Ph and photo-responsive hydrogel actuators
Dunne, Aishling and Mac Ardle, Siobhán and Henessy, Joseph and Florea, Larisa and Diamond, Dermot (2015) Ph and photo-responsive hydrogel actuators. In: 20th international conference on miniaturized systems for chemistry and life sciences (uTas 2016), 9-13 Oct 2016, Dublin, Ireland.
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Hydrogels constitute a group of hydrophilic polymeric materials, capable of holding large amounts of water in
their three-dimensional networks. By incorporating stimuli-responsive units in their structure, hydrogel
actuators can be developed, that respond to a variety of stimuli such as light, pH, electric or magnetic fields
In this study, pH responsive hydrogels were developed using copolymers of acrylic acid (AA) and acrylamide
(AAm) in different molar ratios (30:70, 50:50 and 70:30, respectively). At pH above the pKa of AA (pH>4.5)
the AA dissociates to the more hydrophilic acrylate (A-) form, triggering swelling of the hydrogel (Figure 1).
In contrast, at pH < 4.5, the hydrogel contracts due to the formation of the hydrophilic AA form in the
polymer backbone, which promotes release of water from the gel (Figure 2). In the case of the 50:50
AAm:AA p(AAm-co-AA), circular hydrogels were polymerised that show an area of 0.72 mm2 at pH 3
compared with 3.56 mm2 for the same hydrogel at pH 11. This dramatic increase of 396.14% is due to the
greater amount of the more hydrophilic acrylate ion form being present in basic conditions (Figure 3).
In order to turn this pH response into a photo-response, a reversible photo-acid generator, namely spiropyran
acrylate (SPA), was copolymerised in the p(AAm-co-AA) polymer backbone. In acidic environments, SPA
will spontaneously convert to the protonated hydrophilic merocyanine (MC-H+) form and switch back to the
hydrophobic SPA when exposed to white light, expelling a proton in the process (Figure 4).
The monomer ratio used for the p(AAm-co-AA-co-SPA) hydrogels was 10:10:1 AAm: AA: SPA. When the
hydrogel is immersed in water, in the dark, the AA dissociates and the proton is taken by the SPA to form
MC-H+, which gives the hydrogel a yellow colour. Under these conditions (A-, MC-H+) the polymer chains
are more hydrophilic causing the hydrogel to expand (Figure 5, initial point t=0s). However, when exposed to
white light, the MC-H+ is converted back to the SPA form (colourless) expelling a proton, thus decreasing the
local pH, and protonating the AA. This makes the polymer chains less hydrophilic, causing the hydrogel to
contract (Figure 5, 0-10 min). Upon removal of the white light source, the reverse process occurs and the
initial conditions are restored, resulting in hydrogel expansion (Figure 5, 10-20 min).
For the p(AAm-co-AA-co-SPA) hydrogel, the SPA unit serves as a reversible photo-acid generator. This
ensures a localised pH change under different illumination conditions, determining the ratio of AA/A- present,
and therefore the hydrophilic character of the polymer backbone. In this case, photo-contraction of over 15%
in diameter was achieved within 90 seconds of white light irradiation followed by reswelling to ~95% of its
fully hydrated size after further ~30 seconds in the dark (Figure 6).
In both cases (pH and photo-responsive hydrogels) the stimuli-induced contraction/reswelling processes were
reversible and repeatable over at least 3 cycles with no detectable hysteresis .
The fast actuation of the p(AAm-co-AA-co-SPA) hydrogels demonstrates great potential for their
incorporation in microfluidic systems as reversible photo-controlled micro-valves .
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