A simple microindentation technique for mapping the microscale compliance of soft hydrated materials and tissues Jeffrey G. Jacot, Scott Dianis, Joshua Schnall, Joyce Y. Wong Department of Biomedical Engineering, Boston University, Boston, Massachusetts 02215 Received 23 June 2005; revised 21 January 2006; accepted 1 March 2006 Published online 15 June 2006 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/jbm.a.30812 Abstract: Several recent studies have shown that cells re- spond to the elastic modulus and elasticity gradients on soft substrates. However, traditional macroscale methods for measuring elastic modulus cannot resolve elastic gradients or differences between the macroscale and microscale elastic modulus of layered tissues. Here, we present a technique for measurement of the microscale elastic modulus of soft, hy- drated gels and tissues. This technique requires less equip- ment than equivalent atomic force microscopy (AFM) and can easily measure larger samples with high adhesiveness. We validate this technique by measuring the microscale modulus of a hydrogel with elasticity that does not depend on measurement scale. We show that the elastic modulus measured using microindentation correlates with measure- ments using AFM and the macroscale tensile modulus. We verified the ability of this technique to characterize a hydro- gel with an elastic gradient of 2.2 kPa/mm across 19 mm and to measure the microscale elastic modulus of the endo- thelial side of human greater saphenous vein, which is an order of magnitude less than the whole vein macroscale modulus. This simple, inexpensive system allows the mea- surement of the spatial organization of microscale elastic properties of fully hydrated, soft gels and tissues as a rou- tine laboratory technique. © 2006 Wiley Periodicals, Inc. J Biomed Mater Res 79A: 485– 494, 2006 Key words: microindentation; polyacrylamide; greater sa- phenous vein; compressive modulus; tensile modulus; elas- tic gradient INTRODUCTION Recent advances in microfabrication have allowed the production of soft, thin materials with mechanical properties that vary over length scales on the order of micrometers. These materials have been used in stud- ies probing cell behavior and have demonstrated that variations in substrate mechanical properties influ- ence a wide variety of cellular phenomena, including cell spreading, 1,2 cell proliferation, 3 neuronal branch- ing, 4 and cell differentiation in angiogenesis. 5 In addi- tion, mechanical gradients engineered into substrata and scaffolds have recently been shown to direct cell migration. 6–8 Patterns in the mechanical compliance of tissue-engineered scaffolds could conceivably drive cellular accumulation, orientation, proliferation, and differentiation. 9 However, the development of quan- titative relationships between mechanical compliance and cell behavior is limited by the effectiveness of the characterization of elastic properties on a length scale relevant to cells. In vivo, gradients in the microstructure of biological systems commonly form in regions that experience the greatest stress, 10 but few studies are able to measure the magnitude or effects of these gradients. Further- more, the modulus of specific blood vessel layers can be much different at the microscale when compared with macroscale, 1 and elastic modulus of collagen-rich biological systems can also appear much different at the nanoscale when compared with the microscale. 11 An understanding of the properties of soft tissues, on all scales, can aid researchers in quantitatively charac- terizing local elasticity on the cell-to-tissue scale and could elucidate relationships between tissue mechan- ics and pathologic response. Many studies on the effect of the elasticity of a biomaterial on a biological response have used mac- roscopic tensile techniques of hanging weights on a sample of material, 2,6,12,13 which are unable to resolve Correspondence to: J.Y. Wong; e-mail: jywong@bu.edu Contract grant sponsor: The National Institute of Health; contract grant number: R01 HL72900 – 01 Contract grant sponsor: The Whitaker Foundation; con- tract grant number: RG-98 – 0506 Contract grant sponsor: NASA; contract grant number: NAG9 –1558 © 2006 Wiley Periodicals, Inc.