Microneedles (MNs) are considered to be a novel smart injection system that causes significantly low skin invasion upon puncturing, due to the micron-sized dimensions that pierce into the skin painlessly. This allows transdermal delivery of numerous therapeutic molecules, such as insulin and vaccines. The fabrication of MNs is carried out through conventional old methods such as molding, as well as through newer and more sophisticated technologies, such as three-dimensional (3D) printing, which is considered to be a superior, more accurate, and more time- and production-efficient method than conventional methods. Three-dimensional printing is becoming an innovative method that is used in education through building intricate models, as well as being employed in the synthesis of fabrics, medical devices, medical implants, and orthoses/prostheses. Moreover, it has revolutionary applications in the pharmaceutical, cosmeceutical, and medical fields. Having the capacity to design patient-tailored devices according to their dimensions, along with specified dosage forms, has allowed 3D printing to stand out in the medical field. The different techniques of 3D printing allow for the production of many types of needles with different materials, such as hollow MNs and solid MNs. This review covers the benefits and drawbacks of 3D printing, methods used in 3D printing, types of 3D-printed MNs, characterization of 3D-printed MNs, general applications of 3D printing, and transdermal delivery using 3D-printed MNs. Transdermal drug delivery (TDD) systems cover a broad array of non- and minimally invasive approaches for delivering drugs and vaccines via the skin without the use of injections . The majMicroneedles (MNs) are micron-scale needles ,5 that cMNs’ design can control the drug release. If a bolus release is required, then the drug substance should be integrated into a dissolving MN directly, and the rate of the release in this case is largely dependent on the MN’s dissolution rate. However, since the quantity of the drug is small, this restricts the dose to less than 1 mg, in an MN patch of a few hundred MNs. In case of sustained release, a higher quantity of the drug is needed; thus, the additional amount is added into a backing layer of the dissolving MNs’ array patch .Transdermal MNs have permitted the transport of several active compounds across the skin over the last 20 years. Gerstel and Martin had the first MN patent in 1976, as MNs were presented without pain for a TDD system and prevented tissue damage. In 2004, Mark Prausnitz proposed that MN arrays could increase the transport of both small molecules, macromolecules, and supramolecular complexes. There have been significant advances in MNs’ applications. The phrase “microneedle” (MN) was originally used in 1921  to descrMNs are available in a variety of sizes . The proper size and design for MNs allows them to pierce the skin’s outermost layer to reach the layers underneath. This allows the delivery of loaded substances. These advantages, along with the ability of usage without professional help, make MN approaches patient-friendly ,27.MNs can be fabricated by several conventional methods, such as micromilling process, which includes wet and/or dry cutting ; photoliThe International Organization for Standardization defines “3D printing” as “the fabrication of things by the deposition of a substance utilizing a print head, nozzle, or another printer technology” . Thus, iRecently, a broad variety of 3D printing technologies has opened up an exciting avenue of study to fabricate MNs ,40 and aThere are numerous reviews that have been published about MNs and 3D-printed MNs ,47,48,493D printing is mostly well known for the capability of manufacturing patient-specific tailored products and the easiness of developing prototypes. Although the printers are expensive, 3D printing has an overall low production cost, which compensates for the high price of the printers. Additionally, 3D printing has no storage cost, and it is biocompatible, cost-effective, and has high output volume and high production rates. Extremely important benefits of 3D printing include improving the accuracy, efficacy, convenience, and safety of medicines or medical devices ,42,50,51However, there are drawbacks to 3D printing, which include restrictions on the raw materials used in printing. Additionally, the dimensions of the printed objects are limited, and problems related to patency are present, such as the existence of many patented techniques and methods, which raises the costs of the materials and methods, such as in stereolithography (SLA) and selective laser sintering (SLS). Furthermore, there is a lack of checking of the parallel manufacturing of hazardous materials .3D printing begins with creating  models of the objects that are going to be printed. The design is generated in a CAD file utilizing a 3D modelling tool or a 3D scanner. Three-dimensional designs are often translated to the Standard Triangulation Language (STL) file format, which represents a 3D model’s exterior surface. Then, the 3D printing software will cut the exteriors into discrete printable layers and send the layer-by-layer digital instructions to the printer. The produced items may undergo further dehydration, annealing, polishing, or other post-processing procedures after printing . The difThe American Society for Testing and Materials has classified the 3D printing procedure—or additive fabrication—into seven types, with reference to the basic terms of additive fabrication techniques. The following are the seven main techniques.In this method, inkjet printers spray the prepared formula of a pharmaceutical dosage form or binders  in the form of small droplets at a specified speed and size into a powder bed . The actThere are many types of printers that employ the same technology but differ in the materials and binders used. Sands, ceramics, and metals are among the most typical materials used. However, plastics can also be used. Binder jetting can create parts that have acceptable tolerances, but it can be challenging to predict the final tolerance, because shrinking happens during post-processing .Metal components are incredibly fragile before infiltration and can crumble if not handled carefully. They are nearly fully dense after infiltration, but their mechanical characteristics fall short of those of parts that are made in an old-fashioned way. These metal parts have smoother surfaces than those made with direct metal laser sintering (DMLS) and selective laser melting (SLM). Binder-jetted metal components can still be useful even though their mechanical characteristics do not match the strength or tolerances of powder bed fusion (PBF) prints when they are infiltrated and sintered .This method is even more affordable than vat photopolymerization and PBF. For low-volume runs, print speeds are comparable to PBF and in line with other technologies, but they increase quickly as the volume increases. This method is perfect for full-color prototyping, because it can quickly and cheaply produce complex parts in a variety of colors. Binder jetting is less expensive than material jetting, and despite its limitations in terms of mechanical properties, it can still produce resolutions that are good enough for most prototypes. Binder jetting is particularly appealing for creating intricate sand casts because it can print large, intricate geometries at a reasonable price. Additionally, the procedure is easy enough to incorporate with the majority of conventional foundry processes ,63.This is a different method from the previously mentioned method (binder jetting), in that it is more difficult to apply. The material jet device is presented in Despite the durability of the parts produced by material jetting printers typically being less than what fused deposition modelling (FDM) or PBF can achieve, they are extremely accurate and capable of producing parts with extremely high tolerances. Although the surface finishes are very smooth, printing in a matte setting is an option.The cost of material jetting varies depending on the type of printer. There is a lot of material waste per part because the materials are expensive and the support structures are printed solid. Its speed is comparable to the production speeds of PBF. Although material jetting is an expensive 3D printing technology, it is the only practical option when dimensional accuracy or visually appealing designs are essential, due to its quite high dimensional accuracy and smooth surface finishes. Highly realistic prototypes, anatomical models, intricate and highly precise tooling, jewelry, medical equipment, and surgical tools frequently fall under this category. Prototypes for haptic feedback are frequently created using multi-material printing, such as a rigid case with flexible buttons ,63.The substance is injected through a nozzle that is controlled by a robot. This method does not need a powder bed, and the printing can be carried out on any material. The material extrusion printing device is presented in Although there are occasional exceptions, FDM printers are typically not used to create functional end-use parts. Since the parts that they