To start, the group began analyzing hospital acquired infections, or HAIs, as Spontak said this particular segment of antimicrobial resistance in the U.S. (and all industrialized countries) is very real.
"This is where we started, with HAIs. We found that lurking deep inside hospitals were microbes, some of which proved to be quite deadly," Spontak said, citing 14,000 U.S. deaths last year from C-Difficile alone and the $45 billion in overall associated health care costs from HAIs.
Traditionally, microbes are eliminated through materials and processing; disinfectants, either chemical or via UV radiation; by modifying surface chemistries; or by using cationic hydrogels. However, Spontak said UV radiation is a temporary fix that requires a labor intensive approach, and certain surface metals like silver and copper will "work until they don't," as metals lose their toxicity to microbes over time.
Surfaces that have been chemically modified can crack, and hydrogels are effective but extremely soft "and not mechanically robust," Spontak said.
Enter antimicrobial photodynamic inactivation, where non-coherent visible light stimulates a photosensitizer dispersed in a non-polar TPE, producing a unique molecular oxygen species that can kill microbes.
The approach has its roots in photodynamic therapy on skin cancer and acne, Spontak said.
"The light produces a reactive oxygen species that will kill microbes, and microbes cannot develop a resistance to this," Spontak said.
Benefits to this approach include durability in toxicity toward microbes, in that even if the surface coating is damaged, the photosensitive material is well-dispersed throughout the entirety of the material.
The downside to antimicrobial photodynamic inactivation is that it takes longer compared to anionic inactivation (hours as opposed to minutes), to kill pathogens. Across a range of tested bacteria and viruses, the light-induced elastomer showed a "significant elimination of the pathogens," Spontak said.
Anionic inactivation is faster and more versatile in application (it can be molded to specific forms), but in some cases (such as contact with a person's perspiration), the infused styrenic TPE material can become neutralized.
In this approach, the stimulus to go pathogen hunting is water, Spontak said.
"When water is added to system, the surface pH on the TPE drops dramatically," Spontak said. "We're not talking a small change in pH, but rather a significant change. Staphylococcus won't survive below 1.7 pH, and these materials can get to a pH of less than 1 when water is introduced. This means the material is capable of killing even pH-resistant microbes."
Spontak said a TPE infused with this molecule can be "recharged" by exposing it to a weak acid. Results varied but were promising in this anionic inactivation approach as well, Spontak said.
"In summary, I want to stress that HAIs, drug-resistant bacteria and highly infectious microbes constitute a growing threat to our society," he said. "But we are learning that resistance is futile for microbes in these two approaches.
"The number of confirmed COVID cases in the U.S. has now exceeded one million, and aspects of the global economy are nearing collapse," he continued. "While previous events of similarly worldwide magnitude have adversely affected the global community, the present pandemic highlights society's susceptibility to contagious pathogens, as well as the failure of a responsive strategy to mitigate the rate and extent to which infection spreads."
