In my doctoral thesis, I worked in the exciting field of stimuli-responsive polymers. Scientists have been interested in stimulus-responsive materials for a long time. These materials are so fascinating because they can transform after receiving an external signal. The Nobel Prize in Chemistry 2016 “for the design and synthesis of molecular machines” once again attracted a great deal of attention in this field. There is no doubt that these types of materials will play a major role in the future of materials science.
Stimuli-responsive materials are a class of smart materials and therefore can pick up a signal from outside and convert it into a usable property change. This means that a material can have different properties depending on the external influence it is exposed to.
Since the 1990s stimuli-responsive materials are being extensively developed. According to Web of Science (ISI Web of Knowledge) more than 1,000 research articles are now published each year in peer-reviewed journals related to the term “stimuli responsive”. Especially thermo-responsive polymers were and still are a big part of the research. 
Other stimuli that are often used are pH change, enzymes or light and there are a number of other stimuli. The stimuli are typically divided into three categories: biological (or biochemical), chemical and physical.  In principle stimuli-responsive materials can exhibit both reversible and irreversible reactions. While irreversible reactions like covalent bond cleavage can be used only once, reversible reactions are favoured as they can be used over and over again.
Photo-responsive polymers are a special part of stimuli-responsive materials, which use light irradiation to change. Typically, the light that is being used is in the ultraviolet (UV), visible (vis) or infrared (IR) region. While UV light has the highest energy, vis and IR light are more attractive for solar energy and biomedical research, because vis and IR light responsive polymers fit more to the solar spectrum, show less damage and can penetrate deeply into tissue.
Photo-responsive polymers are particularly interesting because of the following reasons:
- Polymers have advanced and hierarchical structures that offer many possibilities to integrate switchable units.
- Different to switchable small molecules, switchable polymers can have cooperative or collective motions.
- Polymers in general show good processability.
The use of light as a stimulus offers a number of attractive options, such as:
- Local delivery
- Rapid on and off switching
- An easy change in intensity or wavelength
One example where an irreversible reaction is used in connection with photopolymers is photochemical curing by crosslinking. This is used in dental fillings as well as in the photoresist technique.
Top 3 future applications
Photo-responsive polymers find applications in biology and medicine and can be used for controlled and triggered drug delivery.  By incorporating photo-responsive units into polymers, on-demand drug release is triggered by light irradiation.
A drug can be loaded into a photo-responsive polymer carrier system. This carrier can then be injected into the body and the drug can be released by light in the specific region where the drug needs to be applied. This design has the advantage that the drug only attacks in the region where the body is irradiated, while the rest of the body is not affected by the drug and side effects can be reduced. This idea is intended for anti-cancer drugs, for example.  It is important that the light can penetrate deep into the tissue, so accurate wavelength and intensity must be used.
Photo-actuators can convert light energy into mechanical work. Photoresponsive polymers are used to prepare shape memory polymers  or liquid crystalline polymers  that can be used to prepare photo-actuators.
Films made by these photo-responsive polymers are used to prepare actuators that can bend in one direction when irradiated with light. However, also more complicated structures were developed which can bend like springs  or move like a worm.  These polymer photo-actuators can be used as artificial muscles  , in robots  or to harvest and convert solar energy into mechanical work. However, the bending direction and response time must be intelligently designed.
Light controlled reversible adhesives can be realised by using photo-responsive polymers. Conventional adhesives can be used once to glue two things together. However, one-time use is bad for the environment. In addition, glued parts can break when attempted to remove them from each other.
Photo-reactive polymers, which can switch between a sticky and a non-sticky form, can therefore help to produce light-controlled, reversible adhesives. The glue can be stored in a non-sticky form. Irradiated by light, the sticky form can stick two parts together. If the polymer is transferred into the non-sticky form, the parts can be easily removed again. The glued together parts as well as the adhesive can be reused.
In summary, photoresponsive polymers are an exciting and growing field of research. Much has already been done to investigate the behavior of such polymers. But more needs to be done to bring these polymers to market. In the future, more complex polymer structures can be realized by using advanced polymerization methods. In order to realize a commercialization, a cost-effective and mass-suited way of synthesis must be found. Additional emerging applications are phase-change materials, light-healing materials and optical memories.
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