Nano-biosupercapacitors enable autarkic sensor operation in blood

Abstract

Today's smallest energy storage devices for in-vivo applications are larger than 3 mm³ and lack the ability to continuously drive the complex functions of smart dust electronic and microrobotic systems. Here, we create a tubular biosupercapacitor occupying a mere volume of 1/1000 mm³ (=1 nanoliter), yet delivering up to 1.6 V in blood. The tubular geometry of this nano-biosupercapacitor provides efficient self-protection against external forces from pulsating blood or muscle contraction. Redox enzymes and living cells, naturally present in blood boost the performance of the device by 40% and help to solve the self-discharging problem persistently encountered by miniaturized supercapacitors. At full capacity, the nano-biosupercapacitors drive a complex integrated sensor system to measure the pH-value in blood. This demonstration opens up opportunities for next generation intravascular implants and microrobotic systems operating in hard-to-reach small spaces deep inside the human body.

Fig. 1. Fabrication and electrochemical performance of nBSC.

a Microscope image of a completed nBSC before rolling. b Microscope image of a “Swiss-roll” nBSC. Scale bar, 200 µm (a, b). c Microscope image of an array of nBSCs before rolling and (d) after self-assembly into tubular “Swiss-rolls”. Scale bar, 500 µm (c, d). e Schematic illustration showing all active components of “Swiss-roll” nBSC with hollow core for the fluent flow of blood: polymeric rolling stack, gold (Au) top and bottom current collectors, PEDOT:PSS active top and bottom electrode, polyvinyl alcohol (PVA) proton separator and SU8 photoresist passivation. f CV curves of nBSCs in different biological electrolytes (NaCl, blood plasma, blood) at a scan rate of 100 mV ${\mathrm{s}}^{-1}$. g GCD curves at an applied current of 50 nA. h Volumetric specific capacitance as a function of applied current in different electrolytes. i GCD curves at four different potential windows at 100 nA. j Capacitance retention and coulombic efficiency of a device in blood over 5000 cycles. (in Fig. 1h, error bars represent the variation in data over three measured devices. All measurements are performed under ambient conditions at 25 °C).

Fig. 2. Mechanism of bioenhancement in a nBSC.

a–c Schematic illustration of ion transport and energy storage mechanism between two working electrodes in different electrolytes: a Faradaic reactions in presence of Na⁺ and Cl^- ions in medical saline. b Faradaic reactions coupled with bioelectrocatalysis due to the presence of enzymes in blood plasma electrolyte. c ATP synthesis coupled with bioelectrochemical reactions enhancing the electrochemical response of the nBSC in blood. d Schematic illustration of a three-electrode setup with a working electrode (the measured sample), reference electrode (Ag/AgCl) and counter electrode (platinum rod) to determine redox/catalytic reactions. e CV curve of various working electrode configurations measured in blood electrolyte at a scan rate of 30 mV ${\mathrm{s}}^{-1}.$ f CV curve of various working electrode configurations measured in NaCl electrolyte at a scan rate of 30 mV ${\mathrm{s}}^{-1}.$ g Self-discharge/self-charge of nBSCs under different electrolyte fluids for 1.5 h. h Self-discharge profile in blood after 5000x GCD cycles. Inset shows cross-sectional images of tubular nBSC after 5000x GCD cycles. (All measurements are performed under ambient condition at 25 °C.).

Fig. 3. Performance of nBSC under physiologically relevant conditions.

a–c Temperature dependent measurements at static flow condition: a CV of nBSC at different temperatures with blood as electrolyte at a scan rate of 100 mV ${\mathrm{s}}^{-1}$. b GCD curve at different temperatures with blood as electrolyte for an applied current of 50 nA. c Volumetric capacitance of nBSCs in different electrolytes as a function of temperature. d–f Flow dependent measurements at 25 °C: d CV of nBSC at different flow rates with blood as electrolyte at a scan rate of 100 mV ${\mathrm{s}}^{-1}$. e GCD curve of nBSC at different flow rates with blood as electrolyte for an applied current of 50 nA. f Volumetric capacitance of nBSCs in different electrolytes as a function of various flow rates. g Volumetric capacitance of nBSCs in blood as a function of applied pressure. Inset shows nBSC before (top) and after (bottom) compression. Scale bars, 200 µm. h Capacitance retention and coulombic efficiency of a device under repeated compressions at 15 kPa over 100 cycles. Inset shows 100x GCD cycles (each GCD curve was measured after 5 compression cycles) of a nBSC subjected to 15 kPa. (in Fig. 3c, g, the error bars represent the variation in data over three measured devices.), (in Fig. 3f, the error bars represent the variation in data over two measured devices.).

Fig. 4. nBSC as a self-powered pH sensor.

a Microscope image of pH sensor with all integrated components (ring oscillator (RO) with nBSC sensing element connected to nBSC energy storage via drain-to-drain voltage (VDD), ground (GND)) before roll-up and (b), after roll-up, inset shows the operational circuit. Scale bars, 200 µm (a, b). c Volumetric capacitance and relative frequency change of nBSC as a function of electrolyte pH. d Frequency spectral response of the nBSC based pH sensor at various electrolyte pH. e Output voltage swing of a five-stage oscillator working at 3 V DC input provided by the on board nBSC as a function of electrolyte pH. (in Fig. 4c, error bars represent variation in data over three measured devices. All measurements are performed under ambient conditions at 25 °C in artificial plasma flow of 2 ml/min).