Ph.D Thesis

Ph.D StudentMizrahi Natalya
SubjectThe Effect of Therapeutic Ultrasound on Cell Mechanics
DepartmentDepartment of Biomedical Engineering
Supervisors PROFESSOR EMERITUS Eitan Kimmel
Full Thesis textFull thesis text - English Version


Low intensity therapeutic ultrasound (LITUS) is a non-invasive therapeutic tool widely used for clinical applications such as physiotherapy, drug delivery, bone fracture healing, and thrombolysis. Being a non-invasive low-maintenance and highly accessible tool, therapeutic ultrasound has great potential in clinical applications. However, the physical mechanisms responsible for the beneficial effects of LITUS are not understood; this ultimately limits the extent of it applications.

In this study we investigated direct mechanical changes imposed by short exposure to LITUS agitation on three different systems: 1). an inert bio-polymer gel, 2). individual single cells and 3). confluent cell monolayers. At the first step, we used methylcellulose (MC), an inert viscoelastic polymer solution, as a model system for cellular matter. We utilized the nanoparticle microrheology approach to monitor real time changes in MC properties as induced by exposure to LITUS. In this part of the study we demonstrated that 30s acoustic agitation with intensities level of 2w/cm2, which commensurate to intensity levels utilized in clinical settings, transiently decrease viscosity of the polymer solution.

The second part of the study is focusing on direct interaction of LITUS with cells where we characterized immediate mechanical response on the single cell level. We demonstrated prompt and transient fluidization of the cell structure and dramatic acceleration of its remodeling dynamics when exposed to low intensity ultrasound; that are reminiscence of the physical rejuvenation phenomena observed in soft fragile materials. Remarkably, this response caused by very small strains (10-5) at ultrasonic frequencies (106 Hz) is closely analogous to that caused by relatively large strains (10-1) at physiological frequencies (100 Hz).

In the third part of this study, following the findings from the single cell experiments, we focus on the effects of acoustic agitation on a cellular monolayer. In this set of experiments we show that LITUS is capable of causing profound changes in membrane integrity which trigger local disruption of underneath actin network ultimately resulting in decreased contractility of the monolayer. This effect can be directly associated with acoustically induced changes in monolayer permeability utilized in clinical application such as transdermal drug delivery and acoustically facilitated gene transfections.

Altogether, these results highlight the potential of low intensity ultrasound to perturb cytoskeletal dynamics of a single cell as well as structural integrity of the monolayer and demonstrate for the first time a direct and prompt mechanical response of cellular structure to LITUS.