Abstract
This review addresses the potential development of polymer hydrogels in terms of fast stimuli responsiveness and superior mechanical properties. The slow response and mechanical weakness of stimuli-sensitive hydrogels have been considered as the main barriers for their further development and practical application. It has been a dream of scientists to have fast stimuli-responsive hydrogels with superior mechanical performance. In this review, the principal reasons behind the poor stimulus sensitivity and mechanical strength of polymer gels are highlighted, and we present some pioneering physical and chemical efforts aimed at fabricating hydrogels with high stimuli sensitivities and/or superior mechanical properties.
Introduction
Polymer hydrogels (hereafter called ‘hydrogels’) are three-dimensional polymer networks in which the voids are filled with water. These hydrogels are widely used in a variety of industrial and consumer products such as oil dewatering systems, mechanical absorbers, diapers and contact lenses. One of the most remarkable areas of research on hydrogels in the past few decades has been their stimulus sensitivity. Hydrogels composed of stimuli-responsive polymers reversibly alter their shape and volume in response to small variations in the environment, such as pH, temperature, light and electric and magnetic fields, thereby changing their physico-chemical characteristics.1 Among all of the available environmentally responsive hydrogels, temperature-responsive hydrogels have attracted the most attention because of the facile tuning of their properties. In particular, poly(N-isopropylacrylamide) (poly(NIPA)) hydrogels have been widely investigated as thermosensitive hydrogels. Poly(NIPA) has a low critical solution temperature (LCST) at around 32 °C in water. The flexible coil of poly(NIPA) (soluble) converts into a compact globular state (insoluble) at the LCST, and this conformational change is reversible.2 Thus, hydrogels composed of poly(NIPA) undergo a reversible volume change at a similar transition temperature in water. Poly(NIPA) hydrogels have potential applications in drug delivery systems, therapeutic agents, diagnostic devices, biomaterials, cell cultivation, biocatalysts, sensors, microfluidic devices, actuators, optical devices and size-selective separation among others.3, 4, 5
A recent subject that has attracted much attention in the field of gel science is the fabrication of hydrogels with the rapid stimuli responsiveness and superior mechanical properties required for many applications of stimuli-responsive hydrogels. The creation of durable hydrogels that exhibit reversible and rapid changes in shape or volume, the likes of which are found in living organisms such as muscle, has been a challenge for materials chemists. Hydrogels synthesized from monomer solutions by radical polymerization, a general-purpose method for making synthetic hydrogels, however, face several constraints. The inherently weak mechanical properties caused by underlying spatial inhomogeneity during polymerization and extremely slow responsiveness caused by critical slowing down and vitrification during the shrinking process restrict the widespread use of these hydrogels.6 Although some approaches have been developed to improve the stimuli sensitivities and mechanical properties of hydrogels, it remains a challenge to design ideal gel networks with a combination of desired properties.
In this review, we will begin by introducing some pioneering work in the synthesis of hydrogels with fast stimuli responsiveness specifically attributed to chemical modifications. Work based on thermosensitive NIPA as a monomer has been the main focus of attention. Although hydrogels synthesized through these techniques improve stimuli sensitivity to a greater extent, in most cases, not enough attention has been given to the mechanical properties of these hydrogels, and these properties have not been improved. We will therefore review some recently developed novel methods to improve the mechanical properties of hydrogels and discuss the application of these methods for preparing stimuli-responsive hydrogels with superior mechanical performance.
Fast stimuli-sensitive hydrogels
The volume phase transition7, 8, 9 of hydrogels between the swollen state and collapsed state in response to various types of stimuli indicates that hydrogels can be used as smart materials. Here, the term ‘smart’ refers to materials that are sensitive to changes in the environment. Unfortunately, however, the slow response of hydrogels, which is an inherent characteristic of the volume phase transition phenomenon caused by a critical slowing down, has prevented them from being developed for new technologies. Moreover, vitrification during the shrinking process also contributes to the slow change in volume. After a temperature jump, a dense skin layer forms on the surface of poly(NIPA) hydrogels that inhibits water loss from the inner portion and, consequently, slows down the volume change.10 Without the volume phase transition, in which the volume change of hydrogels is drastic but continuous, the characteristic duration of the volume change can be much shorter than in a discontinuous system. The relaxation time of the volume change of hydrogels, however, is highly dependent on the size of the hydrogel, as the swelling and deswelling of hydrogels is a diffusion process. According to the Tanaka-Fillmore theory, the shrinking rate is inversely proportional to the square of the smallest spatial dimension of the hydrogel. As a result, large, bulky hydrogels typically exhibit slow changes in volume.6
To date, many physical processes have been developed to obtain high thermosensitivity in poly(NIPA) hydrogels, including the following: preparations of phase-separated heterogeneous structures11, 12 and macroporous or mesoporous structures,13, 14, 15 gel formation via vacuum synthesis,16 as well as freezing techniques.17, 18 As an example, a poly(NIPA) hydrogel with a phase-separated heterogeneous network structure can be prepared by polymerization at a temperature above the LCST or in particular mixed solvents, including solvents composed of H2O and an organic solvent such as acetone, phenol or tetrahydrofuran. A pore-forming component such as sodium chloride, glucose, poly(ethylene glycol) (PEG), SiO2 or a hydrophilic or hydrophobic additive can be introduced into the pregel solution. The subsequent removal of this pore-forming component from the hydrogel network provides a macroporous network with fast thermoresponsiveness. The basic mechanism of all these processes involves the effect of micro- or macro-level manipulated structures, which are much larger than molecular level structures, on the quick volume change by the expulsion or absorption of water molecules from or into the hydrogel network. These processes, however, are not suited for some practical applications, as the hydrogels lose their mechanical strength, toughness and optical transparency because of their spongy structures. We will not further discuss such porous hydrogels; rather, we will primarily focus on the strategies used to obtain fast thermoresponsive hydrogels using chemical techniques by which the gel networks are modified at the molecular level. Some of the basic techniques used are mentioned below.
