Researchers at the University of Pittsburgh (Pitt), US, achieved a breakthrough in mimicking biological responses in non-living organisms. The study is the first to show that polymeric gels, such as hydrogels, can be moulded and reconfigured through the application of light, a process that, for example, could revolutionise microfluidic devices by making a single system adaptable to a variety of functions — potentially a huge cost savings.
The Pitt team has developed a computational model for photo-responsive polymer gels that contain spirobenzopyran (SP) molecules and showed that these materials can undergo three-dimensional shape changes and simultaneously be driven to undergo self-sustained motion, a uniquely biomimetic behavior.
"The SP moieties are hydrophilic in the dark, but become hydrophobic under illumination with blue light," explains Anna C. Balazs , Distinguished Professor of Chemical and Petroleum Engineering in Pitt’s Swanson School of Engineering . By incorporating these molecules into gels in aqueous solutions, the gel’s shape can be dynamically altered by applying light, e.g., through photomasks to mould a gel into various shapes with features less than a millimetre in size. Furthermore, through repeatedly rastering the light source over a sample, the system exhibits sustained, directed motion. Introducing a temperature gradient can further control this autonomous movement.
"The results point to a robust method for controllably reconfiguring the morphology of soft materials and driving the self-organisation of multiple reconfigurable pieces into complex architectures," Balazs says.
Demonstration of self-sustained motion in polymeric gels could be a paradigm-shifter for light-based technologies. "The ability to remotely manipulate both the shape of the sample and its directed motion is critical for driving multiple samples to ‘recognise’ each other and to ultimately ‘dock’ to form soft, self-assembled structures with distinctive architectures," Balazs says. "Importantly, this light-controlled shape-changing and guided motion opens new routes for fabricating a range of dynamically reconfigurable materials. In particular, this ‘moulding’ technique permits the assembled pieces to be re-shaped and hence, the same sample can be used or reused for a range of different functionalities."
The technique could find application in microfluidic devices, revolutionary hand-held, portable chemical laboratories that can perform biological lab tests in minutes or seconds that used to take days or hours and required a room full of equipment, according to the expert. These microfluidic devices are typically made of polymeric components on glass. Reconfiguring the polymer allows one device to perform multiple functions simply by exposing the setup to different arrangements of light. That is not all. Light makes the surface hydrophobic and can be applied to change a surface’s wetting properties. For example, water could be directed by light to move along a very specific path in the microfluidic device.
The group also showed that the walls or floors of the device could be made to move by applying light. "In effect, you can introduce a micro-scale ‘conveyor belt’ or ‘elevator’ into the chamber just by moving the light over the chambers," Balazs suggests. "Thus, we designed new ways of regulating the traffic — the movement of cells and chemicals — in these microfluidic devices." The fact that systems are reconfigurable means that they are reusable. Such processes would have a dramatic effect on manufacturing and sustainability since the same sample could be used and reused for multiple applications.
What personally excites Balazs the most about the study is that it could give scientists the means to establish rules for making man-made materials behave like biological systems. "The ability to dynamically change shape in response to variations in the environment is a distinctly biological trait," she says, adding that her study was inspired by the adaptability of shape-changing biological species, such as the mimic octopus, who, in the presence of a predator, can contort its body and legs to assume the shape of the predator’s foe to fend off attack. "Through such environmentally driven shape-shifting, these biological species achieve one of their vital functions: survival." Next, Balazs says she and her colleagues "will model the effects of introducing microscopic fibres, which can also respond to light or other stimuli, to determine other means of manipulating the properties of the material." The study, co-authored by Balazs and Olga Kuksenok, research associate professor in the Swanson School, is detailed in the paper "Modeling the Photoinduced Reconfiguration and Directed Motion of Polymer Gels," published in Advanced Functional Materials .