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International Journal of Bioprinting Unique characteristics of 3D-printed microneedles

the cutting speed may be slow. In comparison, laser thinning ink ensures smooth flow during extrusion and
ablation directly burns the substrate material with a laser solid-like behavior post-extrusion. It is worth noting
(e.g., CO laser) in order to carve the desired structure. that the precision of extrusion-based 3D printing is
2
However, when dealing with polymers with low melting relatively low, making it challenging to directly print
point, thermal defects, such as scorching, re-curing, and sharp microneedles.
shrinkage, may occur. Representative works for each type Recently, researchers have used extrusion-based printers
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fabrication method are summarized in Table 1. to directly print drug-loaded microneedles. Researchers
2.2. 3D printing methods for have also combined FDM with etching to enhance the
microneedle manufacturing 3D printing resolution, achieving a microneedle diameter
According to the American Society for Testing and of 100 μm. The mechanical properties and characteristic
Materials (ASTM) standards, 3D printing technology dimensions of the microneedles were further improved
can be divided into seven categories. Among them, by replacing the photopolymer with materials with
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the technologies suitable for microneedle fabrication higher hardness 49,63,64 (Figure 3A). Wu et al. used a dual-
include material extrusion, vat photopolymerization, nozzle Allevi 2 bioprinter to create microneedles with
and powder bed fusion. Table 2 provides an overview of a tip diameter of approximately 20 μm (Figure 3B). By
the characteristics of these 3D printing technologies and mixing the ink with the drug, they were able to bypass the
the types of microneedles they can directly manufacture. secondary drug loading step, but with lower precision. In a
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Table 3 summarizes the microneedle performance achieved recent report, DIW was successfully used to manufacture
by different 3D printing technologies. double-layer microneedles with water-soluble bases and
drug-loaded top layers. The yield strength of the material
2.2.1. Extrusion-based 3D printing was enhanced with added nanosilica particles, which
Extrusion-based 3D printing, which includes fused significantly improved the printing precision by about two
deposition molding (FDM) and direct ink writing (DIW), orders of magnitude.
is a common method of manufacturing microneedles.
FDM prints the model by heating thermoplastic filaments 2.2.2. Vat photopolymerization
and extruding them from a movable nozzle. DIW Vat photopolymerization (VP) creates 3D objects by
extrudes viscoelastic ink from a nozzle, and the shear- solidifying photosensitive resins layer by layer with a light

Table 1. Summary of traditional microneedle manufacturing methods

Fabrication methods Materials Pros Cons
Indirect fabrication Direct molding Polymer, hydrogel, Simple operation, low cost, Complex structures and
ceramics, water 15,23,24 high precision, and reusable material limitations; structure
Reverse molding molds 25 is easily damaged 26
Direct fabrication Microfabrication Silicon, metal 27 Easy to control etching High cost and low speed 30
(dry etching) direction, high precision
(compared with wet
etching) 28,29
Microfabrication Silicon, metal 31 Fast speed and low cost Difficulty in
(wet etching) (compared with dry manufacturing complex
etching) 32 structures and low precision 32
Microfabrication SU-8, PMMA 33,34 Expected to have low mass Limitations in material, harsh
(photolithography) production costs 35 processing conditions 36
Microfabrication Biodegradable polymers, Super sharp needle tip, fast Many influencing factors
(thermal drawing) metal 37,38 speed and can be fabricated (temperature, motion), poor
on curved surfaces 39 uniformity 40
Laser machining (laser cutting) Silicon Have great potential in Limitations in material, slow
(stainless terms of mass productivity speed 43
steel), polymer 41 and economy, sharp needle
tips 42
Laser machining (laser ablation) Poly-lactic acid 44 Easy to operate with high Limitations in materials 43
accuracy, low cost 22
Abbreviations: SU-8, submicron photoresist; PMMA, poly(methylmethacrylate).

Volume 10 Issue 4 (2024) 64 doi: 10.36922/ijb.1896