators of lower extremity stiffness, namely vertical stiffness, leg stiffness, and joint stiffness. Methodological quality was assessed using the Joanna Australian Centre for Evidence-Based Care (JBI). Pearson correlation coefficients were utilized to summarize effect sizes, and meta-regression analysis was used to assess the extent of this association between speed and participant level.Result: In total, thirteen studies involving 272 runners met the inclusion criteria and were included in this review. The quality of the thirteen studies ranged from moderate to high. The meta-analysis results showed a negative correlation between vertical stiffness  and leg stiffness  and RE. Additional, there was a small negative correlation between knee stiffness and RE . Meta-regression results showed that the extent to which leg stiffness was negatively correlated with RE was influenced by speed  and participant maximal oxygen uptake .Conclusion: The results of this study suggest that vertical, leg and knee stiffness were negatively correlated with RE. In addition, maximum oxygen uptake and speed will determine whether the runner can take full advantage of leg stiffness to minimize energy expenditure. Running is a popular sport around the world. Statistics from the US government show that more than 55.9 million people in 2017 were involved in running, jogging, and trail running . KhammasWhen a person is running, bones, muscles, tendons, ligaments and other elements are usually modeled as Spring Mass Model (SMM) to support the body running forward . When a θ). Therefore, the leg spring compression is greater than the COM displacement. Some studies have found that the leg stiffness is always less than the vertical stiffness  in response to vertical ground reaction force during a task performed in the sagittal plane . The vertiffness . Also, wtiffness . Many retiffness .The vertical stiffness and leg stiffness were calculated using the whole lower extremity as a whole SMM , and sucResearchers started exploring the relationship between stiffness and energy expenditure in the 1990s. However, there is still no higher quality evidence for the relationship between lower extremity stiffness and energy expenditure, and some studies have even found opposite results for each other . FurtherThis review followed the Preferred Reporting Items for Systematic Review and Meta-Analyses (PRISMA) statement for reporting systematic reviews.2 OR VO2max OR cost of running OR consumed oxygen OR oxygen uptake) AND  AND (stiffness).The literature search was conducted on three databases, PubMed, Web of Science, and Scopus. Studies published between 1980 and 2022 were screened. Journal article titles, abstracts, and keywords in each database were searched using the following terms and Boolean operators: , likely to be included (maybe) and not meeting the inclusion criteria (no). Finally, disagreements between the two independent reviewers were discussed and resolved in a consensus session. A third reviewer was consulted where consensus could not be reached.2max) > 50 ml/kg/min. 2) The study included the observation of both RE and lower extremity stiffness metrics. 3) Correlations between lower-extremity stiffness and RE, such as Pearson’s correlation coefficient or r-square, are reported.The articles included in this study met the following criteria: 1) Healthy middle-distance runners aged >18 years with a maximal oxygen uptake  were conference presentations, posters, or case studies; 2) included non-runners ; 3) did not provide data or inferred data on RE or biomechanical characteristics; and 4) did not report relevance.2max, and test speed). In addition, Pearson’s correlation coefficient (r) was extracted for the relevant stiffness parameter and energy expenditure. For studies reporting r-square, the square root of the coefficient of determination was taken by its directionality and was transformed into a correlation coefficient.After the full-text screening, a custom table was created to record basic information about the study  Analytical Cross-Sectional Study Critical Assessment Tool . The chep-value of less than 0.05 was considered significantly different. The correlation coefficient was used to summarize results and report 95% confidence intervals. The heterogeneity of studies was assessed using the I2 statistic. Studies with I2 of less than 50% had poor heterogeneity, and a fixed-effects model was considered; otherwise, a random-effects model was used  was used to analyze pooled Pearson’s correlation coefficients. A was used . Accordiwas used . A simplIn total, 1,608 articles were obtained by searching three databases including PubMed, Web of Science, and Scopus. The Endnote software was used to remove duplicates, and 553 articles remained. In addition, papers with only titles and abstracts were filtered, and 27 articles with full texts remained. These articles were further screened. Out of the 27 articles, one longitudinal article included an intervention category, five did not include energy consumption or stiffness metrics, and eight did not include an available correlation coefficient. These articles were excluded. As a result, thirteen articles were included in our systematic review , as show2max of the participants included in the study was 54.02–71.5 ml/kg/min. Ten studies reported correlations between vertical stiffness and RE, ten other studies reported correlations between leg stiffness and RE, and two studies reported correlations between knee stiffness and ankle stiffness and RE.In total, 272 runners were included in this study, 90.44% male and 9.56% female, with a mean age of 24.93 ± 6.48 years. The VOn = 11) of the studies were conducted during running. Barnes and his colleagues used the peak force measured during five squat jumps divided by the vertical displacement as the “leg stiffness”  were of high quality. One study had a low