Biomed Microdevices (2009) 11:421–427 DOI 10.1007/s10544-008-9248-6 Elastic and viscoelastic characterization of microcapsules for drug delivery using a force-feedback MEMS microgripper Keekyoung Kim · Xinyu Liu · Yong Zhang · Ji Cheng · Xiao Yu Wu · Yu Sun Published online: 18 November 2008 © Springer Science + Business Media, LLC 2008 Abstract This paper reports a monolithic, force- feedback MEMS (microelectomechanical systems) microgripper and its application to micro-scale com- pression testing of swollen hydrogel microcapsules at wet state during manipulation. The single-chip micro- gripper integrates an electrothermal microactuator and two capacitive force sensors, one for contact detection (force resolution: 38.5 nN) and the other for gripping force measurements (force resolution: 19.9 nN). With the capability of resolving gripping forces down to 19.9 nN and material deformations with a 20.5 nm res- olution, the system quantified Young’s modulus values and viscoelastic parameters of alginate microcapsules (15–25 μm), demonstrating an easy-to-operate, accu- rate compression testing technique for characterizing soft, micrometer-sized biomaterials. Keywords MEMS microgripper · Micro-scale compression testing · Hydrogel microcapsule · Young’s modulus · Viscoelastic parameters K. Kim · X. Liu · Y. Zhang · Y. Sun (B ) Advanced Micro and Nanosystems Laboratory, University of Toronto, 5 King’s College Road, Toronto, Ontario M5S 3G8, Canada e-mail: sun@mie.utoronto.ca J. Cheng · X. Yu Wu Leslie Dan Faculty of Pharmacy, University of Toronto, 144 College Street, Toronto, Ontario M5S 3M2, Canada 1 Introduction Hydrogel microcapsules used for drug delivery or cell encapsulation are highly deformable with diameters from 1 μm to 100 μm, comparable to the size of most biological cells. The mechanical properties of mi- crocapsules determine whether they can survive the stress in the needle tract during injection, in the blood capillaries, or in the applied tissues. Maintaining their integrity during processing and application is essential for preventing dose dumping, cell death and/or im- munoresponse as well as for achieving desired per- formance. To mechanically characterize soft hydrogel microparticles, a system capable of accurately mea- suring low-magnitude forces and microscopic material deformations is required. Different from local probing techniques such as op- tical tweezers (Fontes et al. 2008), micropipette as- piration (Hochmuth 2000), atomic force microscopy (AFM) (Dulinska et al. 2006), and magnetic bead mea- surement (Bausch et al. 1999), micro-scale compres- sion testing permits the quantification of mechanical parameters globally. Micro-scale compression using a commercial force transducer with an assembled opti- cal fiber probe was demonstrated for investigating the bursting forces of single tomato cells (Blewett et al. 2000). AFM tip with an assembled colloid sphere has also been reported as a micro-scale compression tool for determining Young’s modulus of polyelectrolyte microcapsules (Lulevich and Vinogradova 2004). In the pursuit of monolithic devices for mechanical characterization of biomaterials, many MEMS-based force sensors have been developed. MEMS piezore- sistive force sensors were demonstrated for the mea- surement of contractile forces of individual heart cells