print are weak along the z-axis, they are not among the most precise 3D printers. On all surfaces of the parts, layering is also obvious.Industrial-grade machines can be considerably more costly and beyond the price range of even the most devoted hobbyists. Even though material extrusion machines are inexpensive to buy and simple to operate, outsourcing is still common because one-day lead times are now standard due to the technology’s widespread use. Standard FDM materials are widely accessible, and pricing is kept low by competition. Unlike vat photopolymerization or PBF, single-part printing is quick, but there are no scale-up advantages. As a result, FDM is comparatively slow for high-volume runs and probably not the best option for many different parts ,63.FDM parts are very affordable even though they are not as strong or aesthetically pleasing as parts created in other ways. Due to these qualities, FDM is the most frequently used prototyping technology, particularly at the proof-of-concept stage. If the resolution and surface finish are not crucial considerations, FDM’s availability of a wide range of materials, speed, and affordability make it highly desirable for certain types of production parts. Industrial FDM printers can quickly create working prototypes and finished products out of durable materials such as grips, jigs, and fixtures. FDM manufacturing is more cost-effective than traditional manufacturing when producing these latter components ,63.PBF includes sintering, which means fractional superficial liquefaction and congealing or binding of particulates that have high a melting point with a low-melting-point binder . PrinterPBF technology is utilized by many 3D printers. SLS, DMLS, SLM, HP’s multi-jet fusion (MJF), high-speed sintering (HSS), and electron-beam melting (EBM) are the most widely used types of printers. For plastics, SLS is the most popular, while for metals DMLS and SLM are the most popular. These printers are also capable of printing high-resolution parts. PBF can create parts from extremely complex digital models, because unused powder serves as a support material as the print layers are built up. Both MJF and SLS have a similar potential for complexity, both outperforming SLA. Although SLS always prints at 100 microns and MJF at 80 microns, SLA has even higher resolution (its layer height can go as low as 25 microns). SLS has a wider selection of materials, but MJF can produce slightly better resolutions. DMLS, SLM, and EBM are able to print parts for metal with some of the greatest resolutions currently available .PBF can produce tolerances that are comparable to those produced by vat photopolymerization, but PBF parts are much more robust. PBF is able to create plastic components that are both functional and have the best mechanical qualities of any 3D printing technology. MJF prints have a smoother surface finish and are marginally stronger than SLS prints ,63.EBM systems have a lower potential for distortion because they generate fewer residual stresses than DMLS and SLM. Since the powder particles are only lightly sintered and the parts are still slightly porous, the metal parts produced by DMLS are not as strong as those produced by SLM. SLM parts, however, can match traditional manufacturing processes such as forging and machining in terms of their mechanical properties. Due to the use of powders in their creation, PBF prints all have a slightly rough finish, but they are easily polishable.Although PBF market competition keeps prices down, it is still expensive. The cost of 3D printing metal is still significantly higher than that of computer numerical control (CNC) machining. The price is comparable to that of vat photopolymerization for plastics. Typically, MJF is about 10% less expensive than SLS. SLS and MJF take longer to produce low volumes of plastics than vat photopolymerization and FDM. However, because components are printed directly on the build platform, they are the quickest for large quantities. PBF is the favored method for producing low volumes of plastic parts across all industries, because it can create strong functional parts. One-off industrial hardware—such as machine parts, jigs, grips, and fixtures—and low-volume production runs of specialized plastic parts are typical applications. PBF is the preferred technology for rapid prototyping because it can make extremely complex parts ,63.This method is also known as SLA . A polymSLA, direct light processing (DLP), and continuous liquid interface production (CLIP) are the three most popular subtypes. They are very similar in terms of how a light source directs light at the resin. Overall, SLA is the vat photopolymerization printer technology that is most popular and widely used.Photopolymer resins, the majority of which are proprietary, are used in this method. Standard resins for all-purpose prototyping are some of the many types that are readily available. In addition to these, there are also strong acrylonitrile-butadiene-styrene-like resins, elastic rubber-like resins, transparent castable resins with no ash content after burnout, ceramic-filled resins for extremely rigid prints, and biocompatible resins for medical devices ,63.Vat photopolymerization is capable of printing extremely complex parts, but they cannot produce parts as complex as PBF because they require support structures. These devices can print extremely fine details ,63.Directed energy disposition (DED) uses a direct energy source such as a laser or electron beam to melt down raw substances, after which they are deposited. The DED process involves layering beads of molten material—typically metal. The method of printing is essentially the same as that used in metal extrusion printers for plastic. The feedstock material, which can be wire or a powdered form, is continually pushed via a nozzle and melted at the point of deposition by a laser, electron beam, or arc before cooling and solidifying .The DED 3D printing device is presented in The enormous print-bed sizes of DED are yet another significant benefit. Large manufacturers frequently construct DED printers for customers with build envelopes that are several meters long along any dimension. Because there is no room for overhangs due to the size of the liquid melt pool at the deposition point, support structures are possible but challenging. The same characteristic makes complex geometries impossible. Compared to other metal 3D printers, the resolution is very poor. Powder particle sizes range from 50 to 150 microns, and the diameter of the welding wire is 1 to 3 mm. For instance, sharp corners can only be produced through post-processing . AlthougPart repairs, feature additions, and near-net-shape part production are the three primary uses of DED. This makes it perfect for adding features and fixing damaged parts that cannot be added using other processes. The vast majority of uses involve tool repair, and businesses turn to DED when expensive machinery can be repaired more affordably than it can be replaced, such as in heavy industry ,63.By stacking and laminating sheets of a substance with single horizontal cross-sections, sheet lamination creates parts. Several printers laminate the sheets after they have been cut. In the majority of cases, the sheets are laid, laminated, and then cut to size .This is among the simplest ways to construct 3D models. Even though it is straightforward, there are numerous distinct proprietary technologies based on the material, lamination technique, and cutting technique. Most of the time, the procedure is a straightforward variation of laminating objects from paper. The only radically different technology is ultrasonic consolidation, which employs ultrasonic welding rather than a different bonding substance.There are many materials that can be used with printers of all kinds, including papers, most polymers, fiber-reinforced polymers, ceramics, and almost any metal. All of these materials can be used to create multi-material layers as long as they can all be laminated and shaped using the same techniques .Although the size of sheet lamination print beds varies considerably, they are comparable to those of SLA and SLS printers. The use of large-format printers is uncommon. Highly complex shapes are not possible, because the sheet-cutting techniques are so basic. However, internal structures are possible, because support structures are not required. Embedded wiring between sheets is an additional design choice. Since most processes do not need heat, there is less of a chance that high temperatures will ruin them. The material feedstock is the only factor that influences the resolution. However, because of the brittleness of the bonds between the sheets, these parts cannot be used for structural or functional uses ,63.This technique is cheap and fast, but it has low accuracy and wastes a lot of raw materials . This meNumerous methods are available for 3D printing; however, only a few are used in the medical field due to the need to use materials that are pure and compatible with the body. These techniques are powder-based printing , vat-pol(1)A scheme of the requested design is obtained digitally using the CAD software, and the dimensions are improved using printer specifications.