  Biophotovoltaics has emerged as a promising technology for generating
renewable energy since it relies on living organisms as inexpensive,
self-repairing and readily available catalysts to produce electricity from an
abundant resource - sunlight. The efficiency of biophotovoltaic cells, however,
has remained significantly lower than that achievable through synthetic
materials. Here, we devise a platform to harness the large power densities
afforded by miniaturised geometries. To this effect, we have developed a
soft-lithography approach for the fabrication of microfluidic biophotovoltaic
devices that do not require membranes or mediators. Synechocystis sp. PCC 6803
cells were injected and allowed to settle on the anode, permitting the physical
proximity between cells and electrode required for mediator-free operation. We
demonstrate power densities of above 100 mW/m2 for a chlorophyll concentration
of 100 {\mu}M under white light, a high value for biophotovoltaic devices
without extrinsic supply of additional energy.