rating because it lacked ethical consideration. A common low-scoring item in the entries was “How was the study population selected?” Given that the purpose of this review was to evaluate a simple correlation between lower extremity stiffness and RE, the sampling and selection of the study population did not affect the results of this study. In particular, most studies had clear objectives, valid and reliable data collection, and appropriate statistical methods. Therefore, no study was excluded because of methodological quality issues.I2 = 20.62%). As a result, a meta-analysis was performed using a fixed effects model. The results revealed a large pooled correlation between vertical stiffness and RE , as shown in I2 = 67.93%). As a result, a random-effects model was adopted for meta-analysis. Pooled results showed a large pooled correlation between leg stiffness and RE , as shown in Ten studies reported Pearson correlation coefficients between vertical stiffness and RE. Heterogeneity between vertical stiffness and RE across studies was small . In contrast, ankle stiffness was not associated with RE , and interestingly, the two studies presented opposite results. As shown in Two studies reported pooled results for knee stiffness and ankle stiffness. Pooled results with small heterogeneity showed a small negative correlation between knee stiffness and RE  and test velocity . However, the negative correlation between leg stiffness and RE increased with the VO2max  test velocity , as shown in Further one-way meta-regression analyses were performed for studies to provide clear reporting of test velocity and participant VOWe performed a sensitivity analysis with one-by-one exclusion and the estimated effects would still be within the 95% CI of the pooled results. Funnel plots of effect sizes versus standard errors were generated to determine possible publication bias. Analysis of the funnel plot revealed that the included studies were generally located on the left side of the pooled results. The funnel plot on the leg stiffness and RE correlations was adjusted using Duval and Tweedie’s trim and fill correction to produce a symmetric funnel plot around the Pearson correlation coefficient. This correction shifted the overall effect size to the right but did not change the main results, which still showed a significant trend, as shown in 2max.This study is the first to examine the relationship between lower extremity stiffness and RE systematically. The aim was to investigate whether lower extremity stiffness affects RE. In total, 13 articles were included in this review. The results showed a significant negative correlation between vertical, and knee stiffness and RE in endurance runners. This result suggests that vertical, leg, and knee stiffness influence RE in endurance runners. In addition, the results of the meta-regression showed that the correlation coefficient between leg stiffness and RE was related to the runner’s speed and VOFmax is the peak vertical force, and Δy is the vertical displacement of the COM from ground contact until mid-stance.Current findings showed that vertical stiffness negatively correlated with RE during sub-maximal running. The following formula was used to calculate the vertical stiffness:Many studies have demonstrated that increasing the peak vertical force or decreasing the COM vertical displacement during running will result in better vertical stiffness. Several studies have found that increasing running speed leads to a higher peak vertical force and a decrease in COM vertical displacement Matt . Dutto aThe study found that maintaining a high whole-body vertical stiffness minimizes oxygen consumption, and smaller heterogeneity enhances the certainty of the results. Meta-regression results show that the correlation between vertical stiffness and RE is less susceptible to the effects of runner level and speed. Several studies have demonstrated that trained runners who run to exhaustion maintained initial vertical stiffness  and evenv is running velocity and tc is contact time. Leg length is symbolized as L, which comes into contact with the ground at an angle of θ . tc allows the lower-extremities to generate propulsive force over a longer period of time when in contact with the ground, reducing energy costs  . Therefo2max. This suggests that experienced runners make more rational use of elastic energy to minimize energy costs at faster speeds. Some scholars have suggested that there is a weak negative correlation between knee stiffness and oxygen consumption, but it is not clear how much ankle stiffness runners need. Many factors affect stiffness during running, and these factors are influenced by different tasks, populations, and gender. The optimal stiffness required for a runner needs to be further investigated. The results of this study may, to some extent, inform future training and research.This meta-analysis found a significant negative association of vertical, leg, and knee stiffness with RE. Furthermore, the extent of this correlation was related to speed and the runner’s VO


