Development and Evaluation of 3D‐Printed Dry Microneedle Electrodes for Surface Electromyography

Abstract

electrode in combination with conductive gel. The conductive gel typically has three purposes. [2] It should allow for some movement of the electrode without loss of skin contact, diffuse into the skin, keep it wet, and thereby enhance its conductivity. While hair ideally should be removed, gel may also ensure electrode skin contact in the presence of hair by surrounding it. Overcoming the impedance posed by skin, especially the outermost layer of the epidermis, the stratum corneum, is an inherent issue with noninvasive biopotential recording techniques. Skin impedance reduction methods may involve washing of the skin, application of an abrasive conductive gel, skin shaving, stratum corneum removal by means of tape stripping, as well as the use of penetration enhancers. [2] Skin preparation steps are tedious, timeconsuming, unpleasant to the user or patient, and may even result in adverse skin reactions or infection. [3,4] While skin preparation is considered best practice and likely to continue to be used in research and clinical settings where acquisition of the highest quality data possible is necessary, there are settings where avoidance of skin preparation is desired, if not required. Such settings may include the clinic where time equates cost or signal acquisition by nonexpert users. Even with skin preparation applied, recording may be restricted in time due to drying of the wet gel interface associated with loss in skin contact quality. [1,4,5] This makes conventional wet electrodes unsuitable for long-term applications such as myoelectrically controlled prosthetics or robotic interfaces. To facilitate the widespread translation of sEMG to the clinic and applications involving long-term recording, new types of electrodes are thus required which allow for ease of application (e.g., no need for skin preparation; wet gel electrodes generally are not repositionable), display reduced contact impedance (e.g., overcome the high impedance stratum corneum), and provide consistency during long-term recording as well as delivering repeatable results. One approach to decrease contact impedance is to increase the overall electrode surface area by adding microstructures. Researchers have fabricated microstructured electrodes by mixing a polymer with a conductive filler (carbon nanotubes, acetylene black, carbon black, graphite, or silver). [6-8] Another approach is to first produce Surface electromyography (sEMG) allows for direct measurement of electrical muscle activity with use in fundamental research and many applications in health and sport. However, conventional surface electrode technology can suffer from poor signal quality, requires careful skin preparation, and is commonly not suited for long-term recording. These drawbacks have challenged translation of sEMG to clinical applications. In this paper, dry 3D-printed microneedle electrodes (MNEs) are proposed to overcome some of the limitations of conventional electrodes. Employing a direct-metallaser-sintering (DMLS) 3D printing process, a two-step fabrication ...

3D-printed microneedles in biomedical applications

“…The results of this study corroborated the superiority of MN-based electrodes over conventional wet/dry electrodes for EMG signal acquisition. Furthermore, the advantageous effect of increasing the number of MNs, in an array, on reducing the electrode-skin impedance was emphasized ( Krieger et al., 2020 ).…”

3D Printing—A “Touch-Button” Approach to Manufacture Microneedles for Transdermal Drug Delivery

“…DMLS can be used to produce small, geometrically complex, fully functional metal parts such as MNs from medical-grade materials with great freedom of design, which would be difficult to produce with classical methods [ 112 , 156 ]. The main advantages of these methods are the production of objects with satisfactory accuracy, resolution, and mechanical properties of the finished parts [ 112 ] and without residual stresses and internal defects that can affect conventionally manufactured metal parts [ 113 ].…”

“…( a ) Fabrication of microneedle arrays using DMLS and post-processing by electropolishing. Reproduced with permission from [ 156 ], John Wiley & Sons—Books, 2020. ( b ) Fabrication of hollow microneedle using SLM.…”

“…Krieger et al [ 156 ] used a DMLS 3D printer to produce solid MNs from a medical-grade 316 L stainless steel powder. Although MN dimensions in the specified design input file were 1000 μm in height and 250 μm in base diameter, it was noticeable that printed MNs were significantly shorter (657 ± 16 μm).…”

“…These MNs were further used as an alternative to commercial wet Ag/AgCl electrodes for EMG signal acquisition. The human study involved 14 healthy volunteers, who showed good tolerance of MN electrodes for up to 6 h. Future studies are, however, necessary to ensure the safe handling and disposal of sharp MNs, with regard to safety concerns such as biocompatibility of the materials and risk of skin irritation [ 156 ].…”