(2)The scheme is transformed into a Standard Tessellation Language format, transferred to a computer that manages the 3D printer, and the size and orientation of the printing are selected ,74.(3)The item is printed in the form of continuous layers .The manufacturing of 3D-printed MNs comprises three key stages:3D printing techniques are categorized according to the source of energy, types of substances used, or any other mechanical properties. Three technologies are widely used in the pharmaceutical industry, which are discussed in the following subsections ,76.This approach is also known as extrusion-based FFF, or FDM. It is a common technology in the fabrication of MNs. It is an easy and uncomplicated method that is used for plastic materials that are melted as a filament and liquefied using a liquefier head. The temperature used is higher than the melting point of the plastic material. Afterwards, consecutive layers of the melted plastic are deposited on a plate and through a nozzle. The layers cool and solidify in a rapid manner. The x- and y-axes control the movement of the printer’s head, while the z-axis controls the platform; thus, a 3D object is created .Many features control the quality of the produced MNs, including the process parameters—for instance, the selection of the nozzle diameter and plate diameter, the feeding rate, the speed of the printer, the thickness of the layers, and the positioning of the built object. Therefore, it is important to study these parameters thoroughly in order to reach the best and optimized specifications, giving a final product that has a good surface finishing and is strong and resistant to mechanical stresses, in addition to other properties that are required in the printed object ,79. The Many types of MN arrays have been printed with polylactic acid. They have the advantages of being recycled, biodegradable, FDA-approved, thermoplastic materials. When the final steps of manufacturing were followed by chemical etching using an alkaline solution, MNs with sharp tips were produced, without affecting the other mechanical and chemical features . To solvFDM printing has been used to produce hollow MNs, and through tailored programs the release profiles of various drugs were controlled—for example, in the case of vascular endothelial growth factor, which helps in wound healing, hair growth, and angiogenesis .The photopolymerization-based or photocuring 3D printing technique is among the first invented methods and is a very common technique. It uses laser emissions or light projections in order to polymerize photosensitive polymers . Vat pho(1)Free surface: objects are created from the bottom up in a support platform that resides directly beneath resin surface.(2)Constrained surface: represented as “bat” configuration, where the platform has a building platform that is suspended over a resin bath .Based on the filling method, the SLA procedures can be classified into two major groups:The SLA method is a very common printing technology due to the reported features of being able to manufacture solid MNs that are smaller than 100 µm, with outstanding mechanical strength and penetration capability .Yao et al. (2019) have proven that precision and stiffness can be affected by the exposure time of each layer through creating a high-precision digital light processing (H-P DLP) 3D printing that is based on light curing . The samDLP depends on photopolymerization-based technology. It is similar to the SLA method, but the only difference is the light source. DLP has more speed than SLA, using a smart projector that increases resolution, such as a digital micromirror device (DMD) . It has Hollow MN arrays were made from photolabile acrylate-based polymer resin using DMD™-based micro-SLA . Solid MAnother type of vat polymerization technique is known as liquid-crystal display (LCD). The main approach uses a resin that solidifies the layers using UV light. In LCD 3DP, the fluid crystal is employed as a photo-taking structure. The advantages of the bottom-up 3D printing process over top-down techniques is that it achieves great upright resolution with less curing time and lesser quantities of resin for the fabrication process ,89. AccuCLIP is one of the newest technologies in 3D printing, invented in 2015. This technique, in modern procedures, replaces the original layer-by-layer SLA . The metTwo-photon polymerization (TPP/2PP) is considered to be the most accurate 3D printing technology. It allows for the printing of consecutive layers of objects from a different variety of substances—either fluids or solids—at the micro- or nanoscale. The main energy source is a near-infrared femtosecond laser. TPP has the highest resolution, with the ability to reach up to 100 nm horizontal resolution and 300 nm axial resolution . The met®—a modified ceramic that is biologically safe and non-toxic—which showed important characteristics of mechanical durability when they punctured into the skin [Many types of MNs have been successfully fabricated by researchers. For instance, Moussi et al. (2021) succeeded in printing hollow MNs that could be embedded in the body using this procedure, along with an associated storage chamber . Doraiswthe skin . Ovsianithe skin ,98.This method was first described by Carl R. Deckard, and he obtained a patent for it in 1989 (US 4863538 patent) ,100. It DMLS of 316 L stainless steel (SS).Lost-wax casting of sterling silver using DLP/SLA-printed wax masters.Binder inkjet printing of 316 L SS.The method begins with CAD data from the file formatted as STL and is followed by a uniform dispersion of the powder onto the structure’s platform with the help of a roller and a slot feeder . The finBinder inkjet printing of 316 L SS MNs showed very few in-plane and out-plane deviations. Binder inkjet printing was also able to adjust the final shape with high accuracy. However, DMLS synthesized tiny MNs with finishes that were less accurate compared to the binder inkjet printing .Inkjet printing is a method that is based on Lord Rayleigh’s instability theory, which clarifies the breaking of a liquid into jet droplets . This prThe benefits and drawbacks of various 3D printing methods, in addition to the materials used are summarized, in TDD systems include an extensive range of non-invasive or minimally invasive techniques for administering medications and vaccinations via the skin without the use of needles . MNs canMNs have the characteristic of causing less pain or even no pain due to their microscopic dimensions, in addition to being user-friendly . MoreoveModels are vital for studying diseases. The formation of such models using conventional techniques requires a large number of experimental animals. For instance, patient-derived xenograft models for pharmacological studies are costly and time-inefficient due to the need for a huge quantity of immunodeficient mice to engraft disease cells; thus, 3D printing can overcome this disadvantage through accurate, ideal, biomimetic models with high resolution in a time- and cost-saving matter. The organs include complicated structures such as the kidneys, skin, liver, and various tumor types. For example, 3D livers encapsulated in hybrid hydrogels were printed through vat polymerization technology . AdditioConventional methods have been used to manufacture medical implants and prostheses for hundreds of years, and over the years they have shown some issues, such as structural and functional mismatching, incomplete binding, lack of strength and primary steadiness, low bone development, long-term stability issues, and low cost-effectiveness . These dOn the other hand, 3D printing offers great manufacturing prospects for medical devices . It provPowder bed printing was the first 3D printing method used for tablet fabrication ,138. 