JOURNAL OF MICROELECTROMECHANICAL SYSTEMS 1 Electrothermally Activated SU-8 Microgripper for Single Cell Manipulation in Solution Nikolas Chronis and Luke P. Lee Abstract—The development of a SU-8-based microgripper that can operate in physiological ionic solutions is presented. The electrothermally activated polymer gripper consists of two “hot- and-cold-arm” actuators that are fabricated in a two-mask surface micromachining process. The high thermal expansion coefficient of SU-8 (52 ) compared to silicon and metals, allows the actuation of the microgripper with small average temperature elevations (10 – 32 ) at low voltages (1–2 V). The polymer microgripper can be used for the manipulation of single cells and other biological species in solution with minimal undesired interactions. [1330] Index Terms—Polymer MEMS, polymer microgripper, single cell manipulation, SU-8 actuators. I. INTRODUCTION T HE development of miniaturized systems for manipulating biological samples in solution has become a great tech- nological challenge for the future of genomics and proteomics [1]. Current biomanipulation tools such as optical tweezers [2] and micropipetes [3], although powerful, rely on bulky and ex- pensive setups. Alternatively, the use of microgrippers as the mechanical end-effectors that securely grasp and transport the micro object to the desired location seems to be a promising ap- proach, offering robustness and manipulation flexibility without interfering optically or electrically with the sample. Microgrip- pers capable of being activated in an ionic environment can be used for single cell manipulation and positioning, cell isola- tion, as well as for minimally invasive endoscopic operations. At the same time, biocompatibility issues of the mechanism that converts energy into mechanical motion need to be taken into account. Most of the previously developed microgrippers cannot be operated in physiological solutions because the actuation mech- anism is not compatible with liquid operation. Electrostatic grippers [4], [5] cannot be stimulated in electrolytic media. Thermally driven grippers [6]–[9] operate at extremely high temperatures and high voltages (bubble formation by electrol- ysis typically occurs at in water [10]). Piezoelectric grippers [11] are also activated at high voltages and produce small displacements often requiring multilayer actuators or am- plification mechanisms. Shape memory alloys-based grippers [12], [13] could potentially be used in liquid environment, but they can cycle only a few times before complete immobilization. Manuscript received May 5, 2004; revised October 28, 2004. This work was supported by DARPA under the BioFlips program. Subject Editor N. de Rooij. The authors are with the Berkeley Sensor and Actuator Center, Department of Bioengineering, University of California, Berkeley, CA 94720 USA (e-mail: lplee@socrates.berkeley.edu). Digital Object Identifier 10.1109/JMEMS.2005.845445 In pneumatically driven microgrippers [14], the actuation mechanism is not integrated with the gripper, posing an ad- ditional level of complexity in the design and fabrication of such structures. Pneumatically driven microcages [15], [16], thermally activated polymer-based actuators [17], [18] and electroactive polymer microarms based on ionic absorption and swelling [19], [20], can operate in aqueous solutions but have limited actuation control and are restricted to out of plane motion. This makes it difficult to manipulate and actively position small (5 – 40 in diameter) cells. In contrast to previous works, our approach is based on the de- velopment of a SU-8 microgripper with integrated, electrother- mally activated, in plane, SU-8 actuators. Taking advantage of the structural rigidity, the chemical resistance, as well as the ability to define high aspect ratio structures on SU-8 films, we fabricated a SU-8 microgripper that can operate in both liquid and air environments. A critical element in our design is the large coefficient of thermal expansion of SU-8 that allows the electrothermal activation of the microgripper in ionic physio- logical solutions at low voltages and temperature increases. The proposed actuation mechanism enables the manipulation and positioning of single cells or other biological species with min- imal undesired interactions. This paper demonstrates for the first time the successful manipulation of a 10- in diameter single cell in solution using a MEMS microgripper. II. MICROGRIPPER DESIGN SU-8 is a highly crosslinked epoxy-type photo-pat- ternable polymer with a coefficient of thermal expansion (CTE) of 52 [21], relatively large elastic modulus ( ) [22], [23], and glass transition tempera- ture above 200 [24]. These properties make SU-8 a great material for building rigid mechanical structures for various applications. Recent studies on the growth of human cells on SU-8 substrates [25] also revealed good biocompatibility, making SU-8 useful in a variety of in-vitro BioMEMS appli- cations [26], [27]. However, the long-term stability of SU-8 in cell culture media [28] and its hemocompatibility for in-vivo applications [29] is still questionable. On the other hand, SU-8 is a nonconductive polymer. Developing SU-8-based actuators becomes an extremely challenging task, due to the difficulty of implementing and integrating a reliable actuation mechanism. Based on these features and especially on the high CTE of SU-8 and the high aspect ratio characteristics of SU-8 films, we developed an innovative approach for constructing an elec- trothermal SU-8 microgripper integrated with in plane SU-8 actuators (Fig. 1). The key point is the addition of a thin metal resistor patterned at the bottom of the SU-8 structure. 1057-7157/$20.00 © 2005 IEEE