Aging of plasma-polymerized allylamine nanofilms and the maintenance of their cell adhesion capacity

The Application Progress of Nonthermal Plasma Technology in the Modification of Bone Implant Materials

With the accelerating trend of global aging, bone damage caused by orthopedic diseases, such as osteoporosis and fractures, has become a shared international event. Traffic accidents, high-altitude falls, and other incidents are increasing daily, and the demand for bone implant treatment is also growing. Although extensive research has been conducted in the past decade to develop medical implants for bone regeneration and healing of body tissues, due to their low biocompatibility, weak bone integration ability, and high postoperative infection rates, pure titanium alloys, such as Ti-6A1-4V and Ti-6A1-7Nb, although widely used in clinical practice, have poor induction of phosphate deposition and wear resistance, and Ti-Zr alloy exhibits a lack of mechanical stability and processing complexity. In contrast, the Ti-Ni alloy exhibits toxicity and low thermal conductivity. Nonthermal plasma (NTP) has aroused widespread interest in synthesizing and modifying implanted materials. More and more researchers are using plasma to modify target catalysts such as changing the dispersion of active sites, adjusting electronic properties, enhancing metal carrier interactions, and changing their morphology. NTP provides an alternative option for catalysts in the modification processes of oxidation, reduction, etching, coating, and doping, especially for materials that cannot tolerate thermodynamic or thermosensitive reactions. This review will focus on applying NTP technology in bone implant material modification and analyze the overall performance of three common types of bone implant materials, including metals, ceramics, and polymers. The challenges faced by NTP material modification are also discussed.

Allylamine coating on zirconia dental implant surface promotes osteogenic differentiation in vitro and accelerates osseointegration in vivo

OBJECTIVES

The glow discharge plasma (GDP) procedure has proven efficacy in grafting allylamine onto zirconia dental implant surfaces to enhance osseointegration. This study explored the enhancement of zirconia dental implant properties using GDP at different energy settings (25, 50, 75, 100, and 200 W) both in vitro and in vivo.

MATERIALS AND METHODS

In vitro analyses included scanning electron microscopy, wettability assessment, energy-dispersive X-ray spectroscopy, and more. In vivo experiments involved implanting zirconia dental implants into rabbit femurs and later evaluation through impact stability test, micro-CT, and histomorphometric measurements.

RESULTS

The results demonstrated that 25 and 50 W GDP allylamine grafting positively impacted MG-63 cell proliferation and increased alkaline phosphatase activity. Gene expression analysis revealed upregulation of OCN, OPG, and COL-I. Both 25 and 50 W GDP allylamine grafting significantly improved zirconia's surface properties (p < .05, p < .01, p < .001). However, only 25 W allylamine grafting with optimal energy settings promoted in vivo osseointegration and new bone formation while preventing bone level loss around the dental implant (p < .05, p < .01, p < .001).

CONCLUSIONS

This study presents a promising method for enhancing Zr dental implant surface's bioactivity.

Response of Osteoblasts on Amine-Based Nanocoatings Correlates with the Amino Group Density

Increased life expectancy in industrialized countries is causing an increased incidence of osteoporosis and the need for bioactive bone implants. The integration of implants can be improved physically, but mainly by chemical modifications of the material surface. It was recognized that amino-group-containing coatings improved cell attachment and intracellular signaling. The aim of this study was to determine the role of the amino group density in this positive cell behavior by developing controlled amino-rich nanolayers. This work used covalent grafting of polymer-based nanocoatings with different amino group densities. Titanium coated with the positively-charged trimethoxysilylpropyl modified poly(ethyleneimine) (Ti-TMS-PEI), which mostly improved cell area after 30 min, possessed the highest amino group density with an N/C of 32%. Interestingly, changes in adhesion-related genes on Ti-TMS-PEI could be seen after 4 h. The mRNA microarray data showed a premature transition of the MG-63 cells into the beginning differentiation phase after 24 h indicating Ti-TMS-PEI as a supportive factor for osseointegration. This amino-rich nanolayer also induced higher bovine serum albumin protein adsorption and caused the cells to migrate slower on the surface after a more extended period of cell settlement as an indication of a better surface anchorage. In conclusion, the cell spreading on amine-based nanocoatings correlated well with the amino group density (N/C).