3D Biodegradable implants have been developed for drug delivery . Wu et aThree-dimensional printing is used for the production of implantable dosage forms. In 2007, Huang et al. (2007) created monolithic levofloxacin implants for comparison with compression implants and implants with complicated design for pulsed and bimodal release . BecauseDevices and medical implants, such as stents and catheters, are often coated or spray-coated with materials for localized effects ,147; howTDD systems include an extensive range of non-invasive or minimally invasive techniques for administering medications and vaccinations via the skin without the use of needles . TransdeAn array of solid MNs were inserted through the skin in an earlier stage of microneedle research to overcome the stratum corneum’s barrier effect. After treating the skin surface with silicon wafer-based needles, a medicated patch was placed on it. The interstitial fluid was also extracted using this method to measure the glucose level non-invasively .The development of solid MNs coated with drug solution using a dip-coating method was the main goal of subsequent research in microneedle technology. In this ‘coat and poke’ method, only a small amount of drug (about 1 mg) could be coated on top of MN\s, necessitating extensive optimization for uniform coating.A “poke and release” strategy was created as a result of additional research. The “poke and release” method had the advantage that the drug release could be controlled to meet the needs using a variety of readily available polymers and polysaccharides. In contrast to other physical approaches, the administration of a large dose of medication was still impractical with soluble or biodegradable microneedles, which prompted the creation of hollow microneedles. This method, called “poke and flow” involved permeating the skin and allowing the drug to flow through hollow microneedles from the reservoir in the patch afterward ,150,151.Hollow microneedle arrays with a drug reservoir were created by Wang et al. (2009) . When thFor effective drug delivery, the microneedle’s design must be such that it does not break and does not cause pain or irritation. Drugs can be delivered either systemically or locally using any of the methods mentioned above.The materials used in MN fabrication must be simple to manufacture, strong enough to penetrate the skin barriers, compatible with the active ingredient, biodegradable, and safe. The choice of materials will affect the tensile strength or hardness, loading efficiency, stability, and biocompatibility of the MNs ,154. EacMNs can be divided into hollow and solid in terms of their structure, and they can also be classified into five categories in terms of application.These are manufactured using numerous substances, such as glass, polymers, metals, and ceramics ,156,157.Solid MNs are composed of stainless steel ,164, silIn this approach, the treatment formula is coated onto the solid MNs, and the drug formula dissolves into the punctured skin . This tyDissolving MNs can be composed of several substances, such as polyvinylpyrrolidone, polyvinyl alcohol, carboxymethylcellulose, chondroitin sulfate, and sugars such as maltose, dextran, or galactose . The relSwelling or hydrogel MNs are composed of aqueous blends of polymeric substances such as poly . They coPolymeric MNs are made from degradable polymers with controlled degradation rates, or from non-degradable polymers such as polydimethylsiloxane, polyacrylic acid, and polyvinyl methyl vinyl ether. In order to provide various mechanical features and functions, degradable MNs with varying swelling rates, degradation rates, and biological reactions can be prepared from natural polymers  and synthetic polymers  ,11,12,13Biodegradable polymer MN arrays are a modern class of MNs that are intended for TDD. These needles are composed of thermoplastic polylactic acid, which is a substance that can be recycled and is environmentally friendly, since it degrades naturally. They are manufactured through FDM, which increases their cost. Normal degradation can affect the resolution in the FDM method, which leads to problems in the tips of the MNs. This was solved by Micheal et al. (2018), who created a chemical etching protocol that follows the FDM method. This permits the manufacture of MNs with tips as small as 1 μm. The resulting needles were tested on pig skin, where the MNs easily pierced the skin. The distinctive features of stability and degradability enabled these MNs to be filled with small treatments and then discharged into the skin gradually . The samAny 3D-printed MNs should comply with certain criteria to succeed as delivery systems—for instance, the ability to be inserted into the deep layers of the skin, with mechanical durability that prevents breakage, suitable dimensions and design, and utilizing a proper biocompatible molecule ,183.The substances exploited in MNs must have the ability to withstand mechanical stresses and compression, to allow insertion deep into the skin. Moreover, stability, safety, biocompatibility, medication filling, stretchability, and flexibility are required in the MNs ,154,184.The sheet width, which has a chief impact on accuracy. This impact is particularly important in the construction of bent surfaces, owing to the distinct “staircase” phenomenon. Hence, by increasing the sheet width, the accuracy of the produced material is reduced, and the material is rough .The dimensions of the needle should be carefully identified. If the MN is too lengthy or brittle, it will break during insertion into the skin ,194. ForObtaining high-quality MNs is associated with analyzing and identifying the important factors in the procedure. Optimization of these factors is vital for refining the configuration and dimensions of the product, leading to improved accuracy . To achiThe dimensions and design of the MN have a direct effect on the MN’s performance, irrespective of the fabrication method. The dimensions should be ideal to achieve optimal insertion and skin penetration, in addition to the required rate of drug delivery. The main geometrical dimensions are the MN length, the base and tip width, and the core and shape of the MN ,197. MNsAnother important parameter is the sharpness of the tip. A sharp tip allows for good infiltration of the skin, with the lowest possible force of insertion ,194. DavThe shape of the MN—which is either conical, pyramidal, or cylindrical—has an effect on the MN’s performance. As was discovered by Pere et al. (2018), conical MNs need less strength to pierce experimental pig skin models than the force required with pyramidal shapes . HoweverThe shapes are affected by the type of manufacturing method used. The results of two studies conducted by Sirbubalo et al. (2021) and Camović et al. (2019) showed difficulties in achieving sharp tips with seven shapes through the FDM 3D printing technique, due to the reduced resolution of the printing nozzle, even after alkaline treatment ,181. HowAll of the important parameters—such as the geometry, dimensions, surface morphology, and distribution—of MNs on the array must be assessed. The available techniques for assessment are visual inspection, scanning electron microscopy, and stereomicroscopy . Drop shMNs are subjected to numerous stress conditions during application because of the lack of uniform skin surfaces, along with sudden movements and the mechanical tension applied on the MN once it is taken off the skin . It is vThese tests are essential in determining if the MN has adequate mechanical strength to endure distortions and changes throughout application into the skin.A.Axial Fracture Force TestsThis kind of test estimates the malfunctioning of MNs due to both transverse and axial loading. This test is performed by placing pressure on the MN array parallel to a firm metal surface. The test continues to quantify the force and dislocation while producing pressure against the strain curves . When MNCompression studies include pressing of MNs against a solid metal surface, wherein the whole applied force is focused on the connection surface of the MN tip. The forces utilized in skin penetration by MNs are distributed over a larger MN area, particularly after primary inclusion, as the elastic skin wraps around the MNs .B.Transverse Fracture ForceThese kinds of tests are used to evaluate MNs’ usage. This test is performed on single MNs or on a row MNs, wherein the applied force can be divided by the number of MNs in the row in order to estimate the transverse fracture force per MN. The main drawback of this test is the need for manual orientation of the metal probe with a definite size, which might lead to inaccurate measurements .C.Baseplate Strength and Flexibility TestsBreakage of the baseplate while the MN is inserted by the patient is to be avoided; thus, the strength/breaking force needs to be evaluated. Flexibility is an important parameter that plays an essential part in the MNs piercing into the skin, and it should be adequate for them to penetrate into different skin types without breakage. The test is performed by placing the baseplates between two blocks made from aluminum, and force is then applied to a metal probe. Thus, the force needed to break the baseplate can be calculated by noting the maximum peak value on the force–distance curve. The baseplate’s flexibility can be measured from its bending upon fracture .D.Insertion Force TestsDefining the MNs’ insertion force enables the estimation of the suitable length of the MNs. Calculating the insertion force needed for the MNs to penetrate the skin is vital for assessing fracture forces.The force required for fracturing the needle should be much greater than the force needed for the insertion of the MNs into the skin. The insertion process of MNs is performed either through applicators or manually. However, applicators provide better control of the insertion surroundings and lessen the available inconsistency in comparison with insertion without applicators . UsuallyHistological cryosectioning is a method in which the skin that is treated with MNs is taken away and frozen with liquefied nitrogen. In order to facilitate the identification of the channels formed by the MNs, a dye is used—generally hematoxylin and eosin. The depth of the cutaway tissue ranges between 6 and 12 µm . The draConfocal laser scanning microscopy is a technique that enables the calculation of the measurements of minute holes produced by piercing MNs. The skin sample is exposed to treatment with a florescent dye, which enables the calculation of the thickness of the channels made by the MNs. A disadvantage of this method is the inability to quantify MNs longer than 250 µm, since it only detects ranges between 200 and 250 µm ,193. In Optical coherence tomography can detect the needles’ dimensions accurately, without being invasive. A semi-empirical model was created, derived from the findings of seven distinct water injection tests into pig skin tissue. The model used assumed a spherical fluid flow and tissue deformation and forecasted the flow rate over time using optimal experimental data and model parameters. It was based on the short coherence length of wide-band light sources that execute images of very small dimensions . This approach achieved great success due its many advantages, including the follows: (1) High-quality images due to a small range that lies within 1–10 μm axial resolution, and sometimes smaller—to a level of sub-micrometer (0.5 μm) resolution. (2) Fast imaging—for instance, a temporal image can be taken within a speed of milliseconds. (3) Label-free imaging, which means that the optical coherence tomography imaging can provide high-resolution images without the need for a contrast dye. (4) Cost-effectiveness. (5) Sophisticated functions can be added to the technique, such as the imaging of blood flow by Doppler optical coherence tomography ,212. ThiAn archetypal optical coherence tomography system comprises a low-coherence wide-band light source. The radiated light is joined with an interferometer. The light is allocated into two arms: a reference arm and a sample arm. The reference arm conducts the light in the direction of a reference mirror. The sample arm directs the light into the required tissue. The sample arm encompasses an objective lens that concentrates the light onto the sample tissue, such as the brain or the retina. The light that is backscattered from the tissue assemblies is rejoined with the reference light that is reflected from an extremely reflective (>95%) moving reference mirror, generating an interference pattern that can be detected using a light detector. Building two-dimensional (2D) or 3D cross-sectional items, the stream of light is scanned through the sample surface. Sophisticated systems might contain a charge-coupled device camera and diffraction grating , in addiX-ray transmission computational tomography is a modern process that is also non-invasive and uses a number of scans taken at multiple angles, which permits 3D imaging of MNs’ geometry. A disadvantage of this procedure is that it only works on materials that are suitable for X-ray imaging, such as metals. Skin layers that are pierced cannot be easily differentiated using this method ,215.Slight and momentary arrhythmia occasionally occurs when using MNs, which depends on the type of therapy, used along with their size. This can be measured by a technique called the Draize method. A microscope is used to detect the changes that occur on the skin after treatment, through comparing erythema and edema before and after treatment with MNs .A.In Vitro Studies of PermeationIn vivo tests are implemented on either human or animal skin models to determine the quantity of therapeutic agent received by the cells through diffusion. The therapeutic molecule is introduced into the donor compartment and, through membrane diffusion, is transported to the receptor compartment .Transdermal transport of a certain molecule through MNs can be confirmed through implementing sampling of the receptor compartment and analyzing the sample via high-performance liquid chromatography (HPLC). Moreover, extraction analysis methods can be applied to detect intradermal distribution of a specific molecule from the skin model after penetration . The avaB.In Vivo Studies of PermeationIn vivo studies are performed to evaluate the TDD with regard to the absorption, disposition, and permeation of molecules administered through MNs. Choosing a suitable in vivo model depends on some important features, such as the depth and flexibility of the skin. Pig skin is the model of choice in in vivo TDD studies, despite the variation in structural, histological, and morphological properties between humans and pigs. Rat skin is sufficiently suitable for in vivo TDD studies, especially given that rats are less expensive and easier to handle than pigs, although their skin can provide higher permeability than human skin ,224.MNs have been used in TDD for immunobiological administration of vaccines and antibodies, disease treatment and diagnosis, and cosmetic use. The following are some of the applications of MNs.This is one of the most common applications of MNs, especially for biological agents, peptides, proteins, hormones, and natural agents that cannot be given orally due to first-pass metabolism. Thus, hypodermic administration represents a solution.Cancer chemotherapy has a lot of severe side effects, and one of them is the severe pain associated with the injections. Thus, cancer treatments always provide a wide area for research. MNs can offer a solution to this problem, since they are considered to be negligibly invasive, painless, applicable, and user-friendly . For insImmunology in cancer is now extensively studied, and immune-checkpoint inhibitors are considered to be a potential cancer treatment ,227. MorA hollow MN system was achieved with a large synthetic peptide that works as a cancer vaccine, since it enhances T-cell reactions against cancer. The difference between TTD and intradermal injections is that the amount needed in TDD is less than what is required in intradermal injections, improving the efficacy of the vaccines’ immunogenicity and the effectiveness of tumor vaccine formulations . HindranOn the other hand, many studies have used 3D printing for manufacturing MNs used for the delivery of cancer drugs, such as the study conducted by Uddin et al. (2020), who were able to design a polymeric MN array using SLA and inkjet dispensing to coat the MNs with the oncology product cisplatin. The MNs allowed the delivery of 80–90% of cisplatin over the course of 60 min. In vivo studies confirmed a good response to cisplatin using the MN array, through observing tumor regression. These MNs could be loaded with different chemotherapeutic agents. This new treatment approach gave hope for cancer patients . BhatnagDiabetic patients’ compliance mainly relies on providing a less painful needle, especially considered that the patient is sometimes required to inject themselves multiple times per day. In response to this problem, smart insulin MN array patches that manage blood glucose levels in type 1 diabetic patients were developed. They rapidly responded to glucose levels, and they were painless . In a clWu et al. (2020) conducted a study employing the extrusion-based 3D printing technique to produce a microneedle patch. These MNs were intended for the painless supply of insulin to patients with diabetes mellitus, in response to high glucose levels. To fabricate the MNs, comprehensive examination of suitable bionics was performed, using the additive substance alginate hydroxyapatite. The tips of the MNs in the patch array were formed in a cylindrical shape, whereas the base was a conical shape. These MNs were tested on mice and showed good mechanical strength, since they were able to pierce the mice’s skin without being damaged. This also proved that these MNs had good potential to be used in TDD, as they maintained the glucose serum concentrations in mouse models of type 1 diabetes within normal glucose levels for 40 h, when given one time only. Additionally, they improved the symptoms of diabetes .Vaccines are usually administered through intramuscular injection or intradermal injection. However, some people suffer from a phobia of needles, and MNs are a solution for this drawback. The delivery of vaccines through MNs has been widely studied and is finally in practice ,235. ManMNs are utilized in vaccination, which is extremely important in preventing pandemics; thus, painless injections are required to encourage people to take the vaccines. In a very recent study, 3D printing using the CLIP technique was exploited to fabricate a faceted MN, which was characterized by having an enhanced surface area in comparison with pyramidal structures. In mouse models, a comparison between the conventional injections and MNs was made to evaluate the delivery of the vaccine and the resulting immune effect. It was concluded that MNs not only had a higher load retention time, but also led to higher humeral immune activation and increased the total number of immunoglobulins and IgG. Moreover, the MNs enhanced the numbers of T cells, cytotoxic CD8+ T cells, CD4+ cells, and cytokines .®. These are superior to MN patches in terms of their efficiency in penetrating the skin. Newer rollers are manufactured by 3D printing techniques and also emit light at a certain wavelength; one of these devices is DermaFrac™ [Recently, MNs have been excessively employed in the cosmetics field and achieved great success. The main aim of cosmeceuticals is to deliver different molecules into the skin with the least amount of injury. These molecules could be employed either to enhance wound healing or to increase the permeation of pharmaceuticals . MNs havrmaFrac™ .Lim et al. (2021) developed a delivery system for a small peptide with an anti-wrinkle effect by employing the DLP 3D printing technology. Acetyl-hexapeptide 3, which is the peptide used in the novel system, is known for its safety and efficacy as an anti-aging molecule. Optimization of the resins—mainly vinyl pyrrolidone and polyethylene glycol diacrylate—was performed by examining various concentrations of the polymer and evaluating their effect on important factors such as the swelling rate of the polymer and its durability. The developed MN patch exhibited sufficient capacity to permeate the human skin, while maintaining its shape integral after being stressed. Additionally, the polymer was safe to the dermal fibroblasts, making MNs a potential future anti-aging product .®—the first 3D-printed drug to receive FDA approval—in 2015, this technology has advanced through in-depth research. Some examples of different biomaterials and 3D printing techniques that have been used to create MN patches for TDD applications are presented in Since the release of SpritamRegardless of knowing that existing MN manufacturing processes could produce an acceptable resolution, the constraints of traditional techniques already exist. These include labor-intensiveness, the need for manual steps, the requirement for high proficiency in micro-manufacturing for proper implementation, and cost-effectiveness. The ability to directly design, modify, and fabricate proposed MNs with desired size features using 3D printing shortens the design and prototyping process by doing away with the need for outside manufacturing firms. However, there are significant drawbacks to 3D printing, including slow printing, resolution restrictions, a small material selection, and biocompatibility ,256.The extraordinary comfort and accuracy of 3D-printed MNs and the many types of MNs that 3D printing allows to be fabricated—with reasonably low price, high accuracy, and specific aims or patient design—have made 3D printing revolutionary. The various manufacturing methods of 3D printing and the ability to use multiple inks and raw materials, including biocompatible substances, has allowed an upsurge in the utilization of 3D printing in different fields, such as the pharmaceutical, cosmeceutical, and medical fields. This technology has played a huge role in optimizing the TDD dosage forms, through permitting the manufacturing of numerous types of MNs for the administration of multiple molecules, such as insulin and vaccines.Despite the terms “3D printing” and “rapid-prototyping” having been used interchangeably, the actual 3D printing process is slower than traditional industrial approaches such as injection molding . FurtherThere is continued work to find solutions for the current drawbacks in the 3D printing manufacturing techniques, as well as continuous developments in the available characterization and analysis procedures.Future research may therefore focus on developing 3D printing technologies that are faster, without sacrificing resolution. The ultimate resolution of printed MNs can be achieved by improving the laser beam features in laser-based methods and nuzzle features in extrusion-based methods. Moreover, many studies could be conducted to develop other kinds of MNs used in different applications. Furthermore, clinical trials are needed for determining the safety and efficiency of 3D-printed MNs, because to date there have been no clinical trials conducted on these kinds of MNs, since all of the studies have been conducted on animals and mimicking tissues. There have also been no patents filed regarding 3D-printed MNs. Additionally, since many drugs exhibit restricted solubility in polymer compounds , the properties of the drugs might result in certain limitations. Moreover, when exposed to high temperatures or UV rays throughout the printing process, drugs may be absorbed. Thus, new materials and polymers must be used to fabricate different types of MNs in order to enhance drug-loading capabilities. As a result, to improve the quality of TDD systems and encourage their commercialization, new 3D printing systems must be developed in addition to the existing ones. Another issue at this stage is the commercialization of findings. Therefore, solid scientific foundations and effective commercialization plans should be combined to speed patients’ access to this promising technology.Microneedle printing using 4D technology is another potential future pathway. MNs with greater mechanical strength, tissue adherence, performance, and controlled drug delivery have been created using the 4D printing technique, used for biosensing, wound healing, and other purposes. Thus, in future, hypodermic needles might be replaced with 3D- or 4D-printed MNs ,258,259.


Reduced work ability is relatively common among job seekers and it can hinder future labour market attachment. A commonly used measure to increase employability is the use of active labour market programmes. While vocational labour market training (LMT) has been shown to be an effective way to increase work participation among job seekers, there is still uncertainty about how LMT works in different population groups, for example, among persons with a work disability history.We used nationally representative Finnish register data on 16 062 LMT participants in 2008–2015 aged 25–59 with a history of sickness absence (SA) and propensity score matched participants without such history. For matching, we used information on sociodemographic and work-related factors. We used difference-in-differences analysis to investigate the differential changes in work participation before and after LMT between those with and those without SA history.Having a history of SA was associated with a lower gain in work participation after LMT, but the magnitude varied by sex, employment history and follow-up time. In women, having a history of SA contributed to 3.9–6.2 percentage points smaller increase in work participation 1–3 years after LMT, depending on the employment history. In men, the corresponding numbers were 2.0–4.3 percentage points. The results were more pronounced if the SA was due to mental disorders.The results indicate that work disability, especially due to mental disorders, can hamper work participation after LMT and should be considered when planning employment-enhancing measures for job seekers. However, almost 30% of the LMT participants have been shown to have a work disability background.To the best of our knowledge, this is the first study assessing the contribution of previous work disability on work participation after an LMT or any ALMP. In this study, utilizing nationally representative Finnish register data on individuals who participated in LMT in 2008–2015, we aimed to assess whether having a history of sickness absence (SA) affects work participation after LMT while accounting for the employment history. To reduce the selection bias in having a history of SA, we used propensity score (PS) matching. We used a difference-in-differences (DID) approach to assess the differential changes in work participation before and after LMT between those without and those with SA history due to all causes, mental disorders and other diagnoses. The analysis was stratified by sex and employment history.We used a 70% random sample of the working-age population living in Finland on 31 December 2007. The data consisted of longitudinal individual-level information both prospectively and retrospectively. We obtained information on episodes of employment, unemployment, earnings-related retirement and vocational rehabilitation from the Finnish Centre for Pensions, on episodes of compensated sickness absence (SA) and national pensions from the Finnish Social Insurance Institution, and on sociodemographic factors, work-related factors and employment services including LMT from Statistics Finland. Register linkage was authorized by Statistics Finland (permission TK/141/07.03.00/2022-2) and Findata (permission Dnro THL/6341/14.06.00/2021).In this study, we included job seekers  aged 25–59 years who started vocational labour market training (LMT) between January 2008 and December 2015 and finished it by the end of 2015. Incident LMT was determined as the first episode occurring in one of these years. The participant could have participated in other ALMPs as well.We excluded individuals who received old-age pension, unemployment pension or disability pension three years before the training because they are unlikely to return to work. Only Finnish-born Finns were included in this study as immigrants are more likely to participate in other types of labour market programmes. To ensure a full follow-up time, we excluded individuals who were not living in Finland three years before and three years after the LMT. Moreover, to ensure that we have an employed population with occupational information at least from one relatively recent time point, we excluded those who did not have any information on their occupational status, industrial sector, or employer sector over a period of seven years before the LMT. The eligible study population