When you think of batteries, I bet the image that comes into your mind is of a pink bunny and small metal cylinders. Some of you with recent car issues may even think of the lead-based batteries that we use to start our cars. There are a myriad of batteries with different sizes and characteristics that power a vast number of devices we use in everyday life, such as our smartphones, hearing aids, watches and others. One place these types of batteries are not useful is inside the human body. The chemicals used are largely toxic, and they are not biocompatible or biodegradable.
Batteries have three essential components — an anode, a cathode and an electrolyte. For a battery to produce an electric circuit, electrons that build up at the anode are provided a path so they flow to the cathode. This flow of electrons is what we detect as an electric current.
Scientists have been working on batteries for medical use for many years. Medical devices near the surface of the body can be powered from power supplies outside the body, but devices implanted deep in the body have been a challenge. Implanted devices could monitor a tissue and send back relevant medical data, provide images deep inside the body or dispense drugs as needed.
An ideal biodegradable battery for human use would power a small device temporarily and then be degraded harmlessly by natural processes in the body at the end of its useful life. One prototype has a battery using melanin, which is a pigment in the skin, and manganese oxide as the electrodes and sodium ions as the electrolyte. These are all biocompatible and break down into nontoxic components in the body.
Recently, scientists in Australia have made batteries using silk as the basis for the electrodes. Silk is ideal as a material. It is strong, can be formed into thin films, and is completely biocompatible and biodegradable. For the silk bio-battery, a common biochemical compound called choline nitrate was placed on the silk film as the electrolyte. The anode consisted of a magnesium alloy and the cathode used gold particles. The battery was sealed using additional silk material. Initial devices were the size of a postage stamp, about a quarter the thickness of a credit card and could generate almost one volt — more than enough to power a small device for up to two hours.
More work is needed to allow power generation for longer times and to achieve higher voltages, but this represents an important proof-of-concept for this design. The implanted battery completely degraded in a month and a half. After natural degradation, all that remained from the implanted device were some gold nanoparticles left from the cathode, and those were eventually cleared by the body.
We predict that we will be seeing these types of biodegradable batteries for implanted medical devices soon. These will become a mainstay for remote sensing and drug delivery to parts of the body we could only dream about accessing previously. Hail to the engineers who represent a new frontier in the evolution of medicine.