Optimized Gingiva Cell Behavior on Dental Zirconia as a Result of Atmospheric Argon Plasma Activation

Several physico-chemical modifications have been developed to improve cell contact with prosthetic oral implant surfaces. The activation with non-thermal plasmas was one option. Previous studies found that gingiva fibroblasts on laser-microstructured ceramics were hindered in their migration into cavities. However, after argon (Ar) plasma activation, the cells concentrated in and around the niches. The change in surface properties of zirconia and, subsequently, the effect on cell behavior is unclear. In this study, polished zirconia discs were activated by atmospheric pressure Ar plasma using the kINPen^®09 jet for 1 min. Surfaces were characterized by scanning electron microscopy, X-ray photoelectron spectroscopy (XPS), and water contact angle. In vitro studies with human gingival fibroblasts (HGF-1) focused on spreading, actin cytoskeleton organization, and calcium ion signaling within 24 h. After Ar plasma activation, surfaces were more hydrophilic. XPS revealed decreased carbon and increased oxygen, zirconia, and yttrium content after Ar plasma. The Ar plasma activation boosted the spreading (2 h), and HGF-1 cells formed strong actin filaments with pronounced lamellipodia. Interestingly, the cells' calcium ion signaling was also promoted. Therefore, argon plasma activation of zirconia seems to be a valuable tool to bioactivate the surface for optimal surface occupation by cells and active cell signaling.

Cold atmospheric plasma as an interface biotechnology for enhancing surgical implants

Cold atmospheric plasma (CAP) has been intensively researched for direct treatment of living cells and tissues. Significant attention is now being given to its indirect applications in plasma medicine. Surgical implant is an exemplary conveyor to deliver the therapeutic effects of plasma to patients. There is a constant drive to enhance the clinical performance of surgical implants, targeting at the implant-tissue interface. As a versatile and potent tool, CAP is capable of ameliorating surgical implants using various strategies of interface biotechnology, such as surface modification, coating deposition, and drug delivery. Understanding the chemical, physical, mechanical, electrical, and pharmacological processes occurring at the implant-tissue interface is crucial to effective application of CAP as an interface biotechnology. This preclinical review focuses on the recent advances in CAP-assisted implant-based therapy for major surgical specialties. The ultimate goal here is to elicit unique opportunities and challenges for translating implant science to plasma medicine.

Enhancement of Intracellular Calcium Ion Mobilization by Moderately but Not Highly Positive Material Surface Charges

Electrostatic forces at the cell interface affect the nature of cell adhesion and function; but there is still limited knowledge about the impact of positive or negative surface charges on cell-material interactions in regenerative medicine. Titanium surfaces with a variety of zeta potentials between −90 mV and +50 mV were generated by functionalizing them with amino polymers, extracellular matrix proteins/peptide motifs and polyelectrolyte multilayers. A significant enhancement of intracellular calcium mobilization was achieved on surfaces with a moderately positive (+1 to +10 mV) compared with a negative zeta potential (−90 to −3 mV). Dramatic losses of cell activity (membrane integrity, viability, proliferation, calcium mobilization) were observed on surfaces with a highly positive zeta potential (+50 mV). This systematic study indicates that cells do not prefer positive charges in general, merely moderately positive ones. The cell behavior of MG-63s could be correlated with the materials’ zeta potential; but not with water contact angle or surface free energy. Our findings present new insights and provide an essential knowledge for future applications in dental and orthopedic surgery.

Efficacy of Plasma-Polymerized Allylamine Coating of Zirconia after Five Years

Plasma-polymerized allylamine (PPAAm) coatings of titanium enhance the cell behavior of osteoblasts. The purpose of the present study was to evaluate a PPAAm nanolayer on zirconia after a storage period of 5 years. Zirconia specimens were directly coated with PPAAm (ZA0) or stored in aseptic packages at room temperature for 5 years (ZA5). Uncoated zirconia specimens (Zmt) and the micro-structured endosseous surface of a zirconia implant (Z14) served as controls. The elemental compositions of the PPAAm coatings were characterized and the viability, spreading and gene expression of human osteoblastic cells (MG-63) were assessed. The presence of amino groups in the PPAAm layer was significantly decreased after 5 years due to oxidation processes. Cell viability after 24 h was significantly higher on uncoated specimens (Zmt) than on all other surfaces. Cell spreading after 20 min was significantly higher for Zmt = ZA0 > ZA5 > Z14, while, after 24 h, spreading also varied significantly between Zmt > ZA0 > ZA5 > Z14. The expression of the mRNA differentiation markers collagen I and osteocalcin was upregulated on untreated surfaces Z14 and Zmt when compared to the PPAAm specimens. Due to the high biocompatibility of zirconia itself, a PPAAm coating may not additionally improve cell behavior.

