Chapter 2

Photo-Responsive Hydrogels for Adaptive Membranes

David Díaz Díaz1,2,* and Jeremiah A. Johnson3

1Institut für Organische Chemie, Fakultät für Chemie und Pharmazie, Universität Regensburg, Regensburg, Germany

2IQAC-CSIC, Barcelona, Spain

3Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Mass, USA

*Corresponding author: david.diaz@chemie.uni-regensburg.de

Abstract

Synthetic membranes that show a selective and reversible response to a specific environmental cue (e.g., a change in pH, temperature, ionic strength, electromagnetic field, irradiation) are platform materials for high-tech applications that demand switchable material properties. Mechanistically, stimuli-responsive membranes leverage reversible molecular properties of the membrane components, like conformation, polarity, and/or reactivity, to induce changes in the bulk pore structure. Among the many possible external stimuli, light is particularly attractive as it can be applied both spatially and temporally without diffusion limitations. This chapter describes the permeability properties of a selection of representative photo-responsive hydrogel membranes. The membranes are classified according to their chromophore structures.

Keywords: Hydrogels, polymer gels, photo-responsive, membranes

2.1 Introduction

A membrane is usually defined as an interphase between two bulk phases that acts as a selective and porous barrier for regulating mass transport between two compartments. Membranes are essential for life. For example, natural skin acts as a permeable multifunctional membrane that interacts with the surrounding environment and responds to light, temperature, humidity, chemicals, and mechanical stress. Another example is the nuclear pore membrane, which regulates selective transport of biological molecules and polymers into and out of the nucleus of all eukaryotic cells. Inspired by the incredible material properties of biological membranes, extensive research efforts have focused on the design of artificial membranes that are able to respond, especially in a reversible manner (i.e., valve or gate function), to specific environmental cues. Such materials could have applications in a broad range of modern technologies that demand switchable material properties (e.g., sensors, delivery systems, optoelectronics, transducers, actuators, catalysis, purification). The broad potential of these materials is also reflected in the significant number of published reviews focused on the latest developments and applications of multifunctional responsive polymeric membranes [1–7].

From a mechanistic point of view, stimuli-responsive membranes utilize changes in the conformation, polarity, and/or reactivity of specific responsive functional groups in the membrane bulk or on the membrane surface to induce a change in pore structure and, hence, filtration properties. During the last three decades, functional groups that respond to changes in pH, temperature, ionic strength, specific chemical species, light, electric fields, magnetic fields, and mechanical stresses have all been applied to regulate membrane properties; pH, temperature, and light-responsive membranes have been the most studied [1–7, 8]. Light is a very attractive stimulus for switching material properties because its intensity and wavelength can be easily controlled in a spatial and temporal manner to achieve precise, selective, fast, energy-efficient functional group modification without diffusion limitations. Mother Nature leverages these unique properties of light in the context of biological membranes; photo-induced proton-coupled electron transfer reactions across cellular membranes are responsible for all life on Earth. Again, where Mother Nature leads the way scientists often follow: the incorporation of photo-responsive functional groups (i.e., chromophores) into polymeric membranes has received wide theoretical and experimental attention [3, 9–10]. In these systems, light drives actuation through reversible photo-induced isomerization or ionization reactions of the chromophores, or simply via photo-thermal effects. If the chromophore is attached to a suitable polymeric support, then light-induced changes at the chromophore level (e.g., dipole moment, charge, color, size) can manifest themselves as changes in macroscopic material properties (e.g., wettability, permeability, density, viscosity). Azobenzene, spiropyran, triphenylmethane leuco, diarylethene, and viologen derivatives have arguably been the most investigated chromophores in the context of photo-responsive membranes. These systems have been discussed in several excellent reviews [9, 11, 12].

