Surface Modification Techniques for Zirconia-Based Bioceramics: A Review

Abstract

Zirconia is gaining interest as a ceramic biomaterial for implant applications due to its biocompatibility and desirable mechanical properties. At present, zirconia-based bioceramics is often seen in the applications of hip replacement and dental implants. This article briefly reviews different surface modification techniques that have been applied to zirconia such as polishing, sandblasting, acid etching, biofunctionalization, coating, laser treatment, and ultraviolet light treatment. The potential of surface modification to make zirconia a successful implant material in the future is highly dependent on the establishment of successful in vitro and in vivo studies. Hence, further effort should be made in order to deepen the understanding of tissue response to implant and tissue regeneration process.

INTRODUCTION

Zirconia is rapidly gaining interest as a ceramic biomaterial for implant applications. Zirconia has three different crystalline phases: a monoclinic phase, a cubic phase, and a tetragonal phase. The latter is the one that is clinically used. Yttrium, the chemical element with symbol Y and atomic number 39, is added for aging resistance and so the Yttria Tetragonal Zirconia Polycrystal (YTZP) is formed. This is a bioinert material and is six times harder than stainless steel. Moreover, YTZP has some other eloquent characteristics: (1) electrically neutral, (2) low thermal conductivity, (3) high resistance to high temperature, (4) high thermal shock resistance, (5) chemical stability, (6) color similar to tooