The remote activation of engineered biomaterials had been accomplished in cell culture, but not in animal models—at least, the external control of bioadhesive ligands had not been demonstrated. Aware that the next step was waiting to be taken, researchers at the Georgia Institute of Technology decided to explore the impact of in vivo temporal ligand presentation on cell-material responses. That is, the researchers plotted to sneak disguised biomaterials into living animals. The plan included the use of an external light trigger to activate the biomaterials when and where desired.

The engineered biomaterials contained peptide signaling molecules. And the external light trigger was provided by ultraviolet illumination, which penetrated the skin of test animals.

Details about the plan and its execution appeared December 15 in Nature Materials, in an article entitled, “Light-triggered in vivo activation of adhesive peptides regulates cell adhesion, inflammation, and vascularization of biomaterials.”

“We present a general strategy to temporally and spatially control the in vivo presentation of bioligands using cell-adhesive peptides with a protecting group that can be easily removed via transdermal light exposure to render the peptide fully active,” wrote the authors. “We demonstrate that noninvasive, transdermal time-regulated activation of cell-adhesive RGD peptide on implanted biomaterials regulates in vivo cell adhesion, inflammation, fibrous encapsulation, and vascularization of the material.”

If the light-triggering technique could be made to work in humans, it could help provide more precise timing for processes essential to regenerative medicine, cancer treatment, immunology, stem cell growth, and a range of other areas.

“Many biological processes involve complex cascades of reactions in which the timing must be very tightly controlled,” said Andrés García, Ph.D., a professor in the George W. Woodruff School of Mechanical Engineering at Georgia Tech and the principal investigator for the project. “Until now, we haven’t had control over the sequence of events in the response to implanted materials. But with this technique, we can deliver a drug or particle with its signal in the ‘off’ position, then use light to turn the signal ‘on’ precisely when needed.”

When biomaterials are introduced into the body, they normally stimulate an immune system response immediately. But the researchers used molecular cages like hats to cover binding sites on the peptides that are normally recognized by cell receptors, preventing recognition by the animal's cells. The cages were designed to detach and reveal the peptides when they encounter specific wavelengths of light.

During the five-year project, the research team—which included Ted Lee and Jose Garcia from Georgia Tech and Aranzazu del Campo from the Max Planck Institute—modified peptides that normally trigger cell adhesion to present the molecular cage in order to disguise them. They showed that disguised peptides introduced into animal models on biomaterials could trigger cell adhesion, inflammation, fibrous encapsulation, and vascularization responses when activated by light. They also showed that the location and timing of activation could be controlled inside the animal by simply shining light through the skin.

In the future, photochemists at Max Planck will be working on alternative cages that would be triggered by different wavelengths of light. As much as 90% of the ultraviolet light used in the experiments was lost in passing through the skin of the animal model, limiting the use of that wavelength to locations immediately below the skin.

Development of alternate “hats,” the molecular cages that protect the peptides, could allow sequential activation by light, and light activation of molecules at locations deeper inside the body. “This work,” concluded the authors of the Nature Medicine article, “shows that triggered in vivo presentation of bioligands can be harnessed to direct tissue reparative responses associated with implanted biomaterials.”

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