In this paper, we report on the fabrication and characterization of a new hydrogel-based microvalve. The basic structure is a silicon membrane having an array of orifices with an internal structure designed to anchor the hydrogel while allowing it to gate the flow across the membrane. Each orifice (140 μm diameter) has a central post suspended by four tethers on each side of the membrane. A stimuli-sensitive hydrogel is polymerized inside each orifice. In the swollen state, the hydrogel completely occupies the void space of the orifice, completely blocking pressure-driven fluid flow. In the shrunken state, the hydrogel contracts around the post, allowing fluid to flow through an opened annular gap. Fabrication of the microstructured silicon membrane requires only two masking steps and involves a combination of deep trench and KOH etch. Two different hydrogels, based on N-isopropylacrylamide (temperature-sensitive) and phenylboronic acid (pH and glucose-sensitive) were trapped and tested in this microvalve. The measured response times were 10 s (temperature), 4 min (pH), and 10 min (glucose). The maximum pressure drop the microvalve can sustain before breakage of the hydrogel is 21 kPa and 16 kPa for temperature-sensitive and (pH/glucose)-sensitive hydrogels, respectively.
Bibliographical noteFunding Information:
Ronald A. Siegel is professor and head of the Department of Pharmaceutics, and professor of biomedical engineering at the University of Minnesota. He received his BS degree with honors in mathematics from the University of Oregon, in 1975, and his MS and ScD degrees in electrical engineering and computer science from the Massachusetts Institute of Technology, in 1979 and 1984, respectively. From 1984 to 1998, he was professor of biopharmaceutical sciences and pharmaceutical chemistry at the University of California at San Francisco, and has been at the University of Minnesota, since 1998. His research interests include drug delivery, polymer physical chemistry, microfabrication, membrane transport theory, and application of control theory in the optimization of drug therapy. Dr. Siegel received the Pfizer Young Investigator Grant Award from the American Association of Pharmaceutical Scientists (AAPS) in 1988, and the Young Investigator Award from the Controlled Release Society (CRS) in 1989. He was president of CRS during 1997–1998. In 1999, he was inducted as fellow of the American Institute of Medical and Biological Engineering (AIMBE), and as fellow of AAPS. He presently serves as book review editor of Journal of Controlled Release.
The authors thank the staff of the Nanofabrication Center (NFC) of the University of Minnesota for their assistance. Partial funding for this project was provided by Grant EB003125 from the National Institutes of Health, a grant-in-aid from the Biomedical Engineering Institute at the University of Minnesota, a fellowship from the Spanish Ministry of Education, Culture and Sports for A. Baldi, and a Samuel J. Melendy fellowship to Y. Gu.
Babak Ziaie received his doctoral degree in electrical engineering from the University of Michigan, in 1994. His dissertation was related to the design and development of an implantable single channel microstimulator for functional neuromuscular stimulation. From 1995 to1999, he was a postdoctoral fellow and an assistant research scientist at the Center for Integrated Microsystems (CIMS) of the University of Michigan. He subsequently joined the Electrical and Computer Engineering Department of the University of Minnesota, as an assistant professor (1999–2004). In 2005, he moved to the School of Electrical and Computer Engineering at Purdue University, where he currently is an associate professor. His research interests are mostly related to biomedical applications of MEMS and microsystems. These include implantable wireless microsystems for diagnosis and management of glaucoma, hydrogel-based microsystems for physiological sensing and active flow control, multi-channel wideband wireless interfaces for central nervous system (brain/machine interface), biomimetic structures, and ultra-sensitive sensors for biological (molecular and cellular) applications. Dr. Ziaie is the recipient of the NSF Career Award in Biomedical Engineering (2001) and McKnight Endowment Fund Award for Technological Innovations in Neuroscience (2002). Dr. Ziaie is a member of the IEEE, American Association for the Advancement of Science, and the American Physical Society.