Table 1. Summary of device architectures for wireless, battery-free operation of fully implantable stimulation and recording tools for the central nervous system.

EM, electromagnetic; RF, radio-frequency; SAR, specific absorption rate; RX, receiving; BL, Bluetooth; PV, photovoltaic; US, ultrasonic. Reproduced from Ref.^{98, 100, 134, 141, 149, 158, 184}

Summary of operational modes for wireless, battery-free implantable devices
Power source			Features	Example		Reference
External power source	EM	Far field, passive	Weight: 16-33 mg Available power: ~ 3-5 mW Operational distance: 0.1-3 m Tissue impact: SAR 6150-69 mW/kg	Downlink: RF Uplink: N/A Control: Analog, multi-band RX antenna Embodiment: Subdermal Application: Deep brain/spinal cord/peripheral neural stimulation		98, 101, 122
		Near to mid filed, Magnetic resonant coupling	Weight: ~ 30 mg Available power: ~ 5-10 mW Operational distance: 0.1-1m Tissue impact: SAR ~ < 20 mW/kg	Downlink: RF Uplink: N/A Control: Digital, active Embodiment: Subdermal Application: Brain optogenetics, local oximetry, microfluidic drug delivery		100, 102, 145, 193
				Downlink: RF/BL Uplink: BL Control: closed-loop Embodiment: On organ/nerve Application: Physiological recording, peripheral neural stimulation		141
		PV	Weight: 11 μg – 70 mg Available power: 45 μW – 7 mW Operational distance: ~ 0.5 m Tissue impact: heating of 5 °C with 250-300 mW/mm2 IR radiation for 20 min	Downlink: light (green) Uplink: light (red) Control: Digital Embodiment: N/A Application: Brain neural recording		123, 124, 142, 158,
	US		Weight: 7.6 mg Available power: 0.12 mW Operational distance: ~ 10 cm in tissue, direct body-transducer contact (transducer in contact with body) Tissue impact: local rate of heat generation ~0.3 – 240 mWcm−3 (soft tissue); ~ 8 – 6000 mWcm−3 (bone); ~ 15 – 12000 mW cm−3 (lung)	Downlink: US Uplink: US Control: N/A Embodiment: Deep in tissue / on nerve Application: Physiological/neural recording		125, 126, 184
Internal power source	Triboelectric		Available power: ~ 0.26 mW (peak) Operational distance: N/A	Downlink: N/A Uplink: N/A Control: N/A Embodiment: on organ/nerve Application: Self-triggered electrical neural stimulation		118, 119, 134
	Biofuel		Available power: ~ μW Operational distance: N/A Tissue impact: Biofouling, inflammation	Downlink: RF Uplink: Bluetooth Control: Digital Embodiment: Application: Implantable biofuel cell		136, 149