Plasma Polymerized Allylamine-The Unique Cell-Attractive Nanolayer for Dental Implant Materials

Biomaterials should be bioactive in stimulating the surrounding tissue to accelerate the ingrowth of permanent implants. Chemical and topographical features of the biomaterial surface affect cell physiology at the interface. A frequently asked question is whether the chemistry or the topography dominates the cell-material interaction. Recently, we demonstrated that a plasma-chemical modification using allylamine as a precursor was able to boost not only cell attachment and cell migration, but also intracellular signaling in vital cells. This microwave plasma process generated a homogenous nanolayer with randomly distributed, positively charged amino groups. In contrast, the surface of the human osteoblast is negatively charged at −15 mV due to its hyaluronan coat. As a consequence, we assumed that positive charges at the material surface—provoking electrostatic interaction forces—are attractive for the first cell encounter. This plasma-chemical nanocoating can be used for several biomaterials in orthopedic and dental implantology like titanium, titanium alloys, calcium phosphate scaffolds, and polylactide fiber meshes produced by electrospinning. In this regard, we wanted to ascertain whether plasma polymerized allylamine (PPAAm) is also suitable for increasing the attractiveness of a ceramic surface for dental implants using Yttria-stabilized tetragonal zirconia.

Nanoroughness, Surface Chemistry, and Drug Delivery Control by Atmospheric Plasma Jet on Implantable Devices

Implantable devices need specific tailored surface morphologies and chemistries to interact with the living systems or to actively induce a biological response also by the release of drugs or proteins. These customized requirements foster technologies that can be implemented in additive manufacturing systems. Here, we present a novel approach based on spraying processes that allow to control separately topographic features in the submicron range (∼60 nm to 2 μm), ammine or carboxylic chemistry, and fluorophore release even on temperature-sensitive biodegradable polymers such as polycaprolactone (PCL). We developed a two-steps process with a first deposition of 220 nm silica and poly(lactic- co-glycolide) (PLGA) fluorescent nanoparticles by aerosol followed by the deposition of a fixing layer by an atmospheric pressure plasma jet (APPJ). The nanoparticles can be used to create the nanoroughness and to include active molecule release, while the capping layer ensures stability and the chemical functionalities. The process is enabled by a novel APPJ which allows deposition rates of 10-20 nm·s^-1 at temperatures lower than 50 °C using argon as the process gas. This approach was assessed on titanium alloys for dental implants and on PCL films. The surfaces were characterized by Fourier transform infrared, atomic force microscopy, and scanning electron microscopy (SEM). Titanium alloys were tested with the preosteoblast murine cells line, while the PCL film was tested with fibroblasts. Cell behavior was evaluated by viability and adhesion assays, protein adsorption, cell proliferation, focal adhesion formation, and SEM. The release of a fluorophore molecule was assessed in the cell growing media, simulating a drug release. Osteoblast adhesion on the plasma-treated materials increased by 20% with respect to commercial titanium alloy implants. Fibroblast adhesion increased by a 100% compared to smooth PCL substrates. The release of the fluorophore by the dissolution of the PLGA nanoparticles was verified, and the integrity of the encapsulated drug model was confirmed.

Minimal attachment of Pseudomonas aeruginosa to DNA modified surfaces

Extracellular deoxyribonucleic acid (eDNA) exists in biological environments such as those around medical implants since prokaryotic or eukaryotic cells can undergo processes such as autolysis, necrosis, and apoptosis. For bacteria, eDNA has been shown to be involved in biofilm formation and gene transfer and acts as a nutrient source. In terms of biofilm formation, eDNA in solution has been shown to be very important in increasing attachment; however, very little is known about the role played by surface immobilized eDNA in initiating bacterial attachment and whether the nature of a DNA layer (physically adsorbed or covalently attached, and molecular weight) influences biofilm formation. In this study, the authors shed light on the role that surface attached DNA plays in the early biofilm formation by using Si wafers (Si) and allylamine plasma polymer (AAMpp) coated Si wafers to adsorb and covalently immobilize salmon sperm DNA of three different molecular weights. Pseudomonas aeruginosa was chosen to study the bacterial interactions with these DNA functionalized surfaces. Characterization of surface chemistry and imaging of attached bacteria were performed via x-ray photoelectron spectroscopy (XPS), scanning electron microscopy, and epi-fluorescence microscopy. XPS results confirmed the successful grafting of DNA on the AAMpp and Si surfaces, and surprisingly the results showed that the surface attached DNA actually reduced initial bacterial attachment, which was contrary to the initial hypothesis. This adds speculation about the specific role played by DNA in the dynamics of how it influences biofilm formation, with the possibility that it could actually be used to make bacterial resistant surfaces.

