Summary
Implantable medical devices have found successful applications in the treatment/monitoring of an ailment inside the body, but have restricted limited life time due to the finite capacity of their power supplies, i.e batteries. In contrast, energy harvesters based on exogeneous chemical fuels such as glucose and dissolved oxygen may hold the promise of a long-term energy supply for such implants. Both glucose and oxygen have a relative abundance in all tissues with a limited dependency to other ambient factors. Thus, this project has addressed some selected challenges based on abiotical micro glucose fuel cells (µGFC), which catalyse glucose and oxygen from inorganic materials acting as the catalyst.
The first challenge as well as the most important feature of implantable µGFC has to do with their working condition where both glucose and oxygen are mixed together in the body fluids. A fuel cell normally relies on the reactants being injected in separate compartments to prevent any cross-reactions. In the most severe case, blending the two reactants at the same electrode may cause mixed potentials, or an electrochemical short circuit resulting in no net energy production. The separation of glucose and oxygen from their mixture, a critical condition for the operation of µGFC, was in this project solved by developing a nanoporous oxygen selective cathode that “removed” oxygen from the solution before it came into contact with the glucose selective anode. The cathode was made from nanoporous alumina membranes coated with a palladium catalyst, whereas the anode was made from silicon coated with a Pt-Ni alloy catalyst.
The estimation of the real area of a catalytic surface plays a critical role in assessing the parameters related to its electrochemical properties. This may be independent of the geometrical size due to surface roughness or pores that will increases the real area, and consequently the number of catalytic sites available. Universal measurement methods exist, but these may not be appropriate for a given catalyst system. Thus, a comparable analysis was made between six selected methods, and it was found that the measurements of oxide formation and double layer capacitance gave the most exact result. Hence, it is important to select a method that is compatible with the system being investigated and the experimental conditions used.