In triangular mesh models, the repair of complex hole poses a difficult problem, which always causes serious repair defects. Therefore, it is needed to develop an intelligent identification and classification method of complex holes to reduce repair difficulties. First, the topological structure of the complex hole is studied and all the holes are divided into single holes and continuous holes depending on whether there are intersection points. Second, to tackle the nesting and connecting of complex continuous holes, a decomposition method of multiply connected domains based on intersection points is proposed to partition or reconstruct complex continuous holes into single holes. Based on the different geometric structures, single holes are classified into five common hole types and a corresponding identification method of single holes is presented. Finally, an experiment is carried out to verify the repair quality and efficiency of the proposed method. Compared with Geomagic software, the proposed method can automatically identify and partition complex holes with fewer defects and similar efficiency. It can reduce the difficulties of repairing complex holes and enable the repair of complex holes based on existing methods. It is shown that the method can be applied to complex hole repair of 3D printing models without the participation of technicians. As a rapidly developing manufacturing technology, 3D printing technology  is gradually entering various fields and playing an important role , 2. CompAt present, the identification and repair of triangle mesh holes are mostly carried out manually, with low efficiency and high requirements of operators' skills. Automatic hole identification and repair algorithms often regard all holes as single holes. Most research focuses on the repair method of single holes and disregards the processing and partitioning of complex holes, single hole geometry, and the topological relationship between relevant holes. This results in unsatisfactory model repair and new defects, such as self-intersecting surfaces, highly refracted edges, and degenerate triangles.A decomposition method of multiply connected domains based on intersection points is proposed to partition or reconstruct complex continuous holes into single holes. It can greatly improve the structure and size of the complex holes and can mostly avoid the intersection of various hole lines.An identification method of single holes is researched to divide single holes into five common hole types, which can reduce the difficulties of repairing complex holes and enable the repair of complex holes based on existing repair methods of single holes.The proposed repair process is completely automated, replacing the manual judgment and repair operation, reducing the dependence on professionals. At the same time, it can achieve high efficiency, similar to Geomagic.Therefore, to improve the quality of hole repair and to lower the reliance on specialized technicians, it is necessary to study the intelligent identification and partitioning method of complex holes in triangle meshes. The technical route of the paper is shown in Hole identification and extraction is a critical preliminary process for hole repair. Given the considerable amount of data generated from the conversion from solid model to point cloud or grid, the data processing is time-consuming with low efficiency if holes are identified and extracted through repeated traversal. Therefore, the efficiency and accuracy of identification and extraction will directly affect the efficiency and quality of subsequent repair. The existing hole identification and extraction methods can be divided into two categories according to the data type: hole identification based on point cloud data and that based on grid , 5.There are many methods for hole identification based on point cloud data –9. MilroThe grid-based hole identification method is also a focal point of research. In computer graphics, grid is a very basic representation method, and it can be obtained by point cloud data reconstruction or 3D model transformation. Triangular mesh is the most widely used form because of its simplicity, straightforwardness, and expressiveness. Zhan and Zhang  proposedBased on the above hole identification methods, many scholars have conducted in-depth studies on the hole-filling of triangle mesh models, among which the typical repair methods were as follows: the feature plane is calculated based on the point distribution of holes and the points of holes were projected onto the feature plane for partitioning. Then, implicit function , radial Jun  proposedTo sum up, the repair methods for a specific type of holes, especially for simple holes, are relatively sophisticated, having delivered good repair results in previous research. However, the identification and partitioning of complex holes and the corresponding repair methods vary with each case. Adopting a uniform repair method will lead to self-intersecting surfaces in hole repair and unsatisfactory repair outcomes. At the same time, the human-computer interaction is often adopted in hole repair with ultralow identification and repair efficiency. In this case, the repair outcomes depend on the precision of the repair algorithm and the operator's skills. In order to solve this problem, based on the topological form of holes, ways must be found to intelligently identify and partition complex holes with multiconnected domains and self-intersecting boundaries. Thus, a method of automatically detecting, identifying, and partitioning of holes in triangle mesh models should be proposed, aiming at the nested and crossed characteristics of complex continuous holes. It can make up for the deficiency of various simple hole repair methods in complex holes.Single holes: the points on the hole line can be connected in turn to form a closed loop with a unique solution. This is shown in Continuous holes: a continuous hole consists of multiple single holes, and there are one or more intersection points between the single holes. It offers multiple ways of representing each individual closed loop. As shown in Through compiling and analyzing the data of hole repair failure cases in the triangle mesh model, it is found that the failure often occurs when multiple holes are adjacent or nested to each other. It leads to mapping failure and thus unsatisfactory repair outcomes. In order to solve this problem, it is necessary to analyze the characteristics of such holes and differentiate them from single holes. Through analyzing many cases, it is found that the main difference is whether there are intersection points connecting multiple hole lines. On this basis, all the holes are