Flexible Array of Rigid Porous Microneedles for Bioelectric Skin Patch
Authors: Matsuura Mirai , Tottori Soichiro , Ichinose Sae , Nomaguchi Tomoya , Cho Haechang , Wang Gaobo , Nishizawa Matsuhiko 
Published: December 2024

Electrodermal patches that generate transdermal currents have been widely investigated for wound healing, anti-perspiration, and drug delivery. Our laboratory has recently developed a metal-free, disposable, fully organic, flexible electrodermal patch with potential for a wide range of healthcare applications. The patch has attracted considerable attention because of its ease of use, lack of need for an external power source, improved drug penetration, and, above all, its simplicity. By selecting soft materials for the patch and electrodes, a total thickness of less than 2 mm has been achieved, and the patch remains flexible enough to be used on skin surfaces and other surfaces. However, the application is limited at the present stage due to the high resistance of the stratum corneum of the outer skin layer in addition to the lower output of the on-board enzyme batteries.
Microneedle array can be an attractive solution as it can penetrate through the stratum corneum without reaching blood vessels or nerves. Especially, solid porous microneedles (PMNs), which have microscopic channels throughout the needle structure, exhibit characteristic solution permeability. This serves as a pathway for transdermal iontophoresis drug delivery. Furthermore, modification of the connecting pores with electrically charged materials can generate electroosmotic flow (EOF) inside PMNs, which can enhance drug penetration into the skin without charge or size limitations. Most of the reported microneedle-based skin devices used a hard substrate with high mechanical strength as the base. However, when devices are used on sited of high curvature and deformation, contact with the skin surface is likely to be uneven. This can lead to reduced medical effectiveness, pain, and device breakage. Here, we developed a flexible microneedle array, which provides a stable skin-application effect, by combining a needle portion made of rigid material and a substrate portion made of flexible material.
The flexible PMN array was fabricated by integrating rigid PMNs onto the flexible substrate such as polydimethylsiloxane (PDMS) or polyurethane (PU) sheets. The needle part was fabricated by photopolymerizing a mixture of glycidyl methacrylate, trimethylolpropane trimethacrylate and triethylene glycol dimethacrylate, together with a mixture of polyethylene glycol (PEG) and diethylene glycol monomethyl ether (DEG) as porogen. The porogen was dissolved after photopolymerization to form submicron channels networks inside the PMNs. The PMNs were integrated with flexible substrates by solidifying together directly for mechanical fixation. We then evaluated the developed flexible PMN array as follows. First, it was confirmed that the transcutaneous resistance was reduced by applying the flexible PMN patch, both on the flat and curved pig skins. Second, in combination with a bio battery consisting of an enzyme-modified fabric electrode, it was demonstrated that a stable transcutaneous current was generated from glucose and enzyme even on the curved surfaces. In addition, ex vivo experiments showed skin penetration performance while maintaining flexibility.
In summary, we developed a method to fabricate rigid PMNs directly bond onto the flexible substrates. We then confirmed that the flexible PMN array can conformably fit on and penetrate curved pig skins. Such flexible and disposable skin patches combined with rigid PMNs can serve as a platform for a wide range of medical, healthcare, and cosmetic applications.

Figure 1