Deep tissue injury (DTI) is caused by prolonged mechanical loading that disrupts blood flow and metabolic clearance. A patient simulator that mimics the biomechanical aspects of DTI initiation, stress and strain in deep muscle tissue, would be potentially useful as a training tool for pressure-relief techniques and testing platform for pressure-mitigating products. As a step toward this goal, this study evaluates the ability of silicone materials to mimic the distribution of stress in muscle tissue under concentrated loading.
To quantify the mechanical properties of candidate silicone materials, unconfined compression experiments were conducted on 3 silicone formulations (Ecoflex 0030, Ecoflex 0010, and Dragon Skin; Smooth-On, Inc, Easton, Pennsylvania). Results were fit to an Ogden hyperelastic material model, and the resulting shear moduli (G) were compared with published values for biological tissues. Indentation tests were then conducted on Ecoflex 0030 and porcine muscle to investigate silicone’s ability to mimic the nonuniform stress distribution muscle demonstrates under concentrated loading. Finite element models were created to quantify stresses throughout tissue depth. Finally, a preliminary patient simulator prototype was constructed, and both deep and superficial “tissue” pressures were recorded to examine stress distribution.
Indentation tests showed similar stress distribution trends in muscle and Ecoflex 0030, but stress magnitudes were higher in Ecoflex 0030 than in porcine muscle. All 3 silicone formulations demonstrated shear moduli within the range of published values for biological tissue. For the experimental conditions reported in this work, Ecoflex 0030 exhibited greater stiffness than porcine muscle.
Indentation tests and the prototype patient simulator trial demonstrated similar trends with high pressures closest to the bony prominence with decreasing magnitude toward the interfacial surface. Qualitatively, silicone mimicked the phenomenon observed in muscle of nonuniform stress under concentrated loading. Although shear moduli were within biological ranges, stress and stiffness values exceeded those of porcine muscle. This research represents a first step toward development of a preclinical model simulating the biomechanical conditions of stress and strain in deep muscle, since local biomechanical factors are acknowledged to play a role in DTI initiation. Future research is needed to refine the capacity of preclinical models to simulate biomechanical parameters in successive tissue layers of muscle, fat, dermis, and epidermis typically intervening between bone and support surfaces, for body regions at risk for DTI.
Jessica L. Sparks, PhD, is an Associate Professor of Chemical, Paper, and Biomedical Engineering, Miami University, Oxford, Ohio. Nicholas A. Vavalle, MS, is a doctoral candidate in biomedical engineering, Wake Forest University, Winston-Salem, North Carolina. Krysten E. Kasting is a bioengineering undergraduate student, Miami University, Oxford, Ohio. Benjamin Long, MS, is an Instructor of Physical Therapy, Winston-Salem State University, Winston-Salem, North Carolina. Martin L. Tanaka, PhD, is an Assistant Professor of Engineering and Technology, Western Carolina University, Cullowhee, North Carolina. Phillip A. Sanger, PhD, is a Professor of Electrical and Computer Engineering, Purdue University, West Lafayette, Indiana. Karen Schnell, MSN, owns Blue Sky Health Concepts Consulting, Mebane, North Carolina. Teresa A. Conner-Kerr, PhD, is Dean of the College of Health Sciences, University of North Georgia, Dahlonega, Georgia.
Dr Sparks and Mr Vavalle have disclosed that Wake Forest University is a past recipient of grant funding from the US Department of Education (awarded to T.A.C.-K.).
Mr Long has disclosed that Winston-Salem State University is a past recipient of grant funding from the US Department of Education (awarded to T.A.C.-K.).
Dr Sanger has disclosed that his institution is a past recipient of grant funding from the Golden Leaf Foundation.
Ms Schnell has disclosed that she has previously received an honorarium from Winston-Salem State University, and is a past recipient of payment for writing or reviewing a manuscript from Winston-Salem State University.
Dr Kerr has disclosed that Winston-Salem State University is a past recipient of a Title III US Department of Education grant.
Ms Kasting and Dr Tanaka have disclosed that they have no financial relationships related to this article.
Acknowledgments: The authors acknowledge Nick Ashworth, Isaac Crisp, Andrew York, and Erik Ellington for their assistance with data acquisition and graphical user interface development.
The authors also thank Kristen Pone, Christen Isley, and Peggy Furr, for their assistance with fiberglass casting and material acquisition. Funding for this research was provided by the US Department of Education (grant P031B085015-9 to T.A.C.-K.) and the Goldenleaf Fund (to P.A.S.).
Submitted July 5, 2013; accepted in revised form April 2, 2014.