pubs.acs.org/Langmuir © 2009 American Chemical Society
A Method for Heat-Stimulated Compression of Poly(N-isopropyl acrylamide) Hydrogels Inside Single Giant Unilamellar Vesicles Lidiya Osinkina, Martin Markstr€om, Owe Orwar, and Aldo Jesorka* Department of Chemical and Biological Engineering, Chalmers University of Technology, SE-41296 G€ oteborg, Sweden Received April 6, 2009 In this letter, a hydrogel compression method for the facile generation of high polymer concentration within single giant unilamelar vesicles (GUV) is presented. A GUV with an internalized poly(N-isopropyl acrylamide) solution is heated above the lower critical solution temperature (LCST) until compartmentalization by collapsed hydrogel microaggregates occurs. Subsequently, the volume of the vesicle is decreased by reducing the area of the enclosing membrane. An increase in concentration by a factor of up to 11 is reached while the membrane remains intact, bringing the model system close to the living cell situation with its high concentration of proteins in the cytoplasm. The polymerfilled vesicles are also in a suitable density range to significantly improve the flow control performance of hydrogel valves embedded in soft matter nanofluidic devices.
Introduction Model systems and devices based on hydrogels and selfassembled lipid bilayers are popular both in theoretical studies and in emerging practical applications, such as nanofluidic systems and artificial cell models. For example, the concentration of biomacromolecules, mainly proteins, in the cytoplasm solute can be up to 300 mg/mL (macromolecular crowding).1 To render hydrogel/GUV composite models more similar to the biological cell (i.e., to reach the range of molecular crowding densities that is required for experimental work), the macromolecule concentration inside a vesicle container has to be increased by simple means. Polymers that are water-soluble below their lower critical solution temperature (LCST) and form insoluble but water-rich gel phases above it are particularly suitable for that purpose. The attractive feature of these systems is the possibility to control and modify them by changing the temperature.2-5 This has previously been exploited to create a soft-matter nanofluidic device with an incorporated thermoactuated hydrogel valve.6 Such a device consists of microsized GUV containers, interconnected by flexible lipid nanotubes. It is fabricated by a combination of spontaneous phospholipid self-assembly and micromanipulation-injection procedures.7 Lipid molecules selforganize in physiological buffer solution (PBS) into bilayers that in turn form, under certain conditions, giant unilamellar vesicles (GUV). These vesicles can be envisioned as containers or biomimetic microreactors. In the fabrication concept, multilamellar (or onion shell) vesicles are physically attached to GUVs, supplying them with lipid material during network assembly. By means of a micromanipulation setup with a thin borosilicate needle, lipid nanotubes are pulled out of GUVs under the application of a (1) Zimmerman, S. B.; Trach, S. O. J. Mol. Biol. 1991, 222, 599–620. (2) Long, M. S.; Jones, C. D.; Helfrich, M. R.; Mangeney-Slavin, L. K.; Keating, C. D. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 5920–5925. (3) Jesorka, A.; Markstrom, M.; Orwar, O. Langmuir 2005, 21, 1230–1237. (4) Campillo, C.; Pepin-Donat, B.; Viallat, A. Soft Matter 2007, 3, 1421–1427. (5) Campillo, S. Soft Matter 2008, 4, 2486–2491. (6) Markstrom, M.; Lizana, L.; Orwar, O.; Jesorka, A. Langmuir 2008, 24, 5166–5171. (7) Karlsson, A.; Karlsson, M.; Karlsson, R.; Sott, K.; Lundqvist, A.; Tokarz, M.; Orwar, O. Anal. Chem. 2003, 75, 2529–2537.
Langmuir 2010, 26(1), 1–4
quasi-point force on the membrane. High structural complexity can be reached.8 In the nanofluidic device reported in ref 6, the thermosensitive polymer PNIPAAm, which has its LCST at 32 °C in pure water, is microinjected into such a vesicle’s interior and acts as a valve in response to the temperature change. In this application, the high salt concentration of the utilized buffer solution noticeably decreases the LCST. When the PNIPAAmcontaining vesicle is heated, the encapsulated polymer undergoes a reversible sol-to-gel phase transition, which arises from increased hydrophobicity of the polymer molecules, and eventually forms a physically cross-linked hydrogel. Upon prolonged temperature increase, the gel shrinks further inside the vesicle, dividing it into two microcompartments—a densely aggregated polymer phase and pure PBS. According to cloud-point measurements,9 the concentration of polymer necessary to achieve complete demixing is
