## 4 Power Supply and Wireless Communication Technologies
When implanted in a guinea pig, the chip was able to extract a minimum of 1.12 nW for up to 5 h, enabling a 2.4 GHz radio to transmit measurement of the electrochemical potential every 40-360 s. [PERSON] _et al._ reported the development of an implantable fuel cell that generates power through glucose oxidation, producing 3.4 uW cm\({}^{-2}\) and up to 180 uW cm\({}^{-2}\) steady-state power and peak power, respectively [21]. Glucose is oxidized at the nanostructured surface of an activated platinum anode, and oxygen is reduced to water at the surface of a self-assembled network of single-walled carbon nanotubes embedded in film that forms the cathode [97,98]. The half-opened geometry allowed the researchers to meet the requirement for simultaneous and independent oxidation and reduction and thus avert electrochemical short circuits. The computational investigations found that theoretically, glucose can be harvested from the cerebrospinal fluid to an energy level of \(>\)1 mW without negative physiological consequences, thus confirming the potential of this energy source to power brain-machine interfaces with low energy consumption. Glucose biofuel cells with glucose oxidase and lactose mechanically incorporated into a conductive pure carbon nanotube matrix were demonstrated to deliver a higher power density up to 1.3 mW cm\({}^{-2}\) and an open circuit voltage of 0.95 V [99]. Under physiological conditions of 5 \(\times\) 10\({}^{-3}\) mol\({}^{-1}\) glucose and pH 7, the devices remained stable for one month, delivering 1 mW cm\({}^{-2}\) power density. Connected in series, two \begin{table}
\begin{tabular}{c c c} \hline
**Tissue Type** & **Relative Permittivity \(\varepsilon_{r}\) (\(\times\)10\({}^{3}\))** & **Conductivity \(\sigma\) (S/m)** \\ \hline Bone & 0.28 & 0.0144 \\ Liver & 9.8–14 & 0.15–0.16 \\ Spleen & 3.3 & 0.62 \\ Blood & 2.7–4.0 & 0.55–0.68 \\ Kidney & 10.9–12.5 & 0.24–0.25 \\ Retina & 4.75 & 0.52 \\ Bone (cancellous) & 0.47 & 0.09 \\ Bone (cortical) & 0.23 & 0.02 \\ Bone (marrow) & 0.11 & 0.003 \\ Cartilage & 2.57 & 0.18 \\ Skeletal muscle & 14.4–27.3 & 0.38–0.65 \\ Fat & 0.09 & 0.02 \\ Cerebrospinal fluid & 0.1 & 2 \\ Brain (grey matter) & 3.8 & 0.17 \\ Brain (white matter) & 1.9–3.4 & 0.12–0.15 \\ \hline \end{tabular} \({}^{1}\) Measured _ex vivo_ at 100 kHz, adapted from [92, 93, 94].  
\end{table} 
Table 1: Dielectric properties of tissues \({}^{1}\). of these cells were able to deliver an open circuit voltage of 1.8 V with a maximum power of 3.25 mW at 1.2 V, indicating the possibility of using these cells to power implanted biomedical devices that typically require at least an operating voltage of 0.5-0.6 V. For example, a cytochrome P450--based molecular biosensor used for drug sensing with temperature and pH monitoring was reported to have a power consumption of 48 \(\mu\)W, with 32 \(\mu\)W required for the molecular detection, 2.5 \(\mu\)W for the pH measurement, 1.4 \(\mu\)W for the control over the temperature sensor, and 12 \(\mu\)W for the multiplexing and measurement reading [100]. Although _in vitro_ studies return promising results, _in vivo_ performance of enzymatic biofuel cells is considerably lower. For instance, _in vitro_ (at \(4.7\times 10^{-3}\) mol\({}^{-1}\) glucose, pH 7.2), an intravenous implantable glucose/dioxygen hybrid enzyme-Pt micro-biofuel cell showed high electrocatalytic performance with an open circuit voltage of 0.4 V and a maximum output power of 0.2 mW cm\({}^{-2}\) at 0.25 V [101]. Once implanted into the jugular vein of a living rat, the device was able to deliver an open circuit voltage of 125 mV at a maximum power density of 100 \(\mu\)W cm\({}^{-2}\) at 80 mV. Furthermore, the lifetime of the enzyme, and thus the long-term performance of the device remains an issue, with a notable loss in the power generated with time _in vivo_[102].