Double-network hydrogels improve pH-switchable adhesion

Abstract

For environmentally-switchable adhesive systems to be reused repeatedly, the adhesive strength must not deteriorate after each adhesion cycle. An important criterion to achieve this goal is that the integrity of the interface must be retained after each adhesion cycle. Furthermore, in order to have practical benefits, reversing the adhesion must be a relatively rapid process. Here, a double-network hydrogel of poly(methacrylic acid) and poly[oligo(ethylene glycol)methyl ether methacrylate] is shown to undergo adhesive failure during pH-switchable adhesion with a grafted (brush) layer of polycationic poly[2-(diethyl amino)ethyl methacrylate], and can be reused at least seven times. The surfaces are attached at pH 6 and detached at pH 1. A single-network hydrogel of poly(methacrylic acid), also exhibits pH-switchable adhesion with poly[2-(diethyl amino)ethyl methacrylate] but cohesive failure leads to an accumulation of the hydrogel on the brush surface and the hydrogel can only be reused at different parts of that surface. Even without an environmental stimulus (i.e. attaching and detaching at pH 6), the double-network hydrogel can be used up to three times at the same point on the brush surface. The single-network hydrogel cannot be reused under such circumstances. Finally, the time taken for the reuse of the double-network hydrogel is relatively rapid, taking no more than an hour to reverse the adhesion.

Synthesis of multifunctional high strength, highly swellable, stretchable and self‐healable pH‐responsive ionic double network hydrogels

“…However, the application of hydrogels is often limited by their poor mechanical properties in their swollen state. Thus it is required to synthesize hydrogel materials having high strength, toughness and stiffness which may be suitable for various applications where mechanical properties are important factors, such as in artificial cartilage, in the field of soft mechanics, actuators, artificial nerves and muscles, and as swellable packers in the oil industry and other engineering applications …”

Adhesion between oppositely charged polyelectrolytes

“…MPa at pH 1 to 0.17 MPa at pH 12 [9], whilst the PDEAEMA gel changes from K = 0.37 ± 0.01 MPa at pH 1 to 0.63 ± 0.02 MPa at pH 12. This means that the PDEAEMA and PMAA gels have similar moduli at pH 1, but significantly different values for the maximum force and work of adhesion at this pH.…”

“…The PDEAEMA gel requires both more work and a A c c e p t e d M a n u s c r i p t 11 greater force to be detached from the PMAA brush at pH 6 than the PMAA gel does from the PDEAEMA brush. It is appealing to attribute this behavior to mechanical properties: here the PMAA gel has a modulus of 0.30 ± 0.04 MPa in DI water (pH 6) [9], while for the PDEAEMA, K = 0.48 ± 0.03 MPa. Furthermore, the swollen gels have a smaller modulus than those that are uncharged.…”

“…The use of 'smart' materials for adhesive applications has become an area of some significant effort [3]. Charged polymers are of particular importance in the field of adhesion because of the possibility of using an environmental stimulus, such as salt, temperature, or pH to control the adhesion in situ, including the ability to reverse the adhesion of a system that has already bonded [4,5,6,7,8,9]. A true environmentally switchable adhesive system should be able to detach spontaneously as the relevant environmental parameter is changed, but for practical purposes a sufficient reduction in the adhesion might suffice.…”

Salt Dependence of the Tribological Properties of a Surface-Grafted Weak Polycation in Aqueous Solution

“…There is a tension between the pH and salt dependence of polyelectrolytes, since pH is often considered to convey "smart" properties to the material, with an aim of triggering an environmental stimulus, e.g. for adhesion [17][18][19][20] and drug delivery [21,22]. The addition of salt screens ions and consequently reduces the effect of pH, which is particularly important in physiological environments.…”