English: "Schematic of the non-enzymatic metabolic fuel cell for closed-loop control of blood-glucose homeostasis. A) Operation of the metabolic fuel cell. After implantation, the metabolic fuel cell connects to the bloodstream. At the anode, consisting of cupric oxide (CuO) nanoparticles embedded into multi-walled carbon nanotubes (MWCNTs) and coated with poly-(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) (CuO-MWCNTs-PEDOT:PSS), glucose is catalytically converted to gluconate with concomitant generation of protons (H+) and electrons (e−). While gluconate is either renally cleared or enters the pentose-phosphate cycle, the electronic circuit is supplied with the electrons, and the protons travel to the cathode, consisting of Nafion-coated carbon black (CB)-containing platinum (Pt) nanoparticles (Pt-CB/Nafion), where they reduce oxygen to water (H2O). The electrons travel to the power circuit which provides a wireless interface with wearables for communication, recording and tuning, manages glucose sensing and processes the operation of the metabolic fuel cell at blood-glucose levels above 10 mm glucose. The harnessed electrical energy is used to stimulate engineered human cells to release therapeutic proteins such as insulin in response to light (optostimulation of Opto-β cells) or electric fields (electrostimulation of Electro-β cells). B) Closed-loop blood-glucose homeostasis. The power circuit of the metabolic fuel cell constantly monitors the blood-glucose levels and switches on the metabolic fuel cell when blood-glucose levels are above 10 mm. The metabolic fuel cell not only reduces the blood-glucose by glucose consumption but also uses the harnessed energy for electro- or opto-stimulation of rapid vesicular insulin release by engineered human cells. The combination of these actions reduces the blood-glucose to normal levels and the metabolic fuel cell is shut off. As a consequence, electro- and optostimulation ceases, as does the insulin release, providing time for an insulin refill before the next glucose surge occurs. The metabolic fuel cell and the human designer cells form a closed-loop control circuit that maintains blood-glucose homeostasis in an automatic, seamless, and self-sufficient manner."