before matching included 75 364 individuals.In Finland, LMT is provided as a part of nationwide public employment services. LMT can be organized in many forms, ranging from short courses to the completion of a degree. Generally, job seekers who are unemployed or about to become unemployed and are at least 20 years old can be admitted to LMT. Participants’ health, motivation, prior training, work experience and suitability based on skills and characteristics required by the field in question, are assessed by a team consisting of experts of employment and economic development services and a representative of the training provider. Every year around 8% of job seekers apply for LMT (an average of 90 000 yearly applications) and around half of them are accepted. Of the accepted, most finish their training. There has been a decreasing trend since 2008 in the number of applications mainly due to economic fluctuations, legislative changes and changes related to LMT and other ALMPs.In Finland, all permanent residents aged 16–67 are entitled to sickness allowance. During the first 10 days (weekdays including Saturdays) there is a waiting period that is covered by the employer if the person is employed. After the waiting period, sickness allowance is covered by the Social Insurance Institution of Finland (SIIF) for up to 300 compensated days. A medical certificate is required for sickness allowance.Having a history of SA was defined as having at least one day of SA compensated by SIIF one or two years before LMT. In addition to examining all-cause SA, we also ran the analyses separately for those who received SA due to mental disorders [International Classification of Diseases (ICD-10) codes F00–F99], musculoskeletal diseases (M00–M99) and other diagnoses (the rest of the ICD-10 codes).The following variables were used in calculating the PS: age category , region of residence, place of residence in a rural or urban settlement, family structure, level and field of education, years since last education was completed, occupational class, industrial sector and employment sector . In case information on one of these variables was missing, it was replaced by information from the three previous calendar years, except for occupational class, industrial sector and employment sector which were tracked seven years before the LMT. Student status, participation in a preparatory course, or other ALMP provided by unemployment services (i.e. unemployment placement) were measured over a period of three years before LMT. In the DID analysis, we also adjusted for the duration of LMT.Work participation was defined based on episodes of employment, unemployment, SA, vocational rehabilitation and pensions that were available on a daily level. A person was classified as being at work if he or she was in paid employment and did not simultaneously receive any benefits or pensions. Receiving a partial SA or disability pension benefit in combination with paid employment counted as a half workday. We calculated the proportion of time spent at work based on annually measured workdays three years before the onset of LMT and three years after the end of LMT.N = 8172 in men, N = 7016 in women), (ii) ‘decreasing employment, increasing unemployment’  and (iii) ‘low employment, high unemployment’ . A description of the derived clusters is given in We empirically grouped individuals according to their annual days of having an employment episode and of receiving unemployment benefits over three years before LMT (from here on employment history) applying k-mean cluster analysis.We used PS matching to create as comparable groups as possible between LMT participants who had previous SA and those who did not. We aimed to generate two similar groups in terms of observed characteristics before the LMT and by this to control for possible baseline differences between the groups that may influence work participation after the LMT. We used logistic regression models to calculate the probability of having previous SA, with baseline characteristics as predictors. Then, within 48 strata, i.e. groups that consist of a combination of variables, sex , age group , start year of the LMT  and employment history , we applied 1:1 nearest neighbour matching without replacement to match individuals on the probability of having SA 1–2 years before LMT with a calliper of 0.1 as the maximum tolerance for matching and examined the covariate balance between the PS-matched groups.We analysed differences in the changes in the work participation three years before and after LMT using the DID method. The parallel assumption was tested by visually comparing the trends before and after LMT . The DIDWe further adjusted for municipal-level unemployment ratesIn the study population before matching, 21.3% had previous sickness absence (SA) . Of the The matched vocational labour market training (LMT) participants were most often 40–55 years old, lived with a partner but without children, had a secondary-level education without a high school diploma, and had completed an education in the field of engineering, manufacturing or construction, and had 11–20 years since the last completed education. Typically, they were skilled manual workers, worked in the manufacturing industry and in the private sector, had an annual income of 18 900–24 799 Euros, lived in Western Finland and in an urban area, and had high employment levels before LMT, did not participate in preparatory training or other ALMPs, and were not studying before LMT. Most had an LMT that lasted for 33–180 days. The differences in the distributions of baseline characteristics between those with and those without SA history were small and not statistically significant .Irrespective of previous SA, work participation dropped as the LMT approached and increased immediately after LMT ended, except for those who had high employment levels before LMT . Among tThose with a SA history spent consistently lower work participation after LMT than those without a SA history . Among tThe results of the age- and LMT duration-adjusted DID analyses showed that having a history of SA negatively contributed to work participation after LMT . In men,The results varied greatly by employment history. In general, a history of SA had a small (in men) to non-existent (in women) contribution to work participation among those with high employment before LMT. In men, the strongest negative contribution of SA history to work participation was among those who had increasing levels of unemployment before LMT. The increase in work participation 2–3 years after LMT was over 4 percentage points smaller among those with SA history. In women, the strongest contribution of SA history was among those with high unemployment history with a 5.9–6.2 smaller increase in work participation 2–3 years after LMT.Having a history of SA due to mental disorders led to a considerably smaller gain in work participation after LMT, especially among men and those with high employment history . Among mThe results remained similar when analysing those who had a SA history due to mood disorders (F30–F39) or anxiety, dissociative, stress-related, somatoform and other non-psychotic disorders (F40–F48) . Having Lastly, we analysed whether adjusting for the municipal unemployment rate affects the results, but it only had a negligible effect on the results .To the best of our knowledge, this is the first study assessing the contribution of previous work disability on work participation after vocational labour market training (LMT), or any ALMP. We used a nationwide register-based dataset and applied PS matching and DID analysis to investigate the gain in the annual proportion of time spent at work after participating in LMT among those with and those without sickness absence (SA) history among Finnish job seekers. We found that having SA one or two years before LMT overall had a negative impact on work participation after LMT, but the magnitude varied by sex, employment history, follow-up time and the diagnosis of SA. The negative contribution of SA history to work participation was stronger in women, except if the SA was due to mental disorders.,,,Previous work disability can influence labour market attachment after LMT in two main ways. First, it can deteriorate the success of the LMT, if the individual is unable to fully attend the training due to health problems. In the Finnish system, a job seeker must go through an application process to get into the training. As a part of this process, unemployment services evaluate the fitness and work ability of the applicant, but in practice, work disability can go unnoticed in the unemployment services.,Our results showed that having a history of SA is associated with poorer work participation outcomes, indicating that work disabilities are either unrecognized or otherwise not appropriately accounted for when the job seekers are offered different employment measures. The contribution of SA on work participation was particularly pronounced if the SA was due to mental disorders, resulting in 51–61 and 23–28 workdays per year smaller gain in men and women, respectively. These results support previous findings showing that SA due to mental disorders is associated with a higher risk of exit from the labour market, for example, due to disability retirementThe findings of this study also confirm that high employment before LMT is associated with a better work participation outcome.Many countries spend a considerable amount of public finances in ALMPs. Finland’s spending on ALMP is higher than the OECD average, and a large part of it is spent on training, including LMT.The strengths of this study are the large, nationally representative register data with no missing data, non-response bias or drop to follow-up. Detailed data on employment and sociodemographic factors enabled us to stratify the analyses by sex and employment history. A relatively long follow-up time of three years provided a comprehensive understanding on work participation after LMT and extended beyond the period of possible lock-in effect.ckad154_Supplementary_DataClick here for additional data file.


