Nanoscale strain gauges on flexible polymer substrates

Biological cell force is important for proper cell and tissue function and can be an indicator of disease. Therefore, measuring cell force has potential in disease diagnosis and treatment. However, biological cell force measurement approaches are limited and typically slow due to the analysis of optical images before and after cell application or other methods that have low throughput. This work seeks to overcome this bottleneck by the use of nanoscale strain gauges which can measure cell forces as an electrical signal in real time, as well as being able to be scaled to measure tens of thousands of cells, simultaneously. This paper presents the design, COMSOL simulation, fabrication, as well as electrical and mechanical testing of gold nanometer scale strain gauges embedded in soft polydimethylsiloxane (PDMS) using a sacrificial aluminum layer method. A process flow using an aluminum sacrificial layer is presented, which successfully fabricated gold strain gauges with 100 nm dimensions in soft PDMS polymer and have been used to measure strain applied to the PDMS surface. Compressive strains ranging from 0.4% to 1.7% in the PDMS surface, corresponding to forces of 718 nN to 2.0 μN have been detected with resistance changes of 1%–8%. To the best of our knowledge, these are the smallest metal strain gauges to be made on soft polymers and is a promising new approach for biological cell force measurement.