Comb-type poly(NIPA)-grafted gels
Yoshida et al. prepared comb-type poly(NIPA)-grafted gels as follows. First, a chain transfer agent was used to synthesize various linear poly(NIPA)s with different molecular weights and one amino end group. Next, the amino group was converted into an acrylate group that could polymerize with other vinyl monomers. Finally, the semitelechelic linear poly(NIPA)s with acrylate end groups were polymerized with NIPA and small amounts of a crosslinker. Poly(NIPA) chains with one freely mobile side end were thereby grafted onto the polymer networks (Figure 1a).19, 20 These grafted polymer chains can respond faster than crosslinked poly(NIPA) chains and form hydrophobic nuclei in response to a rise in temperature (Figure 1b). The aggregated nuclei then form many channels for the diffusion of water to enhance the shrinking rate of the crosslinked segment of the hydrogel with respect to non-grafted traditional poly(NIPA) hydrogels.
The volume phase transition7, 8, 9 of hydrogels between the swollen state and collapsed state in response to various types of stimuli indicates that hydrogels can be used as smart materials. Here, the term ‘smart’ refers to materials that are sensitive to changes in the environment. Unfortunately, however, the slow response of hydrogels, which is an inherent characteristic of the volume phase transition phenomenon caused by a critical slowing down, has prevented them from being developed for new technologies. Moreover, vitrification during the shrinking process also contributes to the slow change in volume. After a temperature jump, a dense skin layer forms on the surface of poly(NIPA) hydrogels that inhibits water loss from the inner portion and, consequently, slows down the volume change.10 Without the volume phase transition, in which the volume change of hydrogels is drastic but continuous, the characteristic duration of the volume change can be much shorter than in a discontinuous system. The relaxation time of the volume change of hydrogels, however, is highly dependent on the size of the hydrogel, as the swelling and deswelling of hydrogels is a diffusion process. According to the Tanaka-Fillmore theory, the shrinking rate is inversely proportional to the square of the smallest spatial dimension of the hydrogel. As a result, large, bulky hydrogels typically exhibit slow changes in volume.6
To date, many physical processes have been developed to obtain high thermosensitivity in poly(NIPA) hydrogels, including the following: preparations of phase-separated heterogeneous structures11, 12 and macroporous or mesoporous structures,13, 14, 15 gel formation via vacuum synthesis,16 as well as freezing techniques.17, 18 As an example, a poly(NIPA) hydrogel with a phase-separated heterogeneous network structure can be prepared by polymerization at a temperature above the LCST or in particular mixed solvents, including solvents composed of H2O and an organic solvent such as acetone, phenol or tetrahydrofuran. A pore-forming component such as sodium chloride, glucose, poly(ethylene glycol) (PEG), SiO2 or a hydrophilic or hydrophobic additive can be introduced into the pregel solution. The subsequent removal of this pore-forming component from the hydrogel network provides a macroporous network with fast thermoresponsiveness. The basic mechanism of all these processes involves the effect of micro- or macro-level manipulated structures, which are much larger than molecular level structures, on the quick volume change by the expulsion or absorption of water molecules from or into the hydrogel network. These processes, however, are not suited for some practical applications, as the hydrogels lose their mechanical strength, toughness and optical transparency because of their spongy structures. We will not further discuss such porous hydrogels; rather, we will primarily focus on the strategies used to obtain fast thermoresponsive hydrogels using chemical techniques by which the gel networks are modified at the molecular level. Some of the basic techniques used are mentioned below.
Comb-type poly(NIPA)-grafted gels
Yoshida et al. prepared comb-type poly(NIPA)-grafted gels as follows. First, a chain transfer agent was used to synthesize various linear poly(NIPA)s with different molecular weights and one amino end group. Next, the amino group was converted into an acrylate group that could polymerize with other vinyl monomers. Finally, the semitelechelic linear poly(NIPA)s with acrylate end groups were polymerized with NIPA and small amounts of a crosslinker. Poly(NIPA) chains with one freely mobile side end were thereby grafted onto the polymer networks (Figure 1a).19, 20 These grafted polymer chains can respond faster than crosslinked poly(NIPA) chains and form hydrophobic nuclei in response to a rise in temperature (Figure 1b). The aggregated nuclei then form many channels for the diffusion of water to enhance the shrinking rate of the crosslinked segment of the hydrogel with respect to non-grafted traditional poly(NIPA) hydrogels.
