A new study reports that the shapes and surface patterns of thin films of a stretched material can be modified by shining ultraviolet light at it. The resulting topologies depend on the exposure pattern, the applied stress and the sample thickness.
For applications such as tissue engineering, drug-delivery systems and biosensors, there is a need for stimulus-responsive polymers that can adapt to their environment by converting optical, thermal or mechanical signals into chemical signals, and vice versa1. Writing in Advanced Materials, Bowman and colleagues2 report just such a material, and demonstrate a process in which thin films of the material respond to light and mechanical forces by forming intricate surface patterns.
In traditional polymer networks, crosslinks are used to connect polymer chains. Well-known examples are rubbers (elastomers) and many of the gels that are often used in biomedical applications. Elastomers show large deformations in response to stretching, but only the flexible polymer segments between two crosslinks are affected during deformation — they adopt less coiled, more linear conformations. The crosslinks themselves remain fixed.
Bowman and colleagues2, however, exploit the unique properties of an intriguing class of polymeric material known as a covalent adaptable network3 (Fig. 1a). In these materials, the covalent bonds that hold polymer networks together form reversibly, and fall apart if appropriate external stimuli are supplied. This means that the bonds can be repeatedly and controllably rearranged. Several chemical reactions can be applied to make suitably reversible covalent bonds in these materials3, but Bowman and co-workers used the thiol–ene reaction, in which a thiol compound (R–SH, where R is a hydrocarbon group) is added to a carbon–carbon double bond through a mechanism involving free radicals.
Having prepared their elastomeric polymer network, the authors added a photoinitiator — a compound that generates radicals when exposed to ultraviolet light. These radicals were a crucial part of the adaptable network: once formed, they triggered a reversible 'addition–fragmentation chain transfer' (AFCT) reaction4. The net result of this reaction was to shuffle the connectivity of the network around, breaking and remaking the chemical crosslinks holding the elastomer together.
Bowman and colleagues went on to study what happens if AFCT occurs at selected regions while the elastomer is stretched. Under these conditions, the covalent crosslinks in the affected areas break apart, leaving two reactive ends (Fig. 1b). The authors reasoned that this would cause a sharp drop in the density of crosslinks in those areas, so that the material would turn into a viscous fluid that undergoes plastic flow when the mechanical stress is applied — in much the same way that wet chewing gum flows when stretched. The flow would release the stress until the material reached a new equilibrated morphology, whereupon the reactive ends could find each other again and form new crosslinks, freezing the altered structure in place (Fig. 1c).
The authors found that a thin film of their polymer did indeed behave in this way. When they shone ultraviolet light through a mask onto a stretched thin film of the polymer, the resulting plastic flow caused the material to move away from the light-exposed regions. Overall, the surface of the film developed little bumps of material in areas that remained dark, and shallow depressions in the irradiated regions. This demonstrated that a combination of light irradiation and mechanical deformation can be used to precisely control the topology of light-responsive elastomers, establishing a new technique that Bowman and colleagues dubbed2 mechanophotopatterning (MPP). Remarkably, the intrinsic material properties of the polymer network remained unchanged after deformation, because the total number of crosslinks was unchanged.
Bowman and colleagues demonstrated that a variety of surface topologies can be produced using MPP, making it potentially useful for many applications. An additional feature of the technique makes it even more broadly useful. When the authors irradiated an optically thick sample of their polymer (a sample through which ultraviolet light could not pass completely), a differential stress was generated between the top and the bottom faces of the sheet. To relieve this stress, the sheet bowed into a three-dimensional shape. Such shapes could have promising applications in the manufacture of lenses for advanced optical systems.
The surface patterns made by the authors2 using MPP typically contained features that had lateral dimensions of several hundred micrometres and heights of several micrometres, but there is no reason that micrometre-sized features with heights in the tens of nanometres would not form equally well using suitable masks. Whether the definition of MPP-generated features will be comparable to that of features made using alternative patterning techniques, such as those that rely on buckling and creasing to release mechanical stress5,6, remains to be seen.
Despite the advanced mechanical properties of the material described by Bowman and co-workers, the chemistry involved is straightforward and could be used to make covalent adaptable networks in which the polymer chains have a range of chemical groups attached. I think that the most exciting applications of MPP lie at the interface of cell biology and tissue engineering. One particularly promising application would be to incorporate covalent adaptable networks into flexible scaffolds modified to incorporate ligands that encourage cell binding. Cells are known to exert mechanical forces on substrates to which they bind7. Using substrates amenable to MPP would provide an opportunity to dynamically change the mechanical feedback that the substrate gives to a bound cell. This in turn could provide insights into the impact of mechanical forces on cellular function8, and help in the design of new materials suitable for tissue engineering, especially in cases involving cells that require different mechanical stimuli as a tissue develops9.