Bandgap-enabled ultrasound tissue marking by a biodegradable metastructured hydrogel implant.

Implantation of tissue markers within lesions is standard practice in breast cancer diagnosis and treatment, which provides critical guidance for regular follow-up and precise surgical localization. However, the identification of existing markers is limited by nonspecificity and low resolution of conventional ultrasound imaging. The use of rigid materials to compensate for this constraint has resulted in inadequate biodegradability and biocompatibility. Here, we propose a biodegradable metastructured hydrogel tissue marker with acoustic bandgaps enabling specific ultrasonic reflection spectra. Through frequency-to-color mapping on B-mode images, visualization is achieved, with scalable designs allowing multiple marker distinction. Deliverable via an 18-gauge puncture needle and featuring tissue-mimicking softness, the metagel marker mitigates displacement risks, tissue damage, and inflammation. Live pig experiments demonstrate clear identification and dynamic stability. Six-week rat studies confirm long-term marking ability and biocompatibility. These results support the metagel marker's clinical potential for breast cancer management and precise therapy.