Back pain from spinal disc degeneration affects over 600 million people worldwide. Treatments range from conservative approaches like physiotherapy to invasive procedures such as spinal fusion. Injectable engineered hydrogels offer a promising minimally invasive alternative; however, conventional self-curing lacks control, and optical curing is limited by poor tissue penetration. In contrast, ultrasound-triggered implant formation enables precise spatiotemporal control of gelation at clinically relevant depths. We developed a cavitation-based ultrasound guidance protocol using broadband cavitation emissions as a real-time metric of hydrogel gelation and stiffening. In vitro experiments with hydrogel precursors containing calcium-loaded liposomes showed a rapid drop in broadband cavitation activity within 100–150 s, reflecting gel stiffening and inhibited bubble dynamics. Building on this, an automated feedback system was implemented where ultrasound was modulated based on the running median of broadband power relative to a dynamic threshold, enabling on/off cycling until cavitation ceased, signaling treatment completion. Passive thermometry confirmed safe temperature control. Validation in nine ex vivo bovine spinal units (six treated, three control) confirmed effective ultrasound-triggered gelation without material herniation and partial restoration of biomechanical function. This cavitation-guided approach represents a key advancement in non-invasive, adaptive monitoring for ultrasound-mediated spinal disc repair.
Brans et al. (2025) studied this question.