  Nanotechnology-based microscopic robots could provide accurate in vivo
measurement of chemicals in the bloodstream for detailed biological research
and as an aid to medical treatment. Quantitative performance estimates of such
devices require models of how chemicals in the blood diffuse to the devices.
This paper models microscopic robots and red blood cells (erythrocytes) in
capillaries using realistic distorted cell shapes. The models evaluate two
sensing scenarios: robots moving with the cells past a chemical source on the
vessel wall, and robots attached to the wall for longer-term chemical
monitoring. Using axial symmetric geometry with realistic flow speeds and
diffusion coefficients, we compare detection performance with a simpler model
that does not include the cells. The average chemical absorption is
quantitatively similar in both models, indicating the simpler model is an
adequate design guide to sensor performance in capillaries. However,
determining the variation in forces and absorption as cells move requires the
full model.
