Reversible photoresponsive hydrogels based on spiropyran
Florea, Larisa and Ziółkowski, Bartosz and Theobald, Jannick and Diamond, Dermot (2014) Reversible photoresponsive hydrogels based on spiropyran. In: MRS Spring Meeting 2014, 21-25 Apr 2014, San Francisco, CA..
Full text available as:
The possibility of using photo-stimulus to control actuation is very appealing as light can provide contactless stimulation, is biocompatible and can be applied in a non-invasive and highly precise manner. Photo-responsive hydrogels are highly favoured for making light-stimulated polymer actuators as they have the ability to undergo significant volumetric changes in response to an external light stimulus. Hydrogels possess biocompatibility and a degree of flexibility, very similar to natural tissue, due to their significant water content. Photo-responsive engineered hydrogels can be made to collapse and thereby release a percentage of their water content upon light irradiation.
The most widely used photo-sensitive molecules for photo-actuation of hydrogels are spiropyrans as they allow for reversible conformational, hydrophobicity/hydrophilicity and polarity changes upon irradiation with light of particular wavelengths. Taking advantage of this photo-induced change of hydrophilic/hydrophobic character, photoresponsive gels based on N-isopropylacrylamide and spiropyran have been realised.[1,3]
In this work, a new generation of photoresponsive hydrogels are proposed, based on N-isopropylacrylamide copolymerised with pendant spiropyran groups and acrylic acid. These hydrogels actuate on the following principle: When the hydrogel is immersed in water, the copolymerised acrylic acid dissociates and the proton is taken by the spiropyran unit present in the copolymer causing it to change from spiropyran (SP) to the protonated merocyanine (MCH+) form. When the copolymer is irradiated with light matching the absorbance of MCH+, the MCH+ switches back to the closed hydrophobic SP form, releasing a proton in the process; As result of the formation of the more hydrophobic SP isomer, dehydration of the main polymer chain occurs and the hydrogel shrinks. When the light source is removed, the SP is spontaneously reprotonated and the hydrogel reswells.
Due to the relative pKa values of acrylic acid, and of the spiropyran and merocyanine isomers, the protonation and deprotonation processes occur internally within the gel and there is no need for an external source of protons. In contrast to previous formulations, these gels do not show degradation of their photo-induced shrinking ability after multiple washings in deionised water and repeated switching over a two month period. Moreover, improved reswelling performance of these hydrogels has been realised by inducing porosity through the use of poly(ethylene glycol) as a pore forming agent. In this way, significantly faster kinetics for the hydrogel shrinking and re-swelling processes have been obtained compared to conventional hydrogels.
1. Sugiura, S. et al., Sens. Act. A 2007, 140, 176. 2. Florea, L.; Diamond, D.; Benito-Lopez, F. Macromol. Mater. Eng. 2012, 297, 1148. 3. Ziolkowski, B.; Florea, L.; Theobald, J.; Benito-Lopez, F.; Diamond, D. Soft Matter 2013, 9, 8754.
Archive Staff Only: edit this record