classified into two types.Because of the large difference between repairing single holes and continuous holes in triangle meshes in terms of difficulties and methods, it is an essential step to classify and identify all holes and determine whether they are single holes or continuous holes before hole repair. Before identification, it is necessary to preprocess the input triangle mesh model and quickly establish the topological structure of the model's facets, edges, and vertices. According to the relationship between facets and edges of the triangle mesh model, all simply connected domains of the triangle mesh model are obtained, and each simply connected domain is regarded as a part. Free edges are found for each part, and all free edges are connected from end to end according to the connection between edges and points to obtain hole lines, so the basic information of all holes is obtained, including hole lines, hole directions, and the relationship between holes and parts. Among them, a free edge refers to an edge connected with only one facet. A hole line refers to the closed line formed by connecting the ends of free edges in each set, maintaining the topological relationship of the original triangle mesh model.On this basis, an identification process of single holes and continuous holes is put forward, as shown in   Step 1: detect the boundary of continuous holes.  Based on the information obtained during identification, the boundary of the continuous hole is detected, and the corresponding hole line is formed in the counterclockwise direction. For example, the continuous hole boundary in   Step 2: divide the boundary line.  According to the intersection points of continuous holes, the inner area of continuous holes is divided into several separate holes. For example, the boundary of continuous holes in   Step 3: distinguish the inner holes from outer ones.  The points on the hole line are projected onto the plane, and the positional relations of each hole are calculated according to the relative inner and outer relations between the points and the polygon. If the points of a hole L1 are all outside of another hole L2, it is assumed that L1 is outside of L2. If a hole line is not inside any of the holes, it is an outer hole, and if a hole line is inside one of the holes, it is an inner hole.  Step 4: judge whether it is an outer hole.  If the outer hole does not contain the inner hole, jump to Step 7; otherwise, continue with Step 5. It can be concluded that the outer holes in the continuous holes in   Step 5: identify and determine the new boundary point.  For an outer hole that contains an inner hole, the intersection point in its hole line must be found, and there are three important edges connected to the intersection point, namely, the first edge connecting to the previous index point of the current outer hole line and the second and third edge connecting to two neighbouring index points in the inner hole line. Calculate the included angle between the first edge and the second and third edge. Select the neighbouring index point with the smallest included angle as the new boundary point. For example,   Step 6: form a new boundary.  Starting from the new boundary point, all the points on the inner hole are inserted into the boundary of the outer hole in accordance with the connection order on the boundary, and the outer and inner edges of the hole are combined. In other words, in the example, the inner hole C–P6–P4–C is inserted into the outer hole A-B-C-P3-D-A and the new boundary line is A-B-C-P6-P4-C-P3-D-A.  Step 7: output single holes.  All the holes that meet the conditions are output as a single hole, with the boundary lines given. According to this method, the example outputs four single holes with the boundary lines A-P1-A, B–P5–B, D-P2-D, and A-B-C-P6-P4-C-P3-D-A. The filled area as shown in   Step 8: end.In order to facilitate the repair of triangle mesh models, the complex continuous holes must be divided into single holes to improve the accuracy and repair quality of each single hole repair algorithm. The nesting and connection of the hole lines in continuous holes are likely to cause errors in the partitioning of multiple holes. To solve this problem, a decomposition method of multiply connected domains based on intersection points is proposed to segment and reconstruct the hole lines of continuous holes. The specific steps are shown in When the hole in this case is processed by traditional hole identification methods and software such as Geomagic, C–P6–P4–C can be easily identified and be output as a single hole. If filled based on this, the model obtained will not be desirable and a large number of self-intersecting surfaces will be created. As shown in Crack hole: if the included angle and the cumulative included angle of any two sides forming the hole boundary are less than the set angle threshold, such a single hole is defined as a crack hole. Its shape resembles a straight line. The general rule of thumb is that the angle threshold is set between 5° and 10°, which is used to determine whether the hole resembles a straight line. The cumulative angle refers to the sum of angles between all boundary edges starting from the first edge. Crack holes, which occur in modelling or format conversion, are linear bad edges that are not sewn together at the surface joints. There are no triangles missing, but the adjacent triangles are not connected, as shown in Dislocation hole: if the distance between the points on the hole edge and on the nearest edge is very short (less than the set distance threshold), such holes are called dislocation holes, as shown in Annular hole: it refers to a hole composed of two boundary lines, nearly coplanar or parallel, with similar shapes and without intersection points, as indicated in Simple hole: it refers to a single hole with no triangle surface data inside and only one closed boundary, as shown in Island hole: it refers to the existence of an independent grid area composed of triangle surfaces inside a hole, as shown in After the complex continuous holes are divided into single holes, the geometry of each single hole still varies, posing challenges to the hole repair process. Therefore, by analyzing causes of self-intersecting surfaces based on the geometry of the hole and the current sophisticated hole repair methods, the single hole is further classified into five types: crack hole, dislocation hole, annular hole, simple hole, and island hole. Their respective characteristics are as follows:  Step 1: preprocess the model.  