Implantable biosensors have great potential in the diagnosis, monitoring, and treatment of a variety of disease conditions, showing promise in areas such as diabetic monitoring, heart pacemaker monitoring, and cancer detection. These biosensors currently require a power source such as lithium batteries to function; however, these power sources usually have limited lifetimes and are difficult to miniaturize. To get around these issues, biological cells have been recognized as a novel possibility for generating electric power from potential biosources such as blood glucose or amylum.
What are biofuel cells and how do they work?
In 1791, Galvani showed the link between biology and electricity in his experiments using electrical sparks to make frogs’ legs twitch. Approximately 50 years after this pioneering work, the concept of fuel cells was first introduced in 1839 when Grove reversed the process of water electrolysis. Combining these two concepts, we can define a biological fuel cell (biofuel cell) as a device able to transform electrochemical energy into electricity via biochemical pathways.
They “eat” blood sugar and gain power:
The first application of biofuel cells was glucose biofuel cell (GBFC) technology. In 2010, researchers implanted a GBFC in the retroperitoneal space of freely moving rats. This GBFC was built on composite graphite discs at the anode containing glucose oxidase and ubiquinone, as well as a cathode containing polyphenol oxidase (PPO) and quinone. PPO reduces dioxygen into water under circumstances when some molecules are present at physiological concentrations of pH 7, in the presence of chloride ions and urates. Thus, at the cathode, PPO could regenerate the quinone form reduced previously into hydroquinone by the battery reaction. This GBFC could produce a peak specific power that is more than enough to power a pacemaker. Recently, researchers from Washington State University applied the concept of GBFCs to demonstrate an innovation whereby blood glucose is used as an energy source. The device was shown to operate for at least 30 minutes without replenishment.
Cotton fiber power?
Cotton-based hybrid biofuel cells could be a similar application of converting chemical energy from glucose in the body to electrical energy for energizing implantable biosensors. A team from Korea has demonstrated a technique for electrical communication between an enzyme and an electrode, which can precisely control deposition of both a gold nanoparticle and the enzyme. Gold nanoparticles of about eight nanometers in diameter are coated onto multiple hydrophilic microfibrils composed of cellulose fibers. The layers are assembled with small organic linkers onto cotton fibers to construct metallic cotton fibers. For preparation of the anode for oxidizing the glucose, the glucose oxidase is sequentially layer-by-layer assembled with the same linkers onto the metallic cotton fibers. For preparing the cathode oxygen reduction reaction, gold-covered electrodes are used to provide electrocatalytic capabilities. This biofuel cell performs a power density of 3.7 mWcm−2, which can promote charge transfer through electrodes and provide an important tool to improve the performance of biofuel cells.
Challenges for biofuel cell research:
Research into biofuel cell technology for medical implants has progressed rapidly in recent years, but several challenges remain. One major barrier that must be surmounted is the complexity of the blood matrix that includes various cells and biomolecules. These have been shown to precipitate on the surface of electrodes, which can interfere with electron transfer and inhibit the enzymes employed. Current studies also focus mainly on sensing glucose and lactate, but the use of other enzymes will allow for detection of new biomarkers.
As scientists and engineers continue to develop new biofuel cells that address current challenges, we will begin to realize the full potential of this technology for innovative self-powered implantable devices.
