Elucidating the effects of low-intensity ultrasound on mesenchymal stem cell proliferation and viability

The effects of Low-Intensity Ultrasound (LIUS) on the proliferation, viability and extracellular matrix (ECM) production of Mesenchymal Stem Cells (MSCs) were investigated. Continuous-wave ultrasound was applied at 1 MHz and 350 mW/cm2 to microwells, using a LIUS system assembled in the laboratory. Needle hydrophone mapping showed that pressure amplitudes ranged from 0.015 MPa at the well edge to 0.080 MPa at the centre. The LIUS group received US for 10, 20 and 30 min per day for one week. Assays were performed daily. Relative to control, 10 and 20 min LIUS very significantly stimulated MSC proliferation and ECM synthesis, while 30 min LIUS had a significant adverse effect. The phenomenon that LIUS accelerates MSC proliferation, but only for appropriate exposures, has been noted previously in the literature. However, the actual relationship between the physical forces generated by the LIUS and this phenomenon remains unknown. The fluid flow pattern created by LIUS was studied by injecting dye in the well and Eckart-streaming-like motions were observed, while thermal effects were negligible. By employing LIUS with appropriate focusing and parameters, it might be possible to exploit MSCs for tissue engineering, independently of biochemical stimuli, and in a highly spatially organised manner.