A silicon growth of vapor-liquid-solid method allows us to fabricate high-aspect-ratio nanoscale, microscale needle arrays at precise positions. The Kawano group demonstrated in vivo neural recordings with the silicon-growth–based <7-µm-diameter needle-electrode devices (A. Fujishiro et al., 2011, 2014). Because of the significant advantage of the small needle geometry compared to conventional electrodes, the needle-electrode devices offer high biocompatibility and minimized tissue damage during recording, as well as further long-term and safe chronic recordings, toward the next generation of electrode technology in in vivo electrophysiology.

High stretchability and deformability are promising properties to increase the applications of flexible film electronics including sensors, actuators, and energy harvesters. In particular, they have great potential for applications related to three-dimensional soft biological samples such as organs and tissues that exhibit large and rapid changes in their surface area and volume. The Kawano group introduced a Kirigami-based bioprobe (Y. Morikawa et al., 2016, 2018), which enable one to follow the shape of spherical and large deformable biological samples, such as heart and brain tissues. In addition, its low strain-force characteristic reduces the force induced on organs, thereby enabling minimally invasive biological signal recording.

Compared to electrophysiological methods, optogenetics, which combines optical and genetics methods, offers a higher spatiotemporal resolution for control of neurons and cells. Based on the silicon growth technology, our group developed microscale tube-like probe device, which plays a role in waveguide for the optical stimulation with the high spatial resolution (M. Sakata et al., 2014). Because of the tube geometry, the device also enables electrical recording and stimulation, as well as delivering drugs into the tissue, providing multifunctional measurements of neurons and cells in the same alignment. This optical device also inspired us to develop all parylene flexible waveguide array device, having microelectrodes for the simultaneous optical and electrical measurements (S. Yamagiwa et al., 2015).

We are allowed to detect small electrical signals from the neuron by placing a tiny electrode near the neuron, called extracellular recording method. It seems like that we are standing outside the closed doors of a concert hall, trying to listen to the music being played inside. Microelectronics technology plays a role in the signal amplification, providing the extracellular recording with a high-signal-noise ratio. We have been working on integration of our silicon needle electrode with transistors, using technologies either on-chip process (A. Okugawa et al., 2011) or device package (Y. Kita et al., 2021, T. Banno et al., 2022).

Microscale-electrode technology is a powerful way to explore complex system of brain as well as communicate with neurons for the application to brain–machine interface technology. Based on our silicon growth technology, we developed <5-µm-diameter needle electrode block module devices, enabling stable recording of high-quality neuronal signals of local field potential and spike activity over a long period  (H. Sawahata et al., 2016, K. Yamashita et al., 2022). This electrode technology may be a powerful tool in in vivo electrophysiology that reduce the invasiveness and should improve the stability of neural recordings.

Electrophysiological recording requires further improvements in terms of signal quality, invasiveness, and cable use. Although wireless recording can meet these requirements, conventional wireless systems are heavy and bulky for use in small animals (e.g., mice). We have developed a lightweight (<3.9 g), compact (15 × 15 × 12 mm3), Bluetooth-low-energy-based wireless neuronal recording system, having advantages of high signal quality, good versatility, and low cost, compared to wired recording with a commercial neurophysiology system (S. Idogawa et al., 2021).