2015 IR&D Annual Report

Dynamic Characterization of Soft Biological Tissues, 18-R8549

Principal Investigators
Daniel P. Nicolella
Sydney Chocron
Art Nicholls

Inclusive Dates: 04/01/15 – 11/01/15

Background — Human injury occurs as a result of relatively high-loading-rate events such as falls, automobile accidents, sporting injuries, and blunt and ballistic impact to warfighters. However, the vast majority of human tissue material property data is collected at low-loading rates. Since almost all biological materials are strain-rate dependent, there is a significant gap in our ability to accurately predict injury and therefore design injury mitigation strategies and equipment. Furthermore, very little is actually known about human tissue injury mechanisms and the effect of aging and disease on human injury risk. High-rate dynamic testing of soft biological materials presents significant challenges. SwRI is uniquely positioned to directly address these challenges with the application of a combination of a unique set of technologies and capabilities.

Approach — The objectives of the project are to: 1) develop a methodology to characterize soft biological tissues in tension at moderately high strain rates using a combination of SHPB tension experiments, 3D optical strain measurement, direct force measurement, and numerical modeling, 2) characterize the behavior of select soft biological materials as a function of age and disease over a range of strain rates, and 3) validate the new technique by comparing data collected using an independent test method (traditional materials test machine) at overlapping strain rates and finite element simulations.

Accomplishments — A momentum trap has been designed, manufactured, and implemented on the SwRI tensile SHPB, and a series of preliminary experiments was performed. The momentum trap will enable the intensity of the incident wave on the test specimen to be decreased such that stress equilibrium is achieved prior to specimen failure. The use of a momentum trap will also eliminate the interference of the reflected wave with the incident wave when using the long incident pulses required for use on biological materials that can stretch up to 100-percent strain.

An environmentally controlled test chamber was designed and constructed to provide test environment similar to in vivo conditions. The system includes a rectangular plexiglass chamber, heating element, humidity element, and a temperature and humidity controller. The chamber was designed such that it can be installed on the Bose® ElectroForce test machine as well as the Split Hopkinson Pressure Bar (SPBH) test system. All future soft tissue testing using the Bose and SHPB will be conducted at 370C and >90% humidity.

A total of 29 SSL tests have been performed to date using the quasi-static test machine (Bose ElectroForce 3400). The ligament testing was split into two rounds: the initial round used full thickness lumbar supraspinous ligament (SSL) segments. The ligaments were tested at room temperature at strain rates ranging from 0.01/sec to more than 100/sec. The second round of testing used only the dorsal portion of the SSL and the ligaments were either tested at room temperature and humidity or simulated body conditions within the environmental chamber (370C and >90% humidity) at strain rates ranging from 0.01/sec to more than 100/sec.

Figure 1: Strain rate and temperature effects on the strength of the supraspinous ligament.
Figure 1: Strain rate and temperature effects on the strength of the supraspinous ligament.
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Southwest Research Institute® (SwRI®), headquartered in San Antonio, Texas, is a multidisciplinary, independent, nonprofit, applied engineering and physical sciences research and development organization with 9 technical divisions.