Elsevier

Polymer

Volume 44, Issue 16, July 2003, Pages 4455-4462
Polymer

Photo cross-linkable poly(N-isopropylacrylamide) copolymers III: micro-fabricated temperature responsive hydrogels

https://doi.org/10.1016/S0032-3861(03)00413-0Get rights and content

Abstract

Micro-fabricated temperature responsive poly(N-isopropylacrylamide) (PNIPAAm) hydrogels were produced by photolithographic patterning of photo cross-linkable polymers. These polymers were synthesized by copolymerization of N-isopropylacrylamide (NIPAAm) and 2-(dimethyl maleimido)-N-ethyl-acrylamide (DMIAAm). The patterning process of polymers with 9.2 mol% DMIAAm and film thickness below 5 μm in the dry state was able to depict a lateral resolution of 4 μm with insignificant shape change. In order to increase the adhesion of the swollen hydrogels, and thus, the resolution of a particular pattern, a special adhesion promoter based on a monochlorosilane anchor group and a chromophore head group was synthesized. If a silicon wafer surface was pretreated with the adhesion promoter, the structures were stable and well adhered even at lower cross-linking densities. The hydrogels are suitable as working substances for micro-actuators because of their thermally induced volume changes. The swelling ratio of the pattern at low temperatures increased with a decreased cross-linking density. As expected from the chemical composition of the gels, the phase transition temperature (Tc) decreased with increasing DMIAAm content. The swelling of microstructures in water in comparison to macroscopic objects occured significantly faster. This behavior was attributed to the small gel dimension but it was even more pronounced because of the sponge-like nanostructure of the hydrogels characterized by high-resolution field emission scanning electron microscopy. Suitable applications of these hydrogels are adjusting limbs in fluid micro-systems such as micro-pumps and micro-valves.

Introduction

Cross-linked poly(N-isopropylacrylamide) (PNIPAAm) is a well-known representative of those polymer networks, which show lower critical solution temperature (LCST) behavior. PNIPAAm gels undergo a phase transition in aqueous media at around 32–34 °C [2]. Those hydrogels are promising candidates for the development of micro-actuators because of their thermally induced volume changes [3], [4]. At low temperatures the gel is in a highly swollen state containing more than 95 wt% of water. Above the phase transition temperature, the polymer network chains collapse. This property results in a shrunken volume state of a polymer network at elevated temperatures. Networks based on such material can be used for chemo-mechanical systems, i.e. systems transforming chemical into mechanical energy. A drawback of classical hydrogels, typically gels with mm-dimensions, is their slow response behavior to an applied stimulus. Several attempts have been made to decrease the swelling/deswelling times for such gels by changing the chemical and/or the physical gel structure [5], [6], [7], [8].

Since the kinetics of swelling and deswelling are proportional to the square of the smallest gel dimension, reduction of the gel size to the μm-scale should be very effective in decreasing the response time [9], [10], [11]. For applications of such small hydrogels, e.g. in micro-systems, the functional material has to be separated on or in appropriate microstructures. A promising method for the micro-fabrication of hydrogels is photolithography. Patterned hydrogel structures can be obtained starting from monomers and end functionalized oligomers by using a photo polymerization process [12], [13], [14], [15], [16]. The adhesion of the gels to the support can be enhanced, if coupling agents are used to attach micro-fabricated hydrogel patterns on silicon dioxide surfaces [14], [17], [18]. Recently, the photolithographic synthesis of smart hydrogels from photo cross-linkable polymers has been published [19], [20], [21], [22], [23], [24].

The cross-linking reaction has been performed typically by photodimerization of hydrophobic chromophores. However, when incorporated in a temperature-sensitive polymer, these substituents produce a strong decrease in the phase transition temperature (Tc). Due to the size and the polar structure of the dimethylmaleimide group, temperature-sensitive polymers and gels with high chromophore contents and Tc values higher than room temperature (RT) are accessible [1], [25], [28]. Furthermore, it should be possible to use the same cycloreaction to chemically bind the hydrogels onto a surface.

In this report the micro-fabrication of temperature responsive PNIPAAm hydrogels as well as their swelling behavior is described. Suitable applications of these hydrogels are adjusting limbs in fluid micro-systems such as micro-pumps and micro-valves.

Section snippets

Materials

N-Isopropylacrylamide (NIPAAm, Aldrich) was purified by recrystallization from hexane and dried in vacuum. 2,2′-Azobis(isobutyronitrile) (AIBN) was recrystallized from methanol. Dioxane, tetrahydrofuran (THF), and diethylether were distilled over potassium hydroxide. All other reagents were of analytical grade.

Synthesis of 2-(dimethyl maleimido)-N-ethyl-acrylamide (DMIAAm)

The DMIAAm monomer was prepared according to the literature [25].

1-Allyl-dimethyl-maleimide (2)

5.0 g (39.6 mmol) Dimethylmaleic anhydride and 11.3 g (198 mmol) allylamine were dissolved in 50 ml toluene. The mixture

Copolymer synthesis and characterization

Copolymers of NIPAAm and DMIAAm can easily be prepared by free radical polymerization in dioxane using AIBN as initiator at 70 °C. The molecular weight of the copolymers was about 60,000 g/mol. With increasing DMIAAm content the molecular weight of the polymer decreased [26]. Aqueous solutions of the copolymers showed LCST behavior. The phase transition temperatures of the aqueous solutions of these copolymers decreased with increasing comonomer content and disappeared at DMIAAm contents at or

Conclusion

The photolithographic patterning of PNIPAAm photo polymers is a suitable way to prepare micro-fabricated thermo-responsive hydrogels. The patterning results obtained on a HMDS pretreated Si-wafer were sufficient for the proposed applications. Pattern with lateral dimensions down to 4 μm at reasonable film thickness of this highly swellable material could be obtained. For advanced applications, the resolution properties and the adhesion of the gels were increased by the use of a specially

Acknowledgements

The authors are thankful to U. Keller and G. Kiefermann (Institute of Medical Physics and Biophysics) for expert sample preparation and for performing the majority of FESEM studies as well as for excellent photographic work. The DFG (Deutsche Forschungsgemeinschaft) is gratefully acknowledged for their financial support of this work within the Sonderforschungsbereich 287 ‘Reaktive Polymere’.

References (31)

  • H.G Schild

    Prog Polym Sci

    (1992)
  • Q Yan et al.

    Polymer

    (1995)
  • M.J Lesho et al.

    Sens Actuators, B Chem

    (1996)
  • N.F Sheppard et al.

    Sens Actuators, B Chem

    (1995)
  • J Hoffmann et al.

    Sens Actuators, A Phys

    (1999)
  • M.H Harmon et al.

    Macromolecules

    (2003)
  • K.F Arndt et al.

    Polym Adv Technol

    (2000)
  • X.Z Zhang et al.

    Macromol Chem Phys

    (1999)
  • J Chen et al.

    Macromol Sci, Pure Appl Chem

    (1999)
  • R Yoshida et al.

    Nature

    (1995)
  • S Zhou et al.

    Macromolecules

    (1996)
  • S.H Gehrke

    Adv Polym Sci

    (1993)
  • T Tanaka et al.

    J Chem Phys

    (1979)
  • D.J Beebe et al.

    Nature

    (2000)
  • Cited by (0)

    Part II: [1].

    View full text