A clamp-free micro-stretching system for evaluating the viscoelastic response of cell-laden microfibers

Viscoelastic hydrogel microfibers have extensive applications in tissue engineering and regenerative medicine, however, their viscoelasticity is still difficult to be directly characterized because microfiber-specific measuring system is lacking for quantitative studies. In this paper, we develop a two-probe micro-stretching system to quantitatively investigate viscoelasticity of the microfiber by evaluating the storage and loss modulus: E' and E″. A liquid bridge-based fixation method enables single microfiber to be easily fixed to be stably stretched by a two-probe actuator. Afterward, multi-frequency stretching force loading is automatically implemented based on real-time force control, and the resulting stress and strain in the frequency spectrum are measured to evaluate the E' and E″ of pure GelMA, alginate-GelMA composite and GelMA core-alginate shell microfibers. The measured E' and E″ are verified by the response of NIH/3T3 fibroblast cells to the composite microfibers with different alginate concentrations. Moreover, benefiting from the low-damaged stretching process, our system can also detect the difference of the E' and E″ between two cellular processes including growth and differentiation of the aligned mesenchymal stem cells in the same one core-shell microfiber. These results all show that our proposed system provides a valuable reference tool for biomaterials design, the study of cell-matrix interaction and disease etiology from the perspective of mechanics.