A multi-indent approach to detect the surface of soft materials during nanoindentation
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Barbara L. McFarlin Department of Women Children and Family Health Science, University of Illinois College of Nursing, Chicago, IL 60612, United States
Amy J. Wagoner Johnsona) Department of Mechanical Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801, United States (Received 1 February 2016; accepted 24 June 2016)
There is growing interest in using instrumented indentation to characterize mechanical properties of soft materials, including tissue properties related to damage and disease. However, sample surface detection has been a major challenge. The multi-indent approach (MIA) is a novel method to indirectly detect the surface using data from multiple indents to determine the preload-induced indentation depth. Elastic modulus and shear modulus determined by MIA were equivalent to bulk measurements for 19 and 49 kPa gels. However, the traditional Oliver–Pharr approach significantly overestimated these properties. MIA is also important to accurate characterization of poroelastic properties and allows for much smaller probes and indentation depths to be used for all measurements. This is particularly important for poroelasticity, where the relaxation time scales with the size of the indenter. The novel approach helps to resolve the long-standing challenge of surface detection and has the potential to broaden the use of instrumented indentation for soft materials.
I. INTRODUCTION A. Instrumented indentation
Instrumented indentation is a technique with high force and displacement resolution, less than 1 nN and 1 nm, respectively,1–4 that is used to measure the local mechanical properties of a material.5–7 The technique monitors the real-time load and indentation depth of a probe with well-defined geometry and size, and uses these data combined with models to obtain material properties.2,8 Probes with radius less than 10 lm have been used to characterize harder biological materials, such as cartilage and bone, using instrumented indentation.6,9 Soft gels with modulus ranging from 10 to 100 kPa have been characterized with probe sizes ranging from 100 lm to 1 mm and conical spherical or spherical-shaped probes.2,3,10–12 Instrumented indentation is also increasingly recognized as an efficient way of understanding the biological process of diseased and/or damaged tissues, which are typically soft and heterogeneous on the scale of 10’s to 100’s of micrometers.2,6 It has been used with tissues like Contributing Editor: George Pharr a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2016.265
bone and cartilage,13–15 with fewer studies on softer tissues with modulus less than 1 MPa, in part due to problems with surface detection.6,16 Yao et al. characterized the shear modulus and poroelastic properties of human cervix17; Slaboch et al. characterized rat thrombus relaxation behavior with indentation18; Chhetri et al. studied elastic properties of vocal folds3; and Gupta et al. characterized the porcine cartilage time-independent properties.14 The incr
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