Traverse all the parts of the triangular model and output the basic information of every single hole, for instance, vertexes, free edges, and boundaries. This step provides a data basis for other steps.  Step 2: judge whether there is a crack hole.  Calculate the included angle and the cumulative included angle of any two edges that constitute the single hole. If the value is less than the set threshold value, output the hole as a crack hole; if not, proceed to Step 3.  Step 3: judge whether there is a dislocation hole.  Calculate the distance between points on the single hole and on their nearest edges. If it is less than the set distance, the current point is the dislocation point and output the hole as a dislocation hole; if not, continue with Step 4.  Step 4: judge whether there is an annular hole.  Determine whether there are two boundaries that are the closest and most similar to each other and whether the gravity center of one boundary is within another boundary on the fitting plane. If so, output the hole as an annular hole; if not, proceed to Step 5.  Step 5: judge whether there is an island hole.  Determine whether there are one or more boundaries that are inside another boundary. If so, output it as an island hole; if not, output it as a simple hole.  Step 6: end.After the identification and the segmentation of complex holes into single holes, the shapes of single holes are still different. To prevent various repair defects, different repair methods should be adopted according to hole types. Therefore, an intelligent identification method of single holes is researched to avoid human errors, which can judge the types of five holes in turn and realize the automatic and accurate identification of single holes. The specific judgment process is shown in ®Core i7-4790 processor and 8 GB of RAM on Windows 10. In the process of comparison, various proven methods in repairing different single holes can be chosen. Crack holes and dislocation holes are repaired through the suture or kneading of the corresponding points. Annular holes, simple holes, and island holes are repaired with the variational implicit surface algorithm [As few studies have been conducted on the classification and identification method of the complex holes with intersection points in triangle mesh models, it is difficult to find a targeted algorithm for comparison. For this reason, the well-known commercial software Geomagic is selected to determine the identification and repair effect of the proposed method. All the developed algorithms are implemented in C++ by using Visual Studio 2013 and tested on a PC equipped with an Intellgorithm –30.Taking the backpack model in The repair result of the algorithm proposed in this paper is compared with Geomagic. In terms of the identification and repair of continuous holes, the proposed method can accurately complete the complex hole segmentation and repair, but the commercial software generates multiple spikes and a large number of highly refracted edges, as shown in In terms of the identification and repair of crack holes and dislocation holes, as shown in For the identification and repair of complex holes, the hat model and the ornament model are also chosen for validation, as shown in Figures To accurately check the repair quality, the above two repaired models were scanned by a grid doctor. In terms of the six types of repair defects, including nonmanifold edges, self-intersections, highly-creased edges, spikes, small components, and small holes, Geomagic is found to generate more defects such as spikes, highly creased edges, and self-intersections, while the model repaired by the proposed method has fewer defects and a higher repair quality. The repair time of the proposed method is equivalent to Geomagic; thus, the repair efficiency is acceptable. Specific comparison information is shown in Through the above examples, it can be found that the complex continuous holes containing intersection points can be segmented and all kinds of holes can be accurately identified with the proposed method, as shown in In this work, an intelligent identification and classification method of complex holes is proposed. It can identify, partition, and reconstruct complex holes into single holes based on the intersection points connecting multiple hole lines and classify all the single holes into five types, such as crack holes, dislocation holes, annular holes, simple holes, and island holes. Thus, proper hole repair algorithms can be selected according to the type of the single holes. Compared with Geomagic software, the proposed method can mostly avoid the intersection of various hole lines and reduce the defects of traditional hole repair methods, but it can achieve similar efficiency. The proposed method provides a new idea for the automatic hole repair of triangle meshes without the participation of professionals and can be widely used in 3D printing model repair.In future work, first, the identification method of boundary holes that do not have closed hole lines should be researched. Second, it is recommended to select and study more accurate repair algorithms for some single holes with large curvature variations to further improve the repair results.