Smart hydrogel films produced by electrochemically induced polymerization
Smart Hydrogels
Hydrogels are cross-linked polymer networks swollen in water. Smart hydrogels undergo a swelling-deswelling transition in response to certain stimuli such as a change in temperature or pH. Prominent examples of smart hydrogels are polyacrylic acid (pH-sensitive, Fig. 1A) and poly-N-isopropylacrylamide (pNIPAm, temperature-sensitive, Fig. 1B) with a lower critical solution temperature (LCST) at 32°C[1].
Thin films of smart hydrogels have many applications in sensing[2], microfluidics[3], and life science[4]. In tissue engineering, pNIPAm coatings were used to produce cell sheets (see Fig. 2)[5]: Okano and co-workers grew cells on pNIPAm at 37°C where the film was hydrophobic. They detached the cell sheets by cooling the sample temperature to below 32°C where pNIPAm became hydrophilic. In this context, patterned films of hydrogels with different LCST open up a versatile route to produce two-dimensionally structured implants.
Electrochemically Induced Polymerization
Electrochemically induced polymerization (EIP)[6-10] is an easy and flexible technique to produce functional coatings on conducting substrates. The method makes use of the decomposition of an electro-active initiator (e.g. potassium persulfate[11]) at a positive or at a negative potential (see Fig. 3). The initiator decomposes at the electrode surface and initiates a free-radical polymerization. A polymer coating is formed at the surface. Adhesion is based on strong physisorption of the polymer to the metal.
An electrochemical quartz crystal microbalance (EQCM)[12] is well-suited to investigate EIP in-situ (see Fig. 4). The front electrode of a quartz crystal is used as the working electrode in a three-electrode set-up. The frequency shift of the quartz crystal gives information on the deposited mass and the bandwidth shift yields information on the film’s softness. Simultaneously, the current density is detected.
Current Research Activities
Multi-stimuli responsive hydrogel films
- impact of reaction conditions on film properties
- encapsulation of specific receptors for (bio-)sensing
Patterned Hydrogel Films
- patterned films by electrochemically induced polymerization in a lithographic mode
- investigation of the deposition mechanism for optimal control of structure formation
- production of spots
Methods
- (electrochemical) quartz crystal microbalance
- (cyclic) voltammetry
- atomic force microscopy
- Fourier transform infrared spectroscopy
Literature
[1] H. G. Schild, Prog. Polym. Sci. 1992, 17, 163.
[2] J. Heo, R. M. Crooks, Anal. Chem. 2005, 77, 6843.
[3] M. E. Harmon, M. Tang, C.W. Frank, Polymer 2003, 44, 4547.
[4] N. A. Peppas, J. Z. Hilt, A. Khademhosseini, R. Langer, Adv. Mater. 2006, 18, 1345.
[5] A. Kikuchi, T. Okano, J. Control. Release 2005, 101, 69.
[6] C. Baute, et al., Eur. J. Inorg. Chem. 2001, 1097.
[7] C. S. Lee, J. P. Bell, J. Mater. Sci. 1995, 30, 3827.
[8] S. L. Cram, G. M. Spinks, G. G. Wallace, H. R. Brown, J. Appl. Polym. Sci. 2003, 87, 765.
[9] G. Yildiz, H. Catalgil-Giz, F. Kadirgan, J. Appl. Electrochem. 2000, 30, 71.
[10] J. Reuber, H. Reinhardt, D. Johannsmann, Langmuir 2006, 22, 3362.
[11] Z. Samec, K. Doblhofer, J. Electroanal. Chem. 1994, 367, 141.
[12] D. A. Buttry, M. D. Ward, Chem. Rev. 1992, 92, 1355.
Publications
- Production of Polyacrylic Acid Homo- and Copolymer Films by Electrochemically Induced Free-Radical Polymerization: Preparation and Swelling Behavior
Johanna Bünsow, Diethelm Johannsmann, Macromolecular Symposia, accepted.
Johanna Reuber, Helke Reinhardt, Diethelm Johannsmann, Langmuir 2006, 22, 3362.
Financial Support
This project is funded by the Deutsche Forschungsgemeinschaft (DFG); project number JO 278/12-1.
The project is carried out by Johanna Bünsow.
johanna.buensow@tu-clausthal.de