On the Molecular Level Cavitation in Soft Gelatin Hydrogel

Abstract

We have studied the molecular level cavitation mechanisms and bubble growth kinetics in soft gelatin hydrogel and water. The apparent difference in cavitation threshold pressure between that generates in pure water and that in gelatin hydrogel is considered. Gelatin, which is derived from collagen, is frequently used as a brain simulant material. in liquid, cavitation bubble is created when surrounding pressure drops below the saturation vapor pressure. in principle, a cavitation bubble should continue to grow as long as tensile pressure continues to increase in the system. in our study, using molecular dynamics simulation, we have investigated the pressure requirement for a nanoscale cavitation to grow in water and gel. First, we have modeled a gel like structure with a preexisting bubble of 5 nm radius. A control model containing a 5 nm bubble in pure water is also created. Then, we have applied hydrostatic tensile pressure at two different expansion rates in the gel and water models. The results show that a gel-like structure requires higher pressure for the cavitation to grow, and both gel and water models exhibit strain rate effect on the cavitation threshold pressure. We have also found that the cavitation collapse time is dominated by the viscosity of the medium. Cavitation initiates in a liquid or soft material when pressure drops below the saturated vapor pressure. Soft materials like gelatins are formed when interconnected collagen or collagen-like molecules form a cellular network in bulk water. A slight changes in the applied pressure can collapse a cavitation bubble and induce high pressure to the adjacent cells to initiate damage. Damaging capabilities of collapsing cavitation bubbles are well known. For example, severe erosion damage to propeller blades of a sea vehicle can occur due to high-energy cavitation bubble collapse over the blade surface. By the same mechanism, implosions in fluid-filled biological systems such as soft tissues or brain may experience significant damage 1. Recently, understanding the cavitation mechanisms in soft materials gained attention due to its potential implication in biomedical related applications 2-5. Soft materials such as gelatin hydrogels are widely used as a brain simulant material 6-8. In addition, the stiffness and other physical properties of gelatin gel can be tuned by changing the gel concentration in aqueous solution. Recently, reasonable structural similarities are observed between the gel network and brain's perineuronal net (PNN) 9 (λure 1A,B,D). A high resolution cryo-SEM image shows that a gelatin microstructure consists of both crystalline and coil like amorphous part (Fig. 1C), where the crystalline part is made of collagen-D periodic microfibrillar structure 10 , and the coil part, found only at the network junction, is composed of amorphous collagen chains. Cavitation mechanisms is widely studied for water 11-13 and fairly well understood. However, cavitation mechanism in biological soft matter is not explored well. Cavitation in gel is complic...

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