Eliminating biofilms with self-propelled photothermal nanoswimmers
(Nanowerk Spotlight) Biofilm formation is an important adaptation and survival strategy commonly employed by bacteria. Biofilms are cohesive communities of bacteria that are able to stick to almost any surface and they impact humans in many ways as they can form in natural, medical, and industrial settings.
In biofilm, most microorganisms are protected by self-produced extracellular polymeric substances (EPS). Bacteria cells existing in biofilm are stronger against antibiotics, and they have lower metabolism. It is challenging to completely eliminate matured biofilms because the biofilms can decrease the penetration of antibacterial agents such as antibiotics and disinfectants through the extracellular matrix. Biofilms also tend show resistance against the human immune system.
According to National Institutes of Health estimates, 65% of all microbial and 80% of all chronic infections are associated with biofilm formation. These infections can lead to implant failure and even death.
A common technique to remove biofilm, especially in hospitals, is photothermal therapy (PTT). This method utilizes materials with strong light-absorption ability to convert the energy of near-infrared laser (NIR) light into heat, and subsequently damages the surrounding bacterial cells through local hyperthermia.
One limitation of PTT in fighting – especially thicker – biofilms is the short effective distance. While effective on the surface of biofilms, it cannot kill bacteria in deeper layers of the biofilm.
"Nanotechnology, in particular liposome-based nanocarriers and surface modification of nanoparticles, have been employed to tackle this problem," Xiaogang Qu, a professor of chemistry at Changchun Institute of Applied Chemistry, explains to Nanowerk. "The thermosensitive liposomes that endure phase transition with temperature change enhance the permeability of encapsulated NIR photothermal therapeutic agents and drugs. pH-responsive surface charge transformable gold nanorods have shown good penetration through bacterial biofilms for in situ photothermal ablation of biofilm."
"Unfortunately" he adds, "most current antibiofilm platforms still essentially rely on passive nanostructures that lack the force required for biofilm penetration beyond their passive mass transport limitation."
Micro- and nanoscale synthetic motors that convert diverse energies into movement can potentially penetrate deeply into biofilms and remove biofilms in vivo. However, in vivo applications of artificial motors would require the elimination of external fuels; improved propulsion time in complex biological environments; enhanced cargo loading capacity; higher therapeutic efficacy; and biocompatibility.
The NIR-driven nanoswimmer fabricated by Qu's team is the first demonstration of a self-propelled antibiofilm platform capable of conducting photothermal and antibiotic therapy in the deep layers of biofilm, achieving high therapeutic efficiency in vivo without damaging healthy tissues.
In the past, researchers designed functionalized microswimmers that locally convert diverse energy sources into mechanical movement for antibacterial applications. For instance, urea-driven micromotor with enhance photodynamic toxicity could kill Escherichia coli (E. coli) efficiently (Adv. Funct. Mater. 2019, 29, 1807727). However, a urea fuel solution was required for self-propulsion, which cannot be implemented in vivo due to toxicity concerns.
Another design, biohybrid microswimmers using magnetotactic bacteria propulsion, was capable of penetrating into the E. coli biofilm, and releasing the antibiotic in response to the low pH of the biofilm microenvironment (ACS Nano 2017, 11, 9968-9978).
However, the in vivo antibiofilm applications of these microswimmers is dramatically limited by their relatively large sizes and the requirement for an external power source.
Schematic illustration for synthesis of self-propelled NIR light-driven nanoswimmer (HSMV) and the motion enhanced synergistic antibiofilm therapy upon laser irradiation. (Reprinted with permission by American Chemical Society) (click on image to enlarge)
The team's nanoswimmers have an average diameter of 180 nm and are hollow silica semi-shells with embedded gold nanoparticles and loaded with Vancomycin, an antibiotic used to treat a number of bacterial infections.
In their experiments, the researchers exposed mature S. aureus biofilms to suspensions of their HSMV Nanoswimmers. In the absence of NIR laser radiation, the nanoswimmers move randomly due to Brownian motion and didn't penetrate the biofilms.
If driven by NIR laser light, they performed efficient self-propulsion and penetrated into and diffused throughout the biofilm within 5 minutes. The active motion of HSMV increases effective distance of photothermal therapy and also improves the therapeutic index of antibiotic, resulting in a superior biofilm removal rate.
"The NIR laser irradiation used in our work is 650 nm, which has not enough penetration depths in biological tissues," Qu notes. "As a result, the in vivo performance and functions of our nanomotor may be limited. Compared with the widely studied NIR-I biological window, the laser light in the NIR-II region (1000 to 1350 nm) has a more desirable penetration depth, and possesses higher power safety limit. We plan to construct NIR-II-driven nanoswimmer as an alternative method to solve the penetration depth problem."
Having confirmed the propulsion behavior, cargo delivery ability, high therapeutic efficiency and negligible toxicity profile of this synthetic nanomotors in the treatment of biofilms grown in living mice, the scientists envision the construction of alternative biocompatible motors and fuel or fuel-free actuation to achieve active movement toward different parts of the body for in vivo drug delivery and other biomedical applications.
Unlike other types of nanomaterials commonly used in diagnostics and therapeutics, micro- and nanomotors can move to a predetermined destination through a predefined route, which has the potential to deliver reagents to disease areas with a high efficiency and reduced side effects.
Qu cautions that there are still several issues that need to be addressed prior to applying these artificial motors in practical applications: 1. How to remain their full functionality in vivo during their intended time of operation? 2. How to effectively remove the nanomotors from the living body after they complete their task? 3. How to construct intelligent and multifunctional micro-/nanomotors for diagnostics and therapeutics at the same time?