Time-dependent, microscale mechanical properties of acoustically responsive scaffolds using atomic force microscopy and confocal imaging

Acoustic droplet vaporization (ADV) has been used to modulate delivery of regenerative molecules from acoustically responsive scaffolds (ARSs), which are composite fibrin hydrogels containing payload-carrying, phase-shift emulsions. Here, we studied the micromechanical properties of ARSs including stiffness as well as elastic and shear moduli over 7 days post-ADV. ARSs, containing dextran-loaded perfluorohexane (PFH) emulsions (diameter: 6 mm) embedded in fluorescently labeled fibrin gels, were exposed to acoustic pressures above the ADV threshold (2.2 ± 0.2 MPa ) at 2.5 MHz. The compressive stiffness as well as elastic modulus of fibrin gels (i.e., without emulsions) and ARSs (pre- and post-ADV) were measured via force-spectroscopy and the Hertz model, respectively. The measured stiffness was lowest (1.9 ± 0.5 mN/m) in regions adjacent to the PFH emulsions and highest (16.0 ± 4.4 mN/m) in regions adjacent to the ADV-generated bubbles. Fibrin compaction at the bubble–fibrin interface was observed using time-lapsed confocal imaging and correlated with bubble growth at the time points studied here. In addition, different ADV-generated bubble responses in ARSs will be discussed using time-lapsed confocal imaging. Elucidating the mechanical microenvironment within the ARS could be used to control mechanically induced, cellular processes and further the understanding of ADV-triggered drug delivery for regeneration.