Soft materials for acoustic applications

Abstract

Ultrasound finds wide application in imaging and testing because ultrasound can penetrate tissue and is benign. Gaseous microbubbles strongly scatter ultrasound and are therefore used as contrast agents. Ultrasound responsive materials can be used for many industrial and biomedical applications. Ultrasound can also be used to exert forces and manipulate particles solution and biological cells. In this thesis, material systems are developed for three application areas: 1) models of human organs for the quantitative evaluation of surgical procedures with ultrasound; 2) the fabrication of soft objects by assembling polymeric particles with ultrasound and the acoustic hologram; and 3) the characterization of antibubbles as novel contrast agents that can carry a fluid load. Organ phantoms serve as tools in medical fields to train and plan medical procedures. However, current organ phantoms miss important features or are not realistic. Current models tend to possess a Young’s modulus that is much higher than that of tissue. Furthermore, many of the current models do not show the correct contrast in a medical imaging setting. This thesis presents high fidelity organ phantoms that possess the correct elasticity, compliance, optical appearance, and correct ultrasound contrast. One model is developed for cystoscopy (CY) of the bladder. Another phantom for the transurethral resection of the prostate (TURP). The quality of the phantoms is validated by medical practitioners. For CY, the execution time of the medical practitioners is recorded to completely map the inside of the bladder phantom while localizing tumor models that have been embedded in the bladder wall. For TURP, the quality of the resection is compared with ultrasound imaging before and after the surgical simulation. Parameters are defined to quantify the success of the procedure. The phantoms developed as part of this thesis have received high satisfaction scores from medical practitioners. The parameters reflect the experience of the surgeons. In assembling soft matter, one challenge is that existing 3D printing methods are slow. In contrast, the use of ultrasound patterns shaped with a recently invented acoustic hologram allows objects to be built at once. In this thesis, polydimethylsiloxane (PDMS) particles have been assembled into two-dimensional shapes with ultrasound. To fix the assembly, the PDMS has been physically functionalized with an initiator using swelling. Suitable swelling solutions have been determined based on their solubility. The stability of the physisorbed initiators is evaluated, and the functionalized PDMS particles are fixed via photopolymerization after assembly in aqueous polyethylene glycol dimethacrylate (PEG-DMA) solutions. The fabrication steps can be repeated to increase the thickness of structures that are mechanically stable. The antibubble is an emerging ultrasound contrast agent. It has an inverse form to a conventional bubble in that a substance in the core is surrounded by a gaseous layer. The antibubble is acoustically responsive and, compared to conventional microbubbles, can carry a much greater load. In this thesis, the structure of antibubbles is examined. In particular, the volume of the load is quantified, and the amount of gas per bubble is estimated. The stability of the core substance against diffusion is investigated and shown to be stable for over 11 h.