(Keynote) Soft, Wet and Iontronic Devices
Authors: Nishizawa Matsuhiko 
Published: December 2020

Recent strong demand for wearable and implantable devices has been encouraged researchers to develop biocompatible (soft, wet and ionic) devices that can form a smart interface with biological systems. The conventional medical devices for sensing/treatment are “electronically driven dry systems”, which is in contrast to the “ionically driven wet biosystem”. Therefore, in addition to the improvements in softness and stretchability, the moist and ionic features of electrodes should be considered to realize truly bioconformable interfaces [1]. We have developed original components for such biocompatible devices, including the nanostructured organic electrodes [2], the stretchable enzyme electrodes [3], and the porous microneedle arrays [4]. Here I will present our recent achievements in developping the “soft, wet and iontronic devices” that has been realized by combining above uneque components.
The conducting polymer-based and carbon nanotube-based composite electrodes have huge double-layer capacitance, which is of advantage for the measurement with higher S/N ratio and for the low-invasive stimulation without cytotoxic faradaic reactions such as water electrolysis. These organic electrodes have been utilized for the hydrogel-based subdural electrode [5], the dermal electrodes for electrocardiogram and electromyogram [6], and bioassay [7] and regenerative medicine [8].
The Self-moisturing contact lens [9] was developed by the combination of the hydrogel electrodes and the sugar / O2 enzymatic biobattery to prevent the dry eye syndrome. The ionically produced osmotic flow within the lens showed the anti-dehydration effect.
The stretchable enzymatic biobatteries were also utilized to produce the osmotic flow on the skin for wound healing [10] and through the skin for drug dosing [11]. The combination with the porous microneedle array with randomly distributed microchannels ensured stable transdermal currents of sufficient magnitude by low-invasive puncture of the highly resistive stratum corneum, the outermost layer of skin [12].
References
[1] “Soft, Wet and Ionic Microelectrode Systems” (Award Account), Bull. Chem. Soc. Jpn. 91 (2018) 1141.
[2] “Highly Conductive Stretchable and Biocompatible Electrode-Hydrogel Hybrids for Advanced Tissue Engineering” Adv. Healthcare Mater., 3 (2014) 1919.
[3] “Stretchable Biofuel Cell with Enzyme-Modified Conductive Textiles” Biosens. Bioelectron., 74 (2015) 947.
[4] “Porous Polymer Microneedles for Rapid Fluid Transport by Massively Parallel Microchannels” RSC Advances, 6, (2016) 48630.
[5] “Hydrogel-Based Organic Subdural Electrode with High Conformability to Brain Surface” Sci. Rep., 9 (2019) 13379.
[6] “Totally Shape-Conformable Electrode/Hydrogel Composite for On-Skin Electrophysiological Measurements” Sens. Actuators B, 237 (2016) 49.
[7] “Contractile Skeletal Muscle Cells Cultured with a Conducting Soft Wire for Effective, Selective Stimulation” Sci. Rep., 8 (2018) 2253.
[8] “Hydrogel Microchambers Integrated with Organic Electrodes for Efficient Electrical Stimulation of Human iPSC-derived Cardiomyocytes” Macromol. Biosci., 19 (2019) 1900060.
[9] “Self-Moisturizing Smart Contact Lens Employing Electroosmosis” Adv. Mater. Technol., 5 (2020) 1900889.
[10]” Accelerated Wound Healing on Skin by Electrical Stimulation with a Bioelectric Plaster” Adv. Healthcare Mater., 6 (2017) 1700465.
[11]”Organic Transdermal Iontophoresis Patch with Built-in Biofuel Cell”, Adv. Healthcare Mater., 4 (2015) 506.
[12]” Organic Electrochromic Timer for Enzymatic Skin Patches” Biosens. Bioelectron., 123 (2019) 108.

Figure 1