Wearable electronic devices represent a paradigm change in consumer electronics, on‐body sensing, artificial skins, and wearable communication and entertainment. Because all these electronic devices require energy to operate, wearable energy systems are an integral part of wearable devices. Essentially, the electrodes and other components present in these energy devices should be mechanically strong, flexible, lightweight, and comfortable to the user. Presented here is a critical review of those materials and devices developed for energy conversion and storage applications with an objective to be used in wearable devices. The focus is mainly on the advances made in the field of solar cells, triboelectric generators, Li‐ion batteries, and supercapacitors for wearable device development. As these devices need to be attached/integrated with the fabric, the discussion is limited to devices made in the form of ribbons, filaments, and fibers. Some of the important challenges and future directions to be pursued are also highlighted.
Planar or two-dimensional (2D) microelectrode arrays (MEAs), which are used for in vitro culturing of neurons and tissue slices, have been in existence for over 30 years. However, in order to study complex network morphologies and tissue slices which contain substantial 3D neuronal structures, 3D MEAs with microfluidic ports are required. Integrated fabrication of 3D MEAs with embedded microfluidic ports for nutrient perfusion through these relatively thick tissues typically requires non-planar lithography, which is not easily accomplished. This paper reports a laser-scribing technique coupled with electroplating to fabricate 3D MEAs coupled with microfluidic ports. An excimer laser has been used to define patterns in a polymer mold layer that is conformally vapor-deposited on a 3D microfluidic SU-8 substrate. Metal is electroplated through this mold to fabricate electrodes at multiple heights. To demonstrate 3D MEAs, a standard design was chosen consisting of an array of three-dimensional protrusions (‘towers’) optionally with microfluidic functionality on which electrodes can be formed extending to the top of each tower. Additional electrodes are formed on the substrate resulting in a multi-level electrode structure. Since microfluidics can exist both in the substrate as well as along the towers, a coupled three-dimensional electrical and microfluidic functionality is achieved. The resulting 3D MEAs have been analyzed electrically using impedance spectroscopy and baseline noise measurements. They have further been evaluated fluidically using micro-particle image velocimetry measurements.
The widespread adaptation of 3D printing in the microfluidic, bioelectronic, and Bio-MEMS communities has been stifled by the lack of investigation into the biocompatibility of commercially available printer resins. By introducing an in-depth post-printing treatment of these resins, their biocompatibility can be dramatically improved up to that of a standard cell culture vessel (99.99%). Additionally, encapsulating resins that are less biocompatible with materials that are common constituents in biosensors further enhances the biocompatibility of the material. This investigation provides a clear pathway toward developing fully functional and biocompatible 3D printed biosensor devices, especially for interfacing with electrogenic cells, utilizing benchtop-based microfabrication, and post-processing techniques.
We present a novel benchtop-based microfabrication technology: 3D printing, ink casting, micromachined lamination (3D PICLμM) for rapid prototyping of lab-on-a-chip (LOC) and biological devices. The technology uses cost-effective, makerspace-type microfabrication processes, all of which are ideally suited for low resource settings, and utilizing a combination of these processes, we have demonstrated the following devices: (i) 2D microelectrode array (MEA) targeted at in vitro neural and cardiac electrophysiology, (ii) microneedle array targeted at drug delivery through a transdermal route and (iii) multi-layer microfluidic chip targeted at multiplexed assays for in vitro applications. The 3D printing process has been optimized for printing angle, temperature of the curing process and solvent polishing to address various biofunctional considerations of the three demonstrated devices. We have depicted that the 3D PICLμM process has the capability to fabricate 30 μm sized MEAs (average 1 kHz impedance of 140 kΩ with a double layer capacitance of 3 μF), robust and reliable microneedles having 30 μm radius of curvature and ~40 N mechanical fracture strength and microfluidic devices having 150 μm wide channels and 400 μm fluidic vias capable of fluid mixing and transmitted light microparticle visualization. We believe our 3D PICLμM is ideally suited for applications in areas such as electrophysiology, drug delivery, disease in a dish, organ on a chip, environmental monitoring, agricultural therapeutic delivery and genomic testing.
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