This paper presents for the first time the design of a microwave sensing setup for the potential monitoring and identification of red palm weevil (RPW) gender type. The microwave sensor consists of a planar two-port transmission line (TL) with a single complementary split-ring resonant (CSRR) inclusion etched from the bottom metallic layer. The CSRR sensor is placed on top of a customized non-conductive container. The microwave sensing setup was designed, numerically demonstrated, fabricated and tested experimentally. Simulated results correlate quite well with the experimental data. Moreover, the sensitivity of the CSRR sensor when in close proximity to different RPW genders was evaluated both numerically and experimentally. Based on the measured results from 15 RPW samples with different body sizes, different RPW gender types showed unique microwave signatures. A notable shift in the sensor’s resonance frequency was achieved, where on average a resonant frequency shift of 10% for adult RPWs was achieved, while a 2.4% frequency change was obtained for larvae (young) RPWs. Hence, the proposed microwave sensing setup can be adopted in field trials to examine and differentiate between various RPW genders at various developmental stages. Among many harmful insects, red palm weevil (RPW), also known as Asian palm weevil, is considered a big threat to date palm trees, especially in the Middle East and North Africa (MENA) region. Such pests have the ability to rapidly grow in a short period of time, adopt themselves easily inside the palm trunk in various environmental conditions and can move (travel) from one place to another over several kilometers due to their ability to fly. Unfortunately, the rapid spread of RPW pests results in severe damage to date palm trees, which in turn causes major losses to the economic growth of many developing countries, including many countries within the MENA and Asian regions.One commonly adopted management control technique for RPW pests is the use of traps. The traps are prepared by filling a container with an insect’s pheromone mixed with palm odor and water ,2. The tThere have been numerous research studies and efforts in the literature devoted to managing and controlling the spread of red palm weevil pests, including detecting the presence of RPW pests in palm trunks using acoustic signal processing , opticalDevelop microwave CSRR-loaded transmission line (TL) sensors for the potential identification and detection of RPW gender type through the recording of the transmission strength of the sensor.Present 3D numerical models that mimic the nature of microwave CSRR sensors in the detection of various RPW genders.Present an experimental setup of the microwave identification setup for RPW gender detection. Due to the limited access to a collection of RPWs, only samples were considered in the experimental study.Earlier studies focused on the microwave heating treatment of date palm trunks infested with RPW pests. However, this research work explores deeply the effectiveness of the microwave CSRR probes when interacting with RPW pests towards the identification of different gender types of RPWs.This study also assesses the performance of the microwave CSRR sensor over a population of 17 RPW samples that were collected from a local infected date palm farm: 5 samples for adult male RPW, another 5 for adult female RPWs and 7 samples for larvae insects.This study focuses mainly on isolated RPW samples of various gender types, which is important to quantify the strength and effectiveness of the developed microwave sensor. Future studies will aim to investigate the applicability of machine learning on estimating RPW gender type in a more populated area with different RPW genders and species at different stages. Furthermore, this research study contributed to the field of RPW pest control through the deployment of low-power microwaves to investigate the potential identification of RPW gender instead of the high-power microwave treatments, which were reported in earlier studies; thus, avoiding the need to use harmful radiation that farmers could be exposed to during their daily work.To the best of our knowledge, there have been no major developments or comprehensive studies for the identification of RPW gender type, especially with the help of low-power microwave sensors. The contributions in this research study are summarized below:Due to the vast variations in the electric properties of the RPW at its different life development stages, we believe that microwave CSRR transmission-line-based sensors are attractive to deploy. This is due to their low profile, ease of fabrication and narrowband sensing capability. The microwave sensor is placed on top of the sensed RPW pest with a small controllable stand-off distance. In this study, numerical full-wave simulations are presented and validated with experimental results, where all the materials’ properties of the setup including the RPW types are included in the simulation models.z-direction in L, while the cuts (slots) in the two rings result in a developed distributed capacitance effect C. The resonance frequency of the CSRR can then be estimated usingIn the past twenty years, the integration of small-resonant metamaterial particles, in the form of split-ring resonators (SRRs)  or complsion see b were ins = 1.575 mm, c = 0.45 mm and g = 0.5 mm, where its resonance frequency is 2.45 GHz, within the ISM frequency band. For convenience, an FR-4 square laminate of dimension 6 × 6 mm2 (with a dielectric constant of 4.4 and a loss tangent of 0.02) and thickness of 1.6 mm was chosen as the host medium of the microwave CSRR sensor. In this work, we deploy a microwave CSRR transmission-line-based sensor as a promising near-field probe for identifying the gender of an RPW, taking into account the advantages of such sensors in terms of their high sensitivity and narrowband operational band. L × W × H = 60 × 60 × 10 mm3. An adult RPW is modeled here as an elliptical cylinder  and 7 samples from larva pests were collected in maintained air flow containers, as can be seen from For calibration purposes, the performance of the sensor was first recorded alone, after which, the sensor was assessed when exposed to some selected RPW samples in order to sense both the back and front body surface. The assessment of detecting adult female RPWs’ gender using the CSRR sensor can be seen in Lastly, the measured transmission coefficient of the CSRR sensor when exposed to seven samples of larvae is presented in Although the developed microwave CSRR sensor was able to differentiate between the existence of single and dual RPWs within the aperture area of the sensor, we believe that the use of multiple (cascaded) CSRR rings within the sensor could further enhance the possibility of the sensor differentiating more than two RPW pests when placed in the same aperture area of the sensor. Another issue that could limit the high prediction of the current developed CSRR sensor is the prediction of RPW gender due to the random mobility of RPW pests, which could be enhanced with the deployment of artificial-intelligence-based models.In this paper, a low-cost microwave CSRR sensor was developed and tested for the identification and monitoring of red palm weevil gender for the first time. The sensor is composed of a two-port transmission line loaded with CSRR inclusion that is etched out from a bottom metallic layer. Full-wave numerical simulations were carried out and demonstrated with experimental results for validation purposes. The microwave sensor is fabricated using standard printed circuit board technology, which makes its deployment as an RPW inspection tool very effective and attractive.Based on the the simulated results and measured data, the CSRR probe was able to detect variations in RPW gender types, where the the average percentage frequency shift of the sensor for adult RPW samples resulted in being almost 10%, while it was 2.4% when sensing young larvae samples. From the findings of this numerical and experimental study, microwave CSRR-based sensors are found to be promising candidates for the detection and classification of RPW genders due to the reduced cost of fabrication as well as the simplicity of the sensing system setup.