Fibroblast Morphology on Dynamic Softening of Hydrogels MICHELLE L. PREVITERA, 1 KEVIN L. TROUT, 2 DEVENDRA VERMA, 1 UDAY CHIPPADA, 3 RENE S. SCHLOSS, 1 and NOSHIR A. LANGRANA 1,3 1 Department of Biomedical Engineering, Rutgers University, 599 Taylor Rd., Piscataway, NJ 08854, USA; 2 Department of Chemistry and Biochemistry, The College of St. Scholastica, 1200 Kenwood Ave., Duluth, MN 55811, USA; and 3 Department of Mechanical Engineering and Aerospace, Rutgers University, 599 Taylor Rd., Piscataway, NJ 08854, USA (Received 26 September 2011; accepted 28 November 2011; published online 8 December 2011) Associate Editor Jennifer West oversaw the review of this article. Abstract—Despite cellular environments having dynamic characteristics, many laboratories utilized static polyacryl- amide hydrogels to study the ECM–cell relationship. To attain a more in vivo like environment, we have developed a dynamic, DNA-crosslinked hydrogel (DNA gel). Through the controlled delivery of DNA, we can temporally decrease or increase gel stiffness while expanding or contracting the gel, respectively. These dual mechanical changes make DNA gels a cell–ECM model for studying dynamic mechano- regulated processes, such as wound healing. Here, we characterized DNA gels on a mechanical and cellular level. In contrast to our previous publication, in which we examined the increasing stiffness effects on fibroblast mor- phology, we examined the effects of decreased matrix stiffness on fibroblast morphology. In addition, we quantified the bulk and/or local stress and strain properties of dynamic gels. Gels generated about 0.5 Pa stress and about 6–11% strain upon softening to generate larger and more circular fibroblasts. These results complemented our previous study, where dynamic gels contracted upon stiffening to generate smaller and longer fibroblasts. In conclusion, we developed a biomaterial that increases and decreases in stiffness while contracting and expanding, respectively. We found that the dynamic deformation directionality of the matrix determined the fibroblast morphology and possibly influences function. Keywords—Stress, Strain, ECM, Compliance, Stiffness, DNA gel, Wound healing. INTRODUCTION The cellular microenvironment consists of dyna- mic interactions between the extracellular matrix (ECM) and cells. 3,25,29,34,46 Cells sense and react to various ECM stimuli, including chemical and physi- cal. 27–29,39,44,45 ECM–cell models composed of PDMS, 4 polyacrylamide, 26,39,44,45 collagen, 51 algi- nate, 19,35 Matrigel, 58 and other materials have been used to examine the influences of mechanical cues on the cell’s behavior and physiology. Previous in vitro studies, including those in our laboratory, have used static matrices (where the mechanical properties remain unchanged throughout the culture period) to examine the effects of matrix stiffness on neuron and fibroblast morphology. 11,26,27,39,42,44,45,57 Currently, the effects of matrix mechanics on cells are being studied in vivo. 7,30 Nonetheless, in vivo the ECM is dynamic and is altered due to pathological, develop- mental, and external factors. 22 Cells respond differ- ently to the static and dynamic mechanical cues 28,29 and thus, investigators need to focus their evaluation of cell behavior on dynamic cell–ECM models. We have constructed a DNA-crosslinked hydrogel (DNA gel) as a dynamic microenvironment model. 28,29 DNA gels reversibly increases and/or decreases in ECM stiffness on the fly. 27–29,36,37 DNA gel matrix mechanics are controlled through minimally invasive DNA delivery instead of changing temperature, adjusting pH, applying electric fields, or creating any other modifications that would inhibit in vivo appli- cations. Most importantly, DNA gel design parame- ters can be controlled to mimic the stiffness of biological tissues. 6 This stringent control over design parameters and stiffness makes DNA gels a promising tool for applications in tissue engineering and cell– ECM modeling. The study of in vivo processes using a mimetic material requires a matrix with dual and dynamic mechanical properties. Wound healing is one dynamic process, in which ECM stiffness modulation 12,52 and tissue strengthening 48,53 occur from collagen turnover Address correspondence to Noshir A. Langrana, Department of Mechanical Engineering and Aerospace, Rutgers University, 599 Taylor Rd., Piscataway, NJ 08854, USA. Electronic mail: Langrana@rutgers.edu Annals of Biomedical Engineering, Vol. 40, No. 5, May 2012 (Ó 2011) pp. 1061–1072 DOI: 10.1007/s10439-011-0483-2 0090-6964/12/0500-1061/0 Ó 2011 Biomedical Engineering Society 1061