Another study confirmed that to overcome the high hydrostatic pressures induced on skin during microneedle penetration 8, it is essential to fabricate sharp microneedles. The tip diameter can be as small as 500 nm depending on the manufacturing accuracy. The tip size usually depends on the manufacturing technique and the material used. A force range of 0.1–3 N was reported to be required for microneedle insertion into the skin depending on the area of the tip. 7 showed that there is a linear relationship between the microneedle insertion force and the microneedle tip interfacial area, as the microneedle tip reduces the force of fracture. As increasing the applied force can cause discomfort to the patient and may result in microneedle breakage, it is more effective to increase the tip sharpness for easier penetration of the microneedle into the skin 6. To bypass this problem, either the insertion force may be increased or the microneedle sharpness can be increased. Due to the elastic nature of the skin, the microneedle insertion depth strongly depends on the amount of deformation that occurs around the insertion site on the skin 5. The importance can be better realized when considering that microneedle arrays should bypass the skin layers to access the desired section of the skin. A thorough understanding of the skin structure, anatomy, and cellular and outer surface characteristics as a living unit is crucial for the successful design and fabrication of microneedles. In this regard, penetration and mechanical properties are important aspects that need to be addressed, to determine whether microneedle arrays can pierce the skin without breaking. To be able to penetrate the skin, microneedles should have specific physical properties and precise geometries. Interest in microneedle-based medical devices is growing rapidly as healthcare systems recognize the importance of small, portable medical devices for point-of-care diagnostics and the effective and rapid administration of drugs and vaccines (Fig. The initial microneedle idea was proposed in 1976 3, but due to the limitations of manufacturing techniques, the fabrication of the first microneedle prototypes occurred only in the 1990s, when advancements in micromanufacturing enabled the creation of microstructures 4 (Fig. Microneedles are minimally invasive and have shown capabilities to sample biofluids and deliver a variety of nanoparticles and molecules to the human body for drug and vaccination applications 1, 2. Some of these difficulties include the need for medically trained staff for administration, needle phobia, and needle injuries. Microneedle arrays are micrometer-sized structures designed to reduce the risk and difficulty in the administration of hypodermic needle-based injections. This article presents the fundamentals of 2PP and the recent development of microneedle array fabrication through 2PP as a precise and unique method for the manufacture of microstructures, which may overcome the shortcomings of conventional manufacturing processes. Thus, microstructures are designed according to structural and fluid dynamics considerations rather than the manufacturing constraints imposed by methods such as machining or etching processes. This is a pioneering transformative technology that facilitates the fabrication of complex miniaturized structures that cannot be fabricated with established multistep manufacturing methods such as injection molding, photolithography, and etching. Due to its unprecedented flexibility and high spatial resolution, the use of this technology has been widespread for the fabrication of bio-microdevices and bio-nanodevices such as microneedles and microfluidic devices. 2PP is one of the most versatile and precise additive manufacturing processes, which enables the fabrication of arbitrary three-dimensional (3D) prototypes directly from computer-aided-design (CAD) models with a resolution down to 100 nm. In the last decade, much attention has been paid to using additive manufacturing techniques in both research and industry, such as 3D printing, fused deposition modeling, inkjet printing, and two-photon polymerization (2PP), with 2PP being the most flexible method for the fabrication of microneedle arrays. Microneedles are manufactured using a variety of additive and subtractive micromanufacturing techniques. Microneedle patches have received much interest in the last two decades as drug/vaccine delivery or fluid sampling systems for diagnostic and monitoring purposes.
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