Based on the catalyst used, bio-fuel cells can be classified into microbial fuel (MFC) or enzymatic bio-fuel (EBC) cells; whereas considering the pathway through which electrons move from anode to cathode, we can distinguish between direct or non-direct cells For example, the fuel can come from glucose and oxygen in the blood following these reactions:
At anode: C6H12O6 → C6H10O6 + 2H+ + 2e−
At cathode: O2 + 4H+ + 4e− → 2H2O
The interest in MFCs was triggered in 1960s owing to the USA space program during which possible powering technologies had been sought for a waste disposal system for space flights.
The application of MFCs for IMDs was first initiated in 1960s when cell-free enzyme-based fuel cells were used for implantable artificial hearts. During the 1970s, EBCs using glucose as fuel and oxygen as oxidizer were investigated to provide power for IMDs and since then, various approaches and modifications have been proposed to enhance the performance as well as to target specific applications.
In 2003, Mano and colleagues reported a miniaturized bio-fuel cell capable of operation while implanted in a grape.
They were able to produce a power of 2.4 μW at 0.52 V which was very promising for the applications with compact implants. Bio-fuel cells present several advantages including the use of existing recyclable materials in Nature, the moderate operating conditions for the reactions and the biocompatibility between bio-fuel cells and the human body . Nevertheless, challenges still remain.
Firstly, it is difficult to maintain the biocatalyst over a long period and surgical intervention is of course not a desired option.
Secondly, the microwatt level of bio-fuel cells limits their use in a wide range of applications. Finally, even though they are highly biocompatible, unavoidable biofouling still occurs that can damage the device or harm the patients.
Wastewater treatment: Membrane technology has long been used in wastewater treatment plants and it is now being used more and more in biodiesel production, which produces wash water very high in contaminants. In the past, facility operators could spread this waste stream on land or discharge it untreated to a wastewater treatment plant or local water source such as a river.
Today, this is not permitted in many areas. In addition, water-constrained areas are trying to encourage water conservation and reuse rather than disposal. Anaerobic digestion is often used to remove biochemical oxygen demand and chemical oxygen demand from the waste stream. Ultrafiltration may be used to concentrate the bio sludge, followed by a water recovery and reuse reverse osmosis system. This is especially important in areas where water is limited. For example, Koch Membrane Systems’ membrane filtration technology is being used as part of a membrane bioreactor at an Australian ethanol facility where the waste streams go through biological treatment and use membranes to recover water for reuse.