it seems like they're able to use liposomes (molecules encapsulated in fatty or waxy organic shells) to deliver material to the desired location then use ultrasound to unsheath those molecules and knit them together into larger structures
I have no experience or education in this field so take this with a grain of salt.
Certain compounds like fibrin (in blood clots) and collagen (in extra cellular matrix) self assemble under certain conditions. Small volumes delivered via liposomes being triggered to self assemble in specific locations is fascinating.
I have no experience or education in this field so take this with a grain of salt.