Hyper-elastic behavior of soft-tissue like microgels in two-phase converging microchannel flow

Abstract

Deformation of biological cells, tissues, and similar soft materials is often considered linearly elastic; however, the assumption is only valid in a very limited stress range and often leads to significant errors in mechanical evaluation. We demonstrated the hyper-elastic behavior of ultra-soft poly(N-isopropyl acrylamide) (PNIPAm) microgels (USPNMs) in a converging channel flow, as a representation for biological tissues. The hyper-elasticity of USPNMs in response to a broad range of deformation was characterized at the centerline of the converging flow. We introduced a carrier fluid consisting of baby hydrogels (avg. diameter, 10 μm) and oil that carried the hydrophilic USPNM sample (avg. diameter, 100 μm) on the centerline of oil background fluid. By “baby hydrogel,” we mean small PNIPAm particles obtained during USPNM synthesis, using which, enabled settling-free flow, prevented wall contact, and enhanced carrier fluid viscosity for increased stresses at lower flowrates. Furthermore, drastic reduction of interfacial tension was observed in the converging area due to contact of baby gels with USPNM particles in the carrier fluid. The shear and elongational stresses were balanced with the elastic stress and interfacial Laplace pressure. As a result, we obtained a stress–strain curve from the microscopic images during flow. The non-linear stress–strain curve was characterized by conventional hyper-elastic models. The elastic modulus of the synthesized USPNM was 24 Pa, which is as low as animal brain tissue. This method holds great potential for implementing in similar hyper-elastic systems, enabling accurate mechanical evaluations in the field of soft materials, biology, and medicine.