Hydrogels are molecular networks that swell in water. Responsive hydrogels [7, 13–19] and tunable hydrogel membranes [4, 20–21] have attracted considerable interest in biomedicine due to their unique properties such as environmental-responsiveness to biological cues, reversible phase transitions at physiological temperatures, swellability in water, hydrophilicity, and biocompatibility [22]. The molecular composition of the hydrogel, the chemical methods used for its preparation, the cross-linking density, and the nature of the permeant strongly impact the functionality of hydrogel membranes [1, 23]. As is the case for all responsive membranes, incorporation of pendant stimuli-responsive units into hydrogel networks can factilitate control of the hydrogel membrane’s bulk properties. Once again, pH- and temperature-sensitive materials obtained from synthetic and/or naturally derived hydrophilic building blocks are common platform components of hydrogel-based membranes [4–5, 18, 20, 24–26]. Despite the considerable number of photo-responsive hydrogel systems reported in the literature [27–28], their potential use for adaptive membranes began to receive more attention after the earlier studies carried out with their pH- and thermally-actuated analogues.

This chapter presents key examples of photo-responsive hydrogel membranes. The examples are classified by their chromophore, which is the key molecular specie that interacts with light to induce a bulk material response. In principle, any photo-responsive hydrogel could be fabricated into a planar membrane material; most phoro-responsive hydrogels have not been studied in this context. Therefore, this chapter focuses on examples where the permeability properties of the materials were characterized, i.e., the material was intended to be a membrane. A discussion of photo-thermally sensitive materials is also included. These materials represent a particular subclass of photo-responsive membranes wherein light is used to heat the material and induce a bulk thermal transition. Other photo-responsive polymer systems (i.e., photo-responsive polymer brushes [29]), or hydrogel membranes with responses to external stimuli other than light, are excluded from the scope of this chapter.

2.2 Photo-Responsive Hydrogel Membranes

2.2.1 Photo-Responsive Moiety: Cinnamylidene

Cinnamylidene (CA) is a versatile photo-crosslinkable group that undergoes an efficient [2+2] dimerization reaction in response to visible and long-wavelength UV light. The reaction is photo-reversible; retro [2+2] cycloaddition occurs upon exposure to short-wavelength UV light. Almost two decades ago, Russell and coworkers described the preparation of poly(ethylene glycol) (PEG) hydrogels via photo-initiated end-linking of four-armed PEG star polymers bearing terminal cinnamylidene acetyl (CA) groups (Figure 2.1) [30]. As a proof of concept, tetrakis-CA macromer 1 was obtained in ca. 70% yield by simple esterification of CA acid chloride with hydroxyl terminated four-armed PEG (b-PEG, MW = 15,000 Da). A film of this polymer was cast onto a glass surface and irradiated with light (450 W medium pressure UV lamp, l > 300 nm) for 5 min to 3 h to produce hydrogels. Upon light exposure in solution, the parent trans-CA species undergoes rapid isomerization to the corresponding cis-isomer. Then, end-linking takes place via a photo-induced [2+2] cycloaddition reaction between an excited (*) CA group of one chain with a ground state CA group of another chain to yield cyclobutane products [30].

Figure 2.1 Reversible [2+2] photo-dimerization/photo-scission of b-PEG-CA 1 (λ = 313 nm; ε = 37390 L mol−1 cm−1).

Adapted with permission from Ref. [30]. Copyright (1996) American Chemical Society.

The intermolecular dimerization of CA groups from two different 1 molecules was necessary to form gels; no gels formed with either unmodified b-PEG (i.e., absence of CA groups) or an equimolar aqueous mixture of cinnamylidene acetic acid and b-PEG.

These hydrogels possess many unique features with important practical implications: (1) The gelation occurs without the need for potentially toxic photo-sensitizers and/or photo-initiators; (2) the junction density, and therefore the degree of swelling (DS = (Ws-Wd)/Wd, where Ws = weight of the gel in air after swelling and Wd = weight of the dry gel), of the hydrogels can be fine-tuned by varying the average number of CA moieties on macromer 1 and/or the irradiation time (DS decreases as the degree of substitution or the time of irradiation increase. For instance, DS = 58 ± 4 for a degree of substitution of 25%, whereas DS = 35 ± 5 for a degree of substitution of 64%); (3) The...