Effect of Compressive Strain Rates on Viscoelasticity, Structure, and Water Content in Intact Porcine Stomach Wall Tissues

Abstract

The field of minimally invasive surgery is undergoing a transition towards powered surgical devices operating on automated and robotic systems. Many of these devices mechanically manipulate tissues to perform specific surgical maneuvers. Tissue compression between the jaws of end effectors is used as a key stabilization step for a variety of devices including tissue staplers, RF, and ultrasonic vessel sealers. With the emergence of powered instruments and end effectors, determining the extent and rate of compression when using such devices is of great importance to prevent intra-operative, or post-surgical complications.

Staplers and vessel sealers on the market today rely on tactile feedback and surgeons’ experience and training to determine the rate at which compression is achieved. However, the transition to partially or fully powered and automated surgical systems has eliminated the tactile feedback component. Thus, understanding the mechanical behavior of the target tissue undergoing compression is vital to device manufacturers who aim to develop powered instruments with embedded algorithms to control compression levels and strain rates during tissue manipulation. Additionally, due to the heterogeneity of the tissues in the human body (collagen, muscle, fat composition, etc.), these systems and algorithms will require an in-depth understanding of tissue biomechanics and translation of this information into algorithms that will inform the control laws for these powered instruments. This experiment and analytical research project focus on understanding the biomechanical behavior of the stomach wall, in the context of compressive mechanical loading conditions during the stapling in a sleeve gastrectomy procedure.

The loading profile during the stapling procedure was simulated in experiments as a multi-rate indentation problem. This study observed the impact of indentation rate on the viscoelastic (ramp loading, load/stress relaxation, and recovery/rebound) behaviors of the stomach wall tissue. The stomach wall demonstrated faster and more pronounced relaxation (12%) and strain recovery (25%) when indented at fast rates of 37.5%/s, both of which show statistically significant differences compared to the relaxation (20%) and strain recovery (20%) for slow loading rates (7.5%/s). These highlight strain rate dependence of the viscoelastic behavior of the stomach wall. Additionally, the water content in the indented regions of tissue was measured to be lower (P<0.05) compared to the un-indented regions. However, the experiments also reveal that substantial residual strains (53% - slow rate, 48% - fast rate) remain in the indented region of tissues regardless of the initial compressive strain rate applied during the ramp loading phase. Fluid flow (interstitial) away from indented regions during the compression of tissues was validated and explored through a water content analysis. Finally, a qualitative analysis of the microstructure of the stomach wall highlights a bending/folding and buckling deformation mode in the mucosal layer and residual compressive strain in the muscular layer. Overall, this study provides insights into the compressive viscoelastic behavior, microstructure, and water content of the stomach wall, which will help inform the control laws and algorithms needed to drive compressive surgical instruments in real time during surgical procedures.