Chapter 14 Custom Indentation System for Mechanical Characterization of Soft Matter Chelsey Simmons, Andres Rubiano, Daniel Stewart, and Brandey Andersen Abstract Soft, hydrated samples of biomaterials and live tissues are hard to characterize with standard equipment designed for engineering materials. To accommodate millimeter-scale samples with sub-kPa moduli, we designed a cantilever-based indentation system using piezoelectric actuators and capacitive sensors compatible with a variety of probe tips and sample holders. Embryonic mouse hearts, rat gastrointestinal tissues, human pancreas, and rat brain tissue have all been successfully characterized using this system. Keywords Experimental apparatus • Tissue mechanics • Viscoelasticity • Indentation • Compression testing 14.1 Introduction Common techniques to characterize soft matter include macroscale compression and tension of centimeter-scale tissue samples and nanoscale indentation by atomic force microscopy or nanoindentation instruments. However, the wide variety in indentation methods and material models obfuscate comparisons among the data and do not accommodate millimeterscale samples like embryonic mouse hearts or excised human tumors. To characterize the material properties of a variety of biomaterials and isolated tissues, we designed a cantilever-based indentation system for soft matter. We then fit the indentation data to the common standard linear solid viscoelastic material model using a modified Hertz contact model, which allows us to calculate a steady-state elastic modulus (Ess) and viscosity, as well as an instantaneous (Ea) and a total elastic modulus for comparison to existing data in the literature. We have used this system to experimentally characterize synthetic soft matter, biomaterials, and excised tissue. 14.2 Methods Force-Displacement Quantification with Custom Indenter. We modified a tool designed to measure friction forces to create a custom Multi-Scale Indenter (MSI, Fig. 14.1) [1, 2]. The cantilever-based probe (Fig. 14.1b2) is displaced vertically using a software-controlled piezoelectric stage (Fig. 14.1b1, P-628.1CD, Physik Instrumente). A custom program in LabVIEW (National Instruments) was used to control indentation profile and to read deflection of cantilever tip with capacitive sensor (Fig. 14.1b3, C8S-3.2-2.0 and compact driver CD1-CD6, Lion Precision) through a data acquisition card system (NI 9220 and cDAQ-9171, National Instruments). The probe with a 4 mm-diameter rigid tip (Fig. 14.1b4) is brought C. Simmons (*) Department of Mechanical and Aerospace Engineering, University of Florida, PO Box 116250, Gainesville, FL, USA J. Crayton Pruitt Family Department of Biomedical Engineering, Herbert Wertheim College of Engineering, University of Florida, PO Box 116250, Gainesville, FL, USA Division of Cardiovascular Medicine, Department of Medicine, College of Medicine, University of Florida, PO Box 116250, Gainesville, FL, USA e-mail: css@ufl.edu A. Rubiano Department of Mechanical and Aerospace Engineering, University of Florida, PO Box 116250, Gainesville, FL, USA D. Stewart • B. Andersen J. Crayton Pruitt Family Department of Biomedical Engineering, Herbert Wertheim College of Engineering, University of Florida, PO Box 116250, Gainesville, FL, USA #The Society for Experimental Mechanics, Inc. 2017 C.S. Korach et al. (eds.), Mechanics of Biological Systems and Materials, Volume 6, Conference Proceedings of the Society for Experimental Mechanics Series, DOI 10.1007/978-3-319-41351-8_14 95
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