Polymers can be found everywhere, as each of us is dealing with plastics-containing daily products like water-bottles, cell phones, clothes, etc. Even though polymers are often used as a synonym for plastics, they widely exist also in living organisms. For instance, rubber and cellulose or DNA and proteins, responsible for life, are natural polymers. The utilization of polymers dates back all the way to ancient Egypt where people started using papyrus for writing. However, the chemical nature of polymers was unknown until the 20th century and the work of Hermann Staudinger.30 Staudinger predicted that polymers contain smaller elementary units called monomers. These monomers can be polymerized, i.e., linked with covalent bonds to form long macromolecular chains, which can further associate with each other via chemical or physical interactions. Shortly after this discovery, the first synthetic polymer, Nylon, was fabricated in 1935 by Dupont.31 Triggered by these pioneering findings, researchers have been making groundbreaking advances in the field of
polymer science and a huge amount of different polymers with diverse chemical, mechanical, electrical and optical properties have been synthesized. Polymers appear in various morphologies and they can be produced in the form of linear (homo- or co-) polymers, block copolymers or dendrimers; they can form micelles or capsules via self-assembly. Polymers may be anchored to a surface, generating single/mixed polymer brushes, films, and layer-by-layer (LbL) assemblies. 3D polymer networks can also be generated using chemical or physical crosslinking.
The fast growth of polymer research led also to the development of responsive polymers. Those materials can respond to their environment by altering their chemical and/or physical properties, referred as stimuli-responsive polymers from now on.32,33 In these polymers, the stimuli-responsiveness arises from activation of the functional groups or other responsive elements/domains inside the polymer matrix. When stimuli-responsive polymers meet specific inputs, they undergo structural and conformational change which is then accompanied by variation in their physical properties. In the past decades, many stimuli-responsive polymer systems have been studied with fine-tuned morphologies, utilizing various inputs, and producing wide range of outputs, targeting various applications as illustrated in Fig. 2.1. Rather than giving a comprehensive overview, we direct readers to these reviews16,33–35, and give here only some state-of-the-art examples of applications to show the multiplicity of the field of stimuli-responsive polymers.
Probably the most primitive example of stimuli-responsive polymers is the one exhibiting temperature dependence. A phase transition, a feature causing change in the polymer chain conformation at the solvation state,36 was first demonstrated in the 1960s with Poly(N-isopropylacrylamide) (PNIPAm).37 PNIPAm exhibits a lower critical solution temperature (LCST), above which de-mixing occurs between polymer chains and aqueous solution, yielding phase separation. Today, PNIPAm can be functionalized with various elements having responses to different stimuli (i.e. pH, ionic strength or light), and developed into materials exhibiting various outputs, such as changes of color, wettability, cell interfacing, etc.33
The stimuli-responsive polymers can convert stimulated inputs to readable outputs which makes them suitable to sensor applications. In this context, polymer surfaces with modified nanoparticles or nanocrystals have been attractive choices due to tunable quantum effects they possess.35 Stimulated conformational change of the polymers modifies the chemical environment of the attached particles and therefore changes the optical properties of the polymer matrix. For example, polymers functionalized with pH-responsive molecules can change light absorption and/or emission through pH-modified surface plasmon resonance of gold
nanoparticles38 or Förster resonance energy transfer of quantum dots.39 Also, polymer sensors capable of detecting biological species have been demonstrated. In this case, enzymes produced from bacteria40 or urea41, are often involved in polymer responsiveness to detect the quantity of species.
Figure 2.1 Stimuli-responsive polymers. Polymer structural and conformational change under specific stimulus leading different outputs/applications. Figures in Output/application reproduced with permission: Self-healing42, Copyright 2010, Annual Reviews. Drug delivery43, Copyright 2006, John Wiley and Sons. Sensors39, Copyright 2014, ACS. Artificial muscles44, Copyright 2009, RCS.
As stimuli-responsive polymers can also be made biocompatible, they have been used extensively in bio-medical applications like drug delivery.24 In stimuli-controlled drug delivery, polymer structure is deemed to survive in vivo and in vitro, deliver the cargo and release the drug into the targeted cells. To this extent, polymer capsules and nanofibers have been extensively used. They can, for instance, grab the drug cargos, protect it inside the human body, and release the drug under specific stimuli
(temperature, pH, light)43 or the presence of biological substances (high sugar or allergens level).33
Another interesting application is the capacity of self-healing. Self-healing polymers can recover their properties such as elasticity or surface smoothness, after the structure has been physically damaged.45 This capacity is usually induced by re-formation of chemical bonds under stimulus, and it can be realized by several chemical mechanisms like interchain diffusion, covalent bond re-formation or post-curing of thermoplastic polymers.42 For example, light input has been used to design optically healable supramolecular rubbery polymer, in which the polymer recovers after activating the metal-ligands by UV exposure.46 In another example, a LbL assembly of polyelectrolytes on a metal surface provides a mechanism for corrosion protection: after detection of corrosive ions, the polymer releases inhibitors, buffering the pH of the corrosive area and self-curing of the polymer film, enabling self-healing activity.47
Finally, by targeting applications in the regime of soft robotics, stimuli-responsive polymers able to change their shape under an external stimulus, referred to soft actuators or artificial muscles, are of great interest. These polymers can produce similar or even higher stains and stresses than natural muscles, and provide sophisticated control over shape-morphology.48 In the following, we will introduce macromolecular robotic actuator systems based on crosslinked polymers, and elaborate the topic of soft robotics in the next section.