Left: Ultrasound-induced bubble formation radially compacted the fibrin matrix of the composite. Right: Myofibroblast phenotype was higher proximal to the bubble compared to distal regions.Biophysical conditions – like the stiffness of the surrounding material or matrix – give cells cues that influence their behavior. Matrix stiffness can prompt stem cells to differentiate into other cell types or impact gene regulation. Although these cues are harnessed in hydrogels implanted into the body for biomedical purposes, they cannot be altered once in situ. Now, however, researchers have developed sound-sensitive hydrogels which can change their stiffness locally to affect cellular behavior [Farrell et al., Acta Biomaterialia 138 (2022) 133-143, https://doi.org/10.1016/j.actbio.2021.11.020].
“We designed a composite hydrogel that can be non-invasively modulated using ultrasound,” explains Mario L. Fabiilli of the University of Michigan, who led the work. “Ultrasound induces localized changes to mechanical and morphological properties of the hydrogel, which can be used to control cell behaviors.”
The composite hydrogel, which the researchers label an acoustically responsive scaffold (or ARS), is fabricated from a phase-shift emulsion – in this case, shell-stabilized, liquid droplets containing a perfluorocarbon core – in a fibrin matrix. When exposed to ultrasound, the perfluorocarbon liquid non-thermally vaporizes into gas bubbles, which expand dramatically in volume, squeezing the fibrin network in the vicinity and increasing the local stiffness. This process, which is known as acoustic droplet vaporization, can increase the elastic modulus by a factor of up to 20.
“We applied this composite hydrogel to investigate the differentiation of fibroblasts to myofibroblasts, a transition [that is] correlated with substrate stiffness in 2D,” Fabiilli continues.
Myofibroblasts are critical to wound healing, contracting to close wounds and depositing collagenous material, but if left unregulated can produce scar tissue or cause tissue contraction while too few are associated with ulcers and chronic wounds. To determine how the transition from fibroblast to myofibroblast might be controlled using ultrasound, the researchers seeded normal human fibroblasts from the skin onto an ARS after polymerization.
“In this study, we demonstrate for the first time how ultrasound-induced changes to biophysical signals within an ARS impact cell behavior,” says Fabiilli.
Implanted fibroblasts show higher levels differentiation into myofibroblasts in the areas of ARS with increased stiffness following ultrasound exposure. Ultrasound has the major advantage that it is both non-invasive and can penetrate deep within the body. It can also be focused to pattern ARS with different regions of stiffness with sub-millimeter resolution.
“We are optimistic about the translational potential of our approach,” he adds. “This approach could enable the modulation of biophysical signals in situ.”
The properties of an implanted ARS could be altered non-invasively days after implantation, which could be helpful in developing therapies for chronic wounds and models of fibrosis that consider the impact of biophysical cues.