image: Microscopy images of the photonic crystals after nano-indentation at 30 mN (left), 60 mN (middle) and 90 mN (right) are shown. view more
Credit: Yang lab
BOSTON, Aug. 16, 2015 -- A bomb blast or a rough tackle can inflict brain damage that destroys lives. Yet at the time of impact, these injuries are often invisible. To detect head trauma immediately, a team of researchers has developed a polymer-based material that changes colors depending on how hard it is hit. The goal is to someday incorporate this material into protective headgear, providing an obvious indication of injury.
The team will describe their approach in one of more than 9,000 presentations at the 250th National Meeting & Exposition of the American Chemical Society (ACS), the world's largest scientific society, taking place here through Thursday. A brand-new video on the research is available at http://bit.ly/acsforcepatch.
Recent research and media accounts have indicated that soldiers and professional athletes may suffer long-term complications -- such as memory loss, headaches and dementia -- stemming from past head trauma. In April, a lawsuit filed by a group of National Football League players was settled, requiring the organization to pay retired players with head injuries. And several professional hockey players are now suing the National Hockey League over the same issue. But even children playing contact sports may be at risk.
There is no easy way to tell if someone has just sustained a brain injury, so soldiers and athletes may unknowingly continue to do the very activity that caused the damage and potentially cause more harm. But a force-responsive, color-changing patch could prevent additional injury, says Shu Yang, Ph.D. "If the force was large enough, and you could easily tell that, then you could immediately seek medical attention," she explains.
Yang's team at the University of Pennsylvania used holographic lithography (HL) to create photonic crystals with carefully designed structures to give them a particular color, just like opals. Deforming the crystals with an applied force changes their internal structures, and thus the crystal's color. The material does not require power to detect forces and is lightweight, thus offering an attractive way for medical personnel to identify a damaging force on-site without the use of expensive tools. However, making these crystals is an expensive process that isn't suitable for the mass production, she says.
So the team turned to self-assembly and polymer-based materials that are cheaper to produce over a large area than the earlier HL method. Younghyun Cho, Ph.D., a postdoctoral fellow in Yang's lab, will describe the team's development, which could offer a path to commercialization.
The first step was to mold the polymer into a structure that worked just like the specialized photonic crystals. To make a mold, the researchers mixed up silica particles of various sizes and allowed them to self-assemble into crystals with the desired pattern. They heated the polymer, which infiltrated the mold, allowed it to solidify and then removed the silica mold, leaving behind the inversed polymer crystals.
The researchers then applied varying amounts of force to the polymer crystal and recorded the color change. The results were encouraging. "We were able to change the color consistently with certain forces," Yang says. For example, applying a 30 mN force -- approximately the force of a sedan moving at 80 miles per hour crashing into a brick wall -- caused the crystal to change from red to green. A force of 90 mN -- the equivalent of a speeding truck hitting that same wall -- turned the polymer purple, Cho adds.
"This force is right in the range of a blast injury or a concussion," Yang says.
In future studies, Yang plans to develop materials that can indicate how quickly a force is applied, which affects how damaging a particular trauma is on the brain.
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A press conference on this topic will be held Monday, Aug. 17, at 3 p.m. Eastern time in the Boston Convention & Exhibition Center. Reporters may check-in at Room 153B in person, or watch live on YouTube http://bit.ly/ACSLiveBoston. To ask questions online, sign in with a Google account.
Yang acknowledges funding from the Berkman Opportunity Fund. She also acknowledges her collaborators, Gang Feng, Ph.D., of Villanova University and Jie Yin, Ph.D., of Temple University.
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Title
Power-free mechanochromic sensors from force-recording, elastoplastic inverse opals
Abstract
Athletes, soldiers and even normal people could sustain a concussion or traumatic brain injury as a result of exposure to a certain type of mechanical force in sports or in accidents. It would be ideal if we could develop a power-free, wearable patch that can change color immediately and the appeared color can correspond to the level of the mechanical force. Photonic crystals are dimensionally periodic dielectric structures exhibiting a photonic stopband that determine the wavelength of reflected light. The displayed color can be tuned by varying the particle size, lattice spacing, volume filling fraction and refractive index contrast within the porous network. Many have created polymeric inverse opals that can be responsive to external stimuli to change colors. However, few can record the mechanical force in real-time or the recorded force is rather small.
Here, we created inverse opals as mechanochromic patches by backfilling the colloidal crystals of silica particles (diameter of 320, 285, and 238 nm) with thermoplastic photoresist, SU-8, followed by removal of the templates. Due to elastoplastic deformation of SU-8 films, the deformed inverse opals do not fully recover, allowing us to establish the relationship between the mechanical force and optical responses. The patches are highly sensitive; Δλ/Δε, the ratio of shift in the stopband wavelength to the change in applied strain, is 5.7 nm/%, the highest compared to literature data. When the normal forces of 30, 60, and 90 mN are applied to the pristine inverse opal (320 nm), the stopband blue-shifts to 570, 500, and 440 nm, respectively, corresponding to the pore diameter of 292, 263, and 233 nm, respectively, in good agreement with the pore size in the [111] direction, 297, 262, and 227 nm obtained from SEM images.
To reveal the underlying deformation mechanisms as well as the overall mechanical responses of the inverse opal of SU-8 under different loads, we carry out micromechanical modeling using the finite element method (FEM). The simulation results corroborate well with experiments, suggesting that the uniform variation in pore geometry under the compression play a key role in determining mechanochromic property. The inverse opals prepared from SU-8 can be potentially used as power-free mechanochmic sensors to measure the magnitude of shockwaves without the need of complicated instrumentation for in-situ imaging.