Controlled Attachment of Pseudomonas aeruginosa with Binary Colloidal Crystal-Based Topographies

Micro- and nanotopographies can interfere with bacteria attachment, however, the interplay existing between surface chemistry and topography remains unclear. Here, self-assembled spherical micrometer- silica and nanometer poly(methyl methacrylate) (PMMA)-sized particles are used to make binary colloidal crystal (BCC) topographical patterns to study bacterial attachment. A uniform surface chemistry of allylamine plasma polymer (AAMpp) is coated on the top of the BCCs to study only the topography effects. The uncoated and coated BCCs are exposed to Pseudomonas aeruginosa, and the surfaces and bacteria are characterized using scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and fluorescence microscopy. It is found that bacteria attachment to the uncoated BCCs is delayed and individual cells are attracted to the small particle regions of the patterns. Surprisingly, this phenomenon is also observed for the AAMpp-coated BCCs, with bacteria attaching to the small particle regions of the pattern, in stark contrast to uniform flat films of AAMpp that are highly adhesive toward P. aeruginosa. Also, the overall levels of bacterial attachment are significantly reduced by the BCC patterns compared to controls. Thus, there is a trade-off that exists between chemistry and topography that can be exploited to delay the onset of P. aeruginosa biofilm formation on surfaces.

Enhanced calcium ion mobilization in osteoblasts on amino group containing plasma polymer nanolayer

Background

Biomaterial modifications—chemical and topographical—are of particular importance for the integration of materials in biosystems. Cells are known to sense these biomaterial characteristics, but it has remained unclear which physiological processes bio modifications trigger. Hence, the question arises of whether the dynamic of intracellular calcium ions is important for the characterization of the cell–material interaction. In our prior research we could demonstrate that a defined geometrical surface topography affects the cell physiology; this was finally detectable in a reduced intracellular calcium mobilization after the addition of adenosine triphosphate (ATP).

Results

This new contribution examines the cell physiology of human osteoblasts concerning the relative cell viability and the calcium ion dynamic on different chemical modifications of silicon–titanium (Ti) substrates. Chemical modifications comprising the coating of Ti surfaces with a plasma polymerized allylamine (PPAAm)-layer or with a thin layer of collagen type-I were compared with a bare Ti substrate as well as tissue culture plastic. For this purpose, the human osteoblasts (MG-63 and primary osteoblasts) were seeded onto the surfaces for 24 h. The relative cell viability was determined by colorimetric measurements of the cell metabolism and relativized to the density of cells quantified using crystal violet staining. The calcium ion dynamic of osteoblasts was evaluated by the calcium imaging analysis of fluo-3 stained vital cells using a confocal laser scanning microscope. The positively charged nano PPAAm-layer resulted in enhanced intracellular calcium ion mobilization after ATP-stimulus and cell viability. This study underlines the importance of the calcium signaling for the manifestation of the cell physiology.

Conclusions

Our current work provides new insights into the intracellular calcium dynamic caused by diverse chemical surface compositions. The calcium ion dynamic appears to be a sensitive parameter for the cell physiology and, thus, may represent a useful approach for evaluating a new biomaterial. In this regard, reliable in vitro-tests of cell behavior at the interface to a material are crucial steps in securing the success of a new biomaterial in medicine.

A Cell-Adhesive Plasma Polymerized Allylamine Coating Reduces the In Vivo Inflammatory Response Induced by Ti6Al4V Modified with Plasma Immersion Ion Implantation of Copper

Copper (Cu) could be suitable to create anti-infective implants based on Titanium (Ti), for example by incorporating Cu into the implant surface using plasma immersion ion implantation (Cu-PIII). The cytotoxicity of Cu might be circumvented by an additional cell-adhesive plasma polymerized allylamine film (PPAAm). Thus, this study aimed to examine in vivo local inflammatory reactions for Ti6Al4V implants treated with Cu-PIII (Ti-Cu), alone or with an additional PPAAm film (Ti-Cu-PPAAm), compared to untreated implants (Ti). Successful Cu-PIII and PPAAm treatment was confirmed with X-ray Photoelectron Spectroscopy. Storage of Ti-Cu and Ti-Cu-PPAAm samples in double-distilled water for five days revealed a reduction of Cu release by PPAAm. Subsequently, Ti, Ti-Cu and Ti-Cu-PPAAm samples were simultaneously implanted into the neck musculature of 24 rats. After 7, 14 and 56 days, peri-implant tissue was retrieved from 8 rats/day for morphometric immunohistochemistry of different inflammatory cells. On day 56, Ti-Cu induced significantly stronger reactions compared to Ti (tissue macrophages, antigen-presenting cells, T lymphocytes) and to Ti-Cu-PPAAm (tissue macrophages, T lymphocytes, mast cells). The response for Ti-Cu-PPAAm was comparable with Ti. In conclusion, PPAAm reduced the inflammatory reactions caused by Cu-PIII. Combining both plasma processes could be useful to create antibacterial and tissue compatible Ti-based implants.

Abrogated Cell Contact Guidance on Amino-Functionalized Microgrooves

Topographical and chemical features of biomaterial surfaces affect the cell physiology at the interface and are promising tools for the improvement of implants. The dominance of the surface topography on cell behavior is often accentuated. Striated surfaces induce an alignment of cells and their intracellular adhesion-mediated components. Recently, it could be demonstrated that a chemical modification via plasma polymerized allylamine was not only able to boost osteoblast cell adhesion and spreading but also override the cell alignment on stochastically machined titanium. In order to discern what kind of chemical surface modifications let the cell forget the underlying surface structure, we used an approach on geometric microgrooves produced by deep reactive ion etching (DRIE). In this study, we systematically investigated the surface modification by (i) methyl-, carboxyl-, and amino functionalization created via plasma polymerization processes, (ii) coating with the extracellular matrix protein collagen-I or immobilization of the integrin adhesion peptide sequence Arg-Gly-Asp (RGD), and (iii) treatment with an atmospheric pressure plasma jet operating with argon/oxygen gas (Ar/O₂). Interestingly, only the amino functionalization, which presented positive charges at the surface, was able to chemically disguise the microgrooves and therefore to interrupt the microtopography induced contact guidance of the osteoblastic cells MG-63. However, the RGD peptide coating revealed enhanced cell spreading as well, with fine, actin-containing protrusions. The Ar/O₂-functionalization demonstrated the best topography handling, e.g. cells closely attached even to features such as the sidewalls of the groove steps. In the end, the amino functionalization is unique in abrogating the cell contact guidance.

Free-Radical-Induced Grafting from Plasma Polymer Surfaces

With the advances in science and engineering in the second part of the 20th century, emerging plasma-based technologies continuously find increasing applications in the domain of polymer chemistry, among others. Plasma technologies are predominantly used in two different ways: for the treatment of polymer substrates by a reactive or inert gas aiming at a specific surface functionalization or for the synthesis of a plasma polymer with a unique set of properties from an organic or mixed organic-inorganic precursor. Plasma polymer films (PPFs), often deposited by plasma-enhanced chemical vapor deposition (PECVD), currently attract a great deal of attention. Such films are widely used in various fields for the coating of solid substrates, including membranes, semiconductors, metals, textiles, and polymers, because of a combination of interesting properties such as excellent adhesion, highly cross-linked structures, and the possibility of tuning properties by simply varying the precursor and/or the synthesis parameters. Among the many appealing features of plasma-synthesized and -treated polymers, a highly reactive surface, rich in free radicals arising from deposition/treatment specifics, offers a particular advantage. When handled carefully, these reactive free radicals open doors to the controllable surface functionalization of materials without affecting their bulk properties. The goal of this review is to illustrate the increasing application of plasma-based technologies for tuning the surface properties of polymers, principally through free-radical chemistry.

Continuous cellularization of calcium phosphate hybrid scaffolds induced by plasma polymer activation

The generation of hybrid materials based on β-tricalcium phosphate (TCP) and various biodegradable polymers like poly(l-lactide-co-d,l-lactide) (PLA) represents a common approach to overcoming the disadvantages of pure TCP devices. These disadvantages lie in TCP's mechanical properties, such as brittleness. The positive characteristic of PLA - improvement of compressive strength of calcium phosphate scaffolds - is diametrically opposed to its cell attractiveness. Therefore, the objective of this work was to optimize osteoblast migration and cellularization inside a three-dimensionally (3D) printed, PLA polymer stabilized TCP hybrid scaffold by a plasma polymer process depositing amino groups via allylamine. MG-63 osteoblastic cells inside the 10mm hybrid scaffold were dynamically cultivated for 14days in a 3D model system integrated in a perfusion reactor. The whole TCP/PLA hybrid scaffold was continuously colonized due to plasma polymerized allylamine activation inducing the migration potential of osteoblasts.