A multiaxial bioreactor system that applies targeted magnitudes of strain energy to 3D cellular constructs

Cyclic mechanical stimulation is instrumental in the remodeling and engineering of musculoskeletal tissue, yet the physical mechanisms that regulate this mechanobiological response are poorly understood. A plausible explanation is that extracellular matrix remodeling is governed by the strain energy that develops during tissue distortion and dilation. A major barrier to testing strain energy-based theories is the absence of in vitro experimental methods that can prescribe targeted amounts of strain energy to 3D cellular constructs under different physiological loads for given thermodynamic constraints. Therefore, we designed and built a multiaxial bioreactor that can simultaneously apply cyclic tensile and compressive loads to 3D specimens. Total strain energy is computed and decoupled into distortion and hydrostatic parts using a numerical approach that we developed and verified. A control system adjusts the loads until a user-specified magnitude of total strain energy (per loading cycle) is achieved under uniaxial or biaxial stress conditions. The bioreactor system successfully applied the targeted strain energy of 100 J/m³ to acellular polyurethane scaffolds subjected to uniaxial tension, uniaxial compression, and biaxial tension-compression with errors < 5 %. We then tested the bioreactor's ability to stimulate fibroblast-seeded 3D collagen scaffolds and found that, compared to unstimulated controls, cell viability significantly increased when targeted levels of strain energy (biaxial tension-compression) were periodically applied during two days of culture. By specifically controlling strain energy in 3D cellular constructs, this new testing methodology will allow the investigation of energy-based mechanobiological theories and may assist the advancement of musculoskeletal tissue engineering.