Types of Plasma

There are two main types of plasma for implant surface modification – thermal plasma or atmospheric‐pressure plasma. Thermal, or hot, plasmas have conventionally been used for plasma spraying hydroxyapatite coatings onto implant surfaces, whereas, atmospheric‐pressure, or cold, plasmas have been used to modify implant surface chemistry and increase surface area (Figure 29.1) [17, 18]. The ability of atmospheric‐pressure plasmas to generate reactive oxygen and nitrogen species causes implant surface modification, thereby enhancing its osseointegrative properties.

Compared to thermal plasmas, atmospheric‐pressure plasmas have garnered much attention in the fields of dentistry and biomedicine for their practicality, efficiency, and cost‐effectiveness. Atmospheric‐pressure plasma can drive high‐temperature chemistry at low ambient temperatures while also serving as a suitable benchtop method of implant surface treatment that improves osseointegration immediately prior to insertion [19]. Additionally, atmospheric‐pressure plasma devices do not require the use of vacuum equipment, thus making them more portable and affordable [17]. In contrast, thermal plasmas have limited applications and researchers have argued against its use for treating implant surfaces [20]. It is for these reasons that atmospheric‐pressure plasma has dominated research related to implant surface modification. As a result, the following discussion primarily references studies involving the use of atmospheric‐pressure plasma.

Benefits

Plasma cleaning is a key method of improving the osseointegrative properties of dental implants. Plasma treatment can refine the implant surface for osseointegration by removing organic matter, increasing hydrophilicity, and eliminating microbial challenges. Plasma treatment is an efficient method of cleansing implant surfaces of organic contamination to promote osseointegration. Bone‐to‐implant contact percentages, representative of osseointegration, are commonly less than ideal, ranging between 52 and 78% ​[21]. These values may be attributed to the biological aging of implant surfaces; that is, organic matter and carbon compounds can accumulate on the surface of implants from atmospheric exposure during the manufacture‐to‐insertion period [22]. Studies have shown that plasma cleaning reduces the amount of carbon compound pollutants on the surface of titanium‐based implants, improving osseointegration and bone response. Using atmospheric‐pressure plasma treatment, Danna et al. [23] noted reduced carbon compounds on the surface and increased titanium and oxygen on pure titanium [23]. Tsujita et al. [24] also observed a decrease in the number of carbon pollutants on the surface of titanium screws that were subject to atmospheric‐pressure plasma treatment [24].

The wettability (or hydrophilicity) of implant surfaces improves the absorption of proteins and cell adherence as well as connective tissue network interaction. The role of surface energy and osteoblast cell adhesion is well defined in the literature – higher surface energy correlates with enhanced osteoblast adhesion and proliferation which modulates bone remodeling and osseointegration – and it is favorable to improve the hydrophilicity of implant surfaces to achieve higher surface energy [25–27]. Plasma is capable of conferring hydrophilicity to implant surfaces by modifying the oxide layer, which lies at the interface with the cells of the surrounding tissue, generating reactive species, and thus improving tissue adhesion [15, 28]. Studies from Coelho et al. [19] revealed higher surface hydrophilicity, enhanced cell recruitment, and increased surrounding tissue contact during healing for plasma‐treated titanium implants compared with those that were left untreated [19]. The researchers were also able to observe a higher quantity of bone surrounding roughened plasma‐treated implant surfaces, indicating improved osteoblast adhesion [19]. Duske et al. [15] similarly reported increased hydrophilicity and osteoblast cell attachment to plasma‐treated titanium implant surfaces [15]. The findings from these studies corresponded with current understandings of hydrophilic implant surfaces and cell adhesion reported in the literature [14, 29, 30].

Plasma cleaning of implant surfaces is also an effective method to improve osseointegration whilst disrupting bacterial attachment and aggregation. Titanium and titanium alloy implants that are roughened from sandblasting, grit blasting, and acid‐etching techniques promote bacterial adhesion due to the microscopic pits and grooves that form. These changes to implant surface topography create a crevice for the bacteria to latch onto, ultimately triggering the host‐inflammatory response and increasing the risk for peri‐implant diseases [2, 31]. Plasma treatment can inhibit the attachment of adherent hydrophobic bacteria, like Streptococcus sanguinis, by carbon‐cleaning implant surfaces and producing reactive oxygen and nitrogen species to increase hydrophilicity [32]. The surface energy and chemical properties of implants play a role in facilitating bacterial attachment, that is, materials that have high surface energy generally bind bacteria; however, the hydrophilic or hydrophobic properties of the surface and bacteria dictate attachment to materials with high surface energy [33, 34]. Hydrophobic surfaces bind hydrophobic bacteria rather than hydrophilic bacteria [35]. Interactions between reactive oxygen and nitrogen species from plasma and the implant surface change surface chemistry and topography by improving hydrophilicity and surface energy without altering surface roughness, thus decontaminating the implant, and discouraging further hydrophobic bacterial colonization. In addition to improving the hydrophilicity of the implant surface, the reactive oxygen and nitrogen species generated during plasma cleaning can directly modify bacterial membranes and invoke enzyme inhibition, leading to reactive oxygen and nitrogen species accumulation and bacterial cell death [36, 37]. Through decontamination experiments of titanium‐based implant nanonetwork structures, Zeng et al. further demonstrated that plasma treatment could effectively eradicate the biofilm of biofilm‐contaminated implant surfaces while preserving the nanostructure. Taken together, plasma cleaning is one of the best clinically available surface modification methods to sterilize dental implants and improve osseointegration.

Regarding clinical delivery, plasma cleaning is a cost‐effective, non‐toxic, and efficient means of implant surface modification. At atmospheric pressure, plasma can be obtained from plasma torch, transferred arc, corona discharge, dielectric barrier discharge, or plasma jet devices. From an efficiency standpoint, plasma cleaning is a superior method of surface modification for its ability to disrupt oral biofilm formation and simultaneously generate a hydrophilic surface favorable for osseointegration. Commercially, plasma is advantageous due to its portability, low cost, ease of operation, and versatile applications for implant treatment. For instance, atmospheric‐pressure plasma can be used to treat implants prior to insertion or as a therapeutic approach for peri‐implantitis. Zeng et al. report the benefits of a handheld, nonthermal atmospheric‐pressure plasma device that utilizes piezoelectric technology to eliminate oral biofilm and decontaminate implant surfaces in a more environmentally friendly manner than conventional UV and laser surface modification treatments.

Atmospheric‐pressure plasmas have also been found to be more convenient compared to earlier plasma technology, taking advantage of ambient pressure and temperature to drive plasma cleaning instead of low‐pressure, high‐temperature conditions that were previously required [19, 38]. This improves the functionality and usability of atmospheric‐pressure plasma devices, making them capable of use in the operating room prior to implant insertion. Moreover, a study by Berger et al. [39] investigated the benefits of an on‐site benchtop technique of plasma treatment for implants at the time of placement using a dielectric barrier discharge plasma device, reporting improved hydrophilicity, osteoblast adherence, and implant vitality with increased accessibility from using the dielectric barrier discharge device [39]. Atmospheric‐pressure microplasma devices may also prove to be clinically favorable for their portability and potential to generate plasma that increases implant surface energy under safe atmospheric conditions [19].

Atmospheric‐pressure Plasma Systems and Devices for Cleaning Implant Surfaces

Plasma can be generated at atmospheric pressure using electric transferred arcs, corona, discharge, dielectric barrier discharge, and plasma jet systems. In essence, the input of energy from these systems activates a fraction of gas molecules from the atmosphere, turning them into charged particles (electrons and ions) and reactive oxygen and nitrogen species while the remainder remains inert. It is these charged ions that make plasma active and confer its ability to modify implant surfaces. Traditionally, high temperatures (exceeding 3000 °C) are required to drive atmospheric‐pressure plasma generation (e.g. electric transferred arc plasma); however, modern systems (e.g. dielectric barrier discharge and plasma jet) have been able to generate plasma at lower temperatures (50–400 °C). These systems of atmospheric‐pressure plasma generation will be reviewed in this section.