The Tensile Expansion Microscopy (TExM) cell stretcher: an iris expansion device integrated with automated real-time autofocus and tracking to super-resolve cells

Abstract

Mechanical stretchers are used to physically expand biological samples for microscopic studies of mechanobiology. Existing stretchers suffer from limited strain capacity and non-quantifiable forces, along with focus drift and feature-tracking limitations during microscopy. We develop an iris-based cell stretcher that applies isotropic equibiaxial force to achieve aerial strain of 1664% that corresponds to 4.2x linear expansion for Tensile Expansion Microscopy (TExM), a super-resolution method which increases sample size above the diffraction limit of light. The TExM stretcher uses 3D-printed, printed circuit board (PCB) and cost-effective parts, is portable and automated. We verify the performance of the TExM cell stretcher using image analysis, achieving ~90% mechanical precision, a maximum of four degrees of arm angular deviation, precise speed control down to 0.01 cm/sec, and an average expansion resolution of 3.98 x 10^-3 x. Integrated strain gauge sensors confirm equal application of force by each arm of the stretcher throughout expansion and finite-element simulation and planar-polariscope photoelastic imaging characterize the uniform substrate stress distribution while indicating high stress at the substrate gripping area. An AutoTracking software in communication with the stretcher hardware and microscope enables continuous autofocus and feature tracking during TExM and fiducial markers verify uniform equibiaxial stretch. The stretcher is demonstrated with fixed NIH 3T3 fibroblasts and live HeLa cells, observing ~4x cellular expansion of fixed cell size and separation of live cell clusters, highlighting the potential of TExM cell stretcher for biological imaging.