Cold atmospheric plasma (CAP) has emerged as a powerful tool for modifying the surface properties of nanomaterials to enhance their antimicrobial activity. This approach leverages the reactive species generated during plasma treatment, including reactive oxygen and nitrogen species (RONS), as well as the introduction of functional groups such as amines, to create surfaces that inhibit microbial adhesion and proliferation. The technique is particularly relevant for biomedical applications, including dental implants and orthopedic devices, where preventing bacterial colonization is critical for long-term implant success.
Plasma processing involves the generation of partially ionized gas at near-ambient temperatures, making it suitable for treating heat-sensitive materials. The key parameters influencing the modification of nanomaterials include plasma power, treatment duration, gas composition, and working distance. For instance, power levels between 10-100 W and exposure times ranging from 30 seconds to 10 minutes have been shown to effectively functionalize surfaces without damaging the underlying material. The choice of gas—commonly argon, helium, nitrogen, or oxygen—dictates the type of reactive species produced. Nitrogen-rich plasmas, for example, promote the formation of amine groups, while oxygen plasmas enhance surface oxidation and ROS generation.
Surface modification via CAP introduces amine groups through reactions between plasma-generated radicals and the nanomaterial surface. These amine groups not only improve hydrophilicity but also provide active sites for further chemical conjugation of antimicrobial agents. Additionally, the plasma treatment generates reactive oxygen species such as hydroxyl radicals (•OH), superoxide anions (O₂•⁻), and hydrogen peroxide (H₂O₂), which contribute to oxidative stress on microbial membranes. Studies have demonstrated that CAP-treated titanium dioxide nanoparticles exhibit a 3-4 log reduction in bacterial viability against Staphylococcus aureus and Escherichia coli due to the combined effects of surface amines and ROS.
In dental implants, CAP-treated nanostructured titanium surfaces have shown enhanced resistance to biofilm formation. The modified surfaces reduce the adhesion of Streptococcus mutans and Porphyromonas gingivalis, two primary pathogens associated with peri-implantitis. The amine-functionalized surfaces also promote osteoblast adhesion and proliferation, improving osseointegration while simultaneously preventing bacterial colonization. Similarly, in orthopedic applications, CAP-modified hydroxyapatite coatings on metallic implants exhibit dual functionality: they enhance bone cell attachment while reducing the risk of postoperative infections caused by methicillin-resistant Staphylococcus aureus (MRSA).
The antimicrobial efficacy of CAP-treated nanomaterials is influenced by the stability of the introduced functional groups and the persistence of ROS. While amine groups remain stable for extended periods, the reactive oxygen species tend to diminish over time. To address this, researchers have explored post-plasma chemical grafting of long-lasting antimicrobial agents such as quaternary ammonium compounds or silver nanoparticles. This hybrid approach ensures sustained antimicrobial activity without compromising biocompatibility.
Processing parameters must be carefully optimized to balance antimicrobial performance with material integrity. Excessive plasma exposure can lead to surface etching or degradation of nanostructured coatings, particularly in polymers. For instance, polyetheretherketone (PEEK) nanomaterials used in spinal implants require lower power settings (20-40 W) and shorter treatment times (1-3 minutes) to avoid excessive surface roughness that could compromise mechanical properties. In contrast, ceramic-based nanomaterials like alumina or zirconia tolerate higher energy inputs, enabling deeper functionalization.
Applications in orthopedic devices extend beyond infection prevention. CAP-treated nanostructured surfaces can also modulate immune responses by reducing macrophage-induced inflammation. The presence of amine groups has been shown to downregulate pro-inflammatory cytokine release while promoting anti-inflammatory signaling pathways. This immunomodulatory effect is particularly beneficial for patients with compromised healing capacities, such as those with diabetes or osteoporosis.
The scalability of CAP treatment for industrial applications remains a consideration. Roll-to-roll plasma systems have been developed for continuous processing of nanomaterial-coated medical devices, ensuring uniform treatment across large surface areas. However, batch processing is still prevalent for complex geometries like dental screw implants, where consistent plasma exposure requires precise control over gas flow and sample positioning.
Future directions include the integration of CAP with other surface modification techniques, such as plasma-enhanced chemical vapor deposition (PECVD), to create multilayered antimicrobial nanostructures. Additionally, advances in plasma diagnostics, such as optical emission spectroscopy, enable real-time monitoring of reactive species concentrations, allowing for dynamic adjustment of processing parameters to achieve desired surface properties.
In summary, cold atmospheric plasma treatment offers a versatile and effective method for enhancing the antimicrobial properties of nanomaterials used in dental and orthopedic implants. By optimizing plasma parameters and leveraging the synergistic effects of amine functionalization and ROS generation, researchers can develop next-generation implant surfaces that combine infection resistance with improved biocompatibility and osseointegration. The continued refinement of plasma processing techniques will further expand the applicability of this approach in biomedical nanotechnology.