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The role of a surfactant on the rate of bubble growth via rectified diffusion during acoustic cavitation has been investigated. The charge of the headgroup of ionic surfactants did not affect the bubble growth rate at low surface loadings. However, at higher surface loading, the growth rate was significantly dependent upon the type and charge of the headgroup. Dodecyl trimethyl ammonium chloride (DTAC) caused a greater increase in growth rate than sodium dodecyl sulfate (SDS) or dodecyl dimethyl ammonium propane sulfonate (DDAPS). With charge suppressed by the addition of salt, DDAPS showed the highest growth rate. Particle-image velocimetry (PIV) has been used to characterize the microstreaming around a bubble in aqueous solutions of these surfactants. Results indicate an enhancement of microstreaming velocities in the vicinity of the bubble when surfactant of any form is present in the solution, with the greatest enhancement for the surfactant with the most bulky headgroup. A clear difference could also be observed when bubbles underwent surface mode oscillations that dramatically increased the streaming velocities and resulted in a chaotic flow. The enhancement in surface oscillations and microstreaming was greatest for DTAC reflecting the bulkiness and charge of the headgroup. The resistance to mass transfer also played a role in enhancing the rectification of gas into the bubble. Surfactants with a longer chain length provide greater mass transfer resistance, and this contributes to higher growth rates.

It is hypothesized that microstreaming might play a role in the sonoporation process, inducing shear stresses which create tension and stretching over the cell membrane and thus lead to its transient permeabilization. In this study, the results of microscopic particle-image velocimetry (PIV) for large bubbles are presented and compared to those obtained using a numerical model. Air bubbles were created in a water solution and then attached to a wall. For each bubble, the microstreaming was measured over a plane located at a distance of 50 µm from the bubble wall. Bubbles were excited using a single element transducer at 28 kHz and 7 kPa. The acoustic microstreaming generated by the air bubble was calculated for the same excitation signal by using a theoretical model based on an analytical solution of the time-averaged Navier-Stokes equation. This approach was also applied to estimate the flow around an encapsulated contrast microbubble using an excitation signal centered at 2.87 MHz with a pressure amplitude of 50 kPa. PIV measurements show that a maximal average velocity of 0.25 mm/s occurs near the bubble resonant size (244 µm diameter bubble). For this bubble, the shear stress is also maximal. Theoretical data are in good qualitative agreement with PIV measurements. Using the contrast agent microbubble model, a maximal flow velocity and shear stress of 4 mm/s and 19 Pa respectively are obtained for a 2.5 µm diameter bubble. The shear stresses are much higher than those produced by normal blood flow (0.5 -2 Pa). These results suggest that bubbles are capable of exerting significant shear stresses on the cell membrane, affecting likely the sonoporation process.

Therapeutic hypothermia is already widely acknowledged as an effective neuroprotective intervention, especially within the acute care setting in relation to conditions such as cardiac arrest and neonatal encephalopathy. Its multifactorial mechanisms of action, including lowering metabolic rate and reducing acute inflammatory cellular processes, ultimately provide protection for central nervous tissue from continuing injury following ischaemic or traumatic insult. Its clinical application within acute traumatic spinal cord injury would therefore seem very plausible, it having the potential to combat the pathophysiological secondary injury processes that can develop in the proceeding hours to days following the initial injury. As such it could offer invaluable assistance to lessen subsequent sensory, motor and autonomic dysfunction for an individual affected by this devastating condition. Yet research surrounding this intervention's applicability in this field is somewhat lacking, the majority being experimental. Despite a recent resurgence of interest, which in turn has produced encouraging results, there is a real possibility that this potentially transformational intervention for treating traumatic spinal cord injury could remain an experimental therapy and never reach clinical implementation.

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