Biointegrated and Wirelessly Powered Implantable Brain Devices: A Review <s> D. Solar-Powered Wireless System <s> Objective. Wireless control and power harvesting systems that operate injectable, cellular-scale optoelectronic components provide important demonstrated capabilities in neuromodulatory techniques such as optogenetics. Here, we report a radio frequency (RF) control/harvesting device that offers dramatically reduced size, decreased weight and improved efficiency compared to previously reported technologies. Combined use of this platform with ultrathin, multijunction, high efficiency solar cells allows for hundred-fold reduction of transmitted RF power, which greatly enhances the wireless coverage. Approach. Fabrication involves separate construction of the harvester and the injectable µ-ILEDs. To test whether the presence of the implantable device alters behavior, we implanted one group of wild type mice and compared sociability behavior to unaltered controls. Social interaction experiments followed protocols defined by Silverman et al. with minor modifications. Main results. The results presented here demonstrate that miniaturized RF harvesters, and RF control strategies with photovoltaic harvesters can, when combined with injectable µ-ILEDs, offer versatile capabilities in optogenetics. Experimental and modeling studies establish a range of effective operating conditions for these two approaches. Optogenetics studies with social groups of mice demonstrate the utility of these systems. Significance. The addition of miniaturized, high performance photovoltaic cells significantly expands the operating range and reduces the required RF power. The platform can offer capabilities to modulate signaling path in the brain region of freely-behaving animals. These suggest its potential for widespread use in neuroscience. <s> BIB001 </s> Biointegrated and Wirelessly Powered Implantable Brain Devices: A Review <s> D. Solar-Powered Wireless System <s> This paper presents a novel inductor-less switched capacitor (SC) DC–DC converter, which generates simultaneous dual-output voltages for implantable electronic devices. Present dual output converters are limited to fixed ratio gain, which degrade conversion efficiency when the input voltage changes. The proposed power converter offers both step-up and step-down conversion with 4-phase reconfigurable logic. With an input voltage of 1 V provided by photovoltaic (PV) cells, the proposed converter achieves step-up, step-down, and synchronised voltage conversions in four gain modes. These are 1.5 and 0.5 V for normal mode, 2 and 1 V for high mode, 2 V for double boost mode, as well as 3 and 2 V for super boost mode with the ripple variation of 14–59 mV. The converter circuit has been simulated in standard 0.18- $\mu \text{m}$ CMOS technology and the results agree with the state-of-the-art SC converters. However, our proposed monolithically integrated PV powered circuit achieves a conversion efficiency of 85.26% and provides an extra flexibility in terms of gain, which is advantageous for future implantable applications that have a range of inputs. This research is, therefore, an important step in achieving truly autonomous implantable electronic devices. <s> BIB002 </s> Biointegrated and Wirelessly Powered Implantable Brain Devices: A Review <s> D. Solar-Powered Wireless System <s> In this work, we present a simple complementary metal-oxide semiconductor (CMOS)-controlled photovoltaic power-transfer platform that is suitable for very small (less than or equal to 1–2 mm) electronic devices such as implantable health-care devices or distributed nodes for the Internet of Things. We designed a 1.25 mm × 1.25 mm CMOS power receiver chip that contains integrated photovoltaic cells. We characterized the CMOS-integrated power receiver and successfully demonstrated blue light-emitting diode (LED) operation powered by infrared light. Then, we integrated the CMOS chip and a few off-chip components into a 1-mm3 implantable optogenetic stimulator, and demonstrated the operation of the device. <s> BIB003 </s> Biointegrated and Wirelessly Powered Implantable Brain Devices: A Review <s> D. Solar-Powered Wireless System <s> Implantable electronic devices are emerging as important healthcare technologies due to their sustainable operation and low risk of infection. To overcome the drawbacks of the built-in battery in implantable devices, energy harvesting from the human body or another external source is required. Energy harvesting using appropriately sized and properly designed photovoltaic cells enable implantable medical devices to be autonomous and self-powered. Among the challenges in using PV cells is the small fraction of incident light that penetrates the skin. Thus, it is necessary to involve such physical properties in the energy harvesting system design. Consequently, we propose a novel photodiode model that considers skin loss in different ethnic groups. Our physical simulations have been implemented using COMSOL and MATLAB. Circuit and system modelling have been performed using Cadence 180nm TSMC technology. Our results show that the transmittance of near infrared light is almost the same in three skin types: Caucasian, Asian and African. Maximum power delivery of 12 μW (African skin) and 14 μW (Caucasian and Asian skin) were achieved at 0.45 V. With the help of a power management unit, an output voltage of 1.8–2 V was achieved using the PV cells. <s> BIB004
Photovoltaic energy harvesting from light BIB001 BIB002  BIB004 is another enthralling tool to wirelessly power the implantable devices. In BIB001 , to replace the batteries for uninterrupted functioning of implants, an implantable device made of tiny, thin solar cells (gallium arsenide, 5 mg) along with a wireless logical control module based on RF signal to activate µ-LEDs, was introduced and depicted in Fig. 8(d) . The wireless control of this device was enabled by using a rectifier circuit that converts the RF signal to direct current to drive a low-power logic circuit, which is integrated with solar cells and LEDs. As compared to the identical system without photovoltaics, this combination of solar and RF wireless system improves the wireless power transfer range capability (∼3 m) and simultaneously lower the RF power requirements substantially (almost by a factor of 10). As a result, this system significantly reduces electromagnetic exposure to animals as well as enabling free, natural behaviour in animals. This is a head-mounted wireless system, where the solar cells are placed on top of the head of a rat. In another work BIB003 , a photovoltaic wireless power transfer system based on CMOS (complementary metal-oxide semiconductor) applicable for tiny (≤1-2 mm) implantable electronic devices is introduced. To integrate the photovoltaic cells, the implant contains a CMOS power receiver chip having surface area of 1.25 × 1.25 mm 2 . By using the infrared light, this implant successfully powered a blue LED. Then, the CMOS chip as well as a few off-chip parts were integrated to develop an implantable optogenetics device (1 mm 3 ). However, light sources nature, proximity, and direction limits the solar cells wireless powering ability for implantable neural devices.