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

diagnostics. In addition to monitoring human health, microneedles designed for wound management. 11,14,110
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3D-printed microneedles can be used for the detection of Moreover, there have been a few studies focusing on
salicylic acid and residual pesticides in vegetables and effectively controlling drug release in wound management.
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fruits. In addition to printing technologies such as TPP For instance, temperature-sensitive hydrogels can detect
and CLIP, the vast majority of 3D-printed microneedles whether there is an inflammatory reaction in the wound
have a stepped surface structure. However, as for surface- and precisely regulate the targeted release of vascular
enhanced Raman scattering (SERS) sensors, this stepped endothelial growth factor (VEGF). Different wettability
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surface structure actually enlarges the specific surface area levels determine the release kinetics, making it possible to
of the microneedles and improves the sensing performance. meet the specific biomedical needs of various diseases. 112
Intelligent responsive microneedles are important for
on-demand drug delivery or environment-specific sensing 4. Unique benefits and challenges of 3D
upon stimulation. 98-100 However, hardly any of these types printing microneedles
of microneedles were created directly by 3D printing, 3D printing is capable of creating intricate functional
probably because most novel intelligent materials are 3D structures that are unattainable through traditional
incompatible with existing 3D printing techniques. manufacturing methods. Nonetheless, 3D-printed
microneedles face a number of challenges stemming from
3.3. Dry electrode the technical limitations of existing 3D printing techniques.
Dry electrodes play a crucial role in the acquisition of This section examines the unique benefits and challenges
bioelectrical signals such as electrocardiogram (ECG), associated with 3D-printed microneedles in depth and
electromyography (EMG), and electroencephalogram summarizes in Table 4.
(EEG). These bioelectrical signals are important for
diagnosing and monitoring our physiological and 4.1. Novel hollow microneedles
pathological conditions. Wet electrodes are unsuitable The hollow microneedle is a significant improvement over
for long-term monitoring due to the gradual drying of the conventional syringe needle. Compared with solid and
the conductive gel. Griss et al. first introduced the coated microneedles, hollow microneedles have superior
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use of microneedles to detect physiological signals. The drug delivery capabilities because they can store a larger
microneedles penetrated the epidermis and were sensitive amount of drugs within their inner cavities. Additionally,
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to human movement. Compared to wet electrodes applied they can also be used in a reverse manner to extract
to the skin surface, microneedle electrodes provide interstitial fluid and blood, serving as a minimally
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stable long-term signal detection (Figure 5D). A invasive means of sample acquisition in health monitoring.
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3D-printed physiological signal detection sensor known as
microneedle array electrode (MAE), which preserves skin Traditional manufacturing methods 1,28,115-119 , such
integrity, has been proposed. Researchers have also used as lithography, laser machining, DRIE, etching, and
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magnetorheological mapping photolithography (MRDL) injection molding, can fabricate microneedles with good
to efficiently produce flexible microneedle electrodes that physical properties at a large scale and at low cost but
are more in line with the surface of the human body, have a number of drawbacks. Traditional methods involve
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yielding markedly improved electrode–skin interface complex processes, produce a suboptimal hole quality, and
impedance (EII) measurement results compared to are constrained to simple structural designs, limiting their
ordinary dry electrodes and wet electrodes (Figure 5E). applicability.
In contrast, 3D printing has micron-level resolution
3.4. Wound management and a fully automated manufacturing mode, solving the
Microneedles can serve the dual purpose of wound repair aforementioned challenges. 3D printing not only enables
and monitoring in wound management. In addition to the production of holes with varying morphologies, but
delivering drugs, creating a stable electric field around can also produce microneedles with diverse structures,
the wound and providing oxygen are alternative thus enhancing the diversity and functionality of
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wound treatment strategies. Indirect indicators, such as hollow microneedles. 52,120
temperature, resistance, and pH changes, can also provide
valuable insights into the wound healing process. Shao Traditional manufacturing methods impose restrictions
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et al. blended a hydrogel with a new two-dimensional on the shape and design of hollow microneedles. In
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transition metal material, MXene, and 3D-printed it contrast, when employing 3D printing as a manufacturing
directly on a microneedle mold. While 3D printing of platform, researchers can accurately and directly prepare
microneedles for such applications is relatively uncommon, microneedles with various aperture configurations based
traditional methods have been utilized for creating on prior simulation models. A hollow microneedle created

Volume 10 Issue 4 (2024) 70 doi: 10.36922/ijb.1896