Chameleons can famously change their colors to camouflage themselves, communicate and regulate their temperature. Scientists have tried to replicate these color-changing properties for stealth technologies, anti-counterfeiting measures and electronic displays, but the materials have limitations. Now, researchers have developed a flexible film that changes color in response to stretching, pressure or humidity. They report their results in ACS Applied Materials & Interfaces.

By tensing or relaxing their skin, chameleons can change the way light reflects from guanine crystals under the surface, producing what's known as structural coloration. These structural colors are different from the pigments that give many other creatures their hues. Scientists have mimicked the crystalline nanostructures of chameleon skin in various color-changing materials, but they're typically difficult to produce, or they rely on non-renewable petroleum resources. In contrast, cellulose nanocrystals are a renewable material that can self-assemble into a film with iridescent structural colors. However, the films are typically fragile and, unlike chameleon skin, can't be stretched without breaking. Fei Song, Yu-Zhong Wang and colleagues wanted to develop a highly flexible film made of cellulose nanocrystals that changes color when stretched.

To increase the flexibility of cellulose nanocrystals, the researchers added a polymer called PEGDA and used UV light to crosslink it to the rod-shaped nanocrystals, producing films with bright iridescent colors ranging from blue to red, depending on the PEGDA amount. The films were both strong and flexible, stretching up to 39% of their original length before breaking. During stretching, the color of one film gradually changed from red to green, and then changed back when relaxed. According to the researchers, this is the first time that stretching- and relaxing-induced, reversible structural color changes that are brilliant and visible to the naked eye have been realized for cellulose nanocrystal materials. The film also changed color with pressure and humidity, allowing the team to show or hide writing made by an inkless pen. The new bio-based smart skin could find applications in strain sensing, encryption and anti-counterfeiting measures, the researchers say.

Delivering proteins to testes could someday treat male infertility

Delivering a protein (red fluorescence) to mice testes with a fibroin nanoparticle-encapsulated cationic lipid complex (green) restored male fertility.
Courtesy: Adapted from ACS Nano 2020, DOI: 10.1021/acsnano.0c04936.

According to the Mayo Clinic, about 15% of couples are infertile, and male infertility plays a role in over one-third of these cases. Often, problems with sperm development are to blame. Now, researchers reporting in ACS Nano have found a way to deliver a protein important for sperm cell production directly to mouse testicles, where it restored normal sperm development and allowed previously infertile mice to father pups.

Male infertility often happens because of a lack of sperm in the semen, which can result from damage to the blood-testis barrier (BTB). This barrier protects reproductive cells from harmful toxicants and drugs, and a protein called PIN1 is important for its function. Mice genetically engineered to lack PIN1 are infertile, with small testes, depleted sperm stem cells and a low sperm count. Although scientists have considered gene therapies to treat male infertility, these procedures are risky because they could cause unwanted genetic changes in reproductive cells that might be passed onto offspring. Hyun-Mo Ryoo and colleagues wanted to develop a system to deliver proteins (such as PIN1) instead of genes to the testes, but first they had to find a way to get proteins through the complex tubes of the testicles and into cells.

The researchers developed a delivery system called Fibroplex, which consisted of spherical nanoparticles made of silk fibroin and a coating of lipids. They loaded PIN1 into Fibroplex, and showed that the particles appeared safe and didn't show signs of toxicity or testicular damage in mice. When the team injected the PIN1-loaded Fibroplex into the testes of young mice with PIN1 deletions, the treatment restored nearly normal PIN1 levels and sperm stem cell numbers and repaired the BTB. Treated mice had normal testicular weight and size and about 50% of the sperm count of wild-type mice. Until about 5 months after treatment, when the protein degraded, the PIN1-Fibroplex-treated mice fathered a similar number of pups as wild-type mice, whereas untreated mice with PIN1 deletions remained infertile. This is the first demonstration of direct delivery of proteins into the testis to treat male infertility, the researchers say.