Ultrasound-Responsive Block Copolymer Micelles Based on a New Amplification Mechanism

A new approach for amplifying the effect of high-intensity focused ultrasound (HIFU) in disassembling amphiphilic block copolymer (BCP) micelles in aqueous solution was investigated. The diblock copolymer is comprised of a water-soluble poly(ethylene oxide) (PEO) block and a block of poly(2-(2-methoxyethoxy)ethyl methacrylate) (PMEO(2)MA) that is hydrophobic at temperatures above its lower critical solution temperature (LCST). We show that by introducing a small amount of HIFU-labile 2-tetrahydropyranyl methacrylate (THPMA) comonomer units into the PMEO(2)MA that forms the micelle core at T > LCST, an ultrasound irradiation of a micellar solution could induce the hydrolysis of THPMA groups. As a result, the LCST of the thermosensitive polymer increases due to the conversion of hydrophobic THPMA comonomer units onto hydrophilic methacrylic acid. Consequently, the BCP micelles disassemble without actually changing the solution temperature. In addition to the characterization results of transmittance measurements, variable-temperature NMR, SEM, and DLS, a C-13 NMR spectral analysis provided critical evidence for the hydrolysis reaction of THPMA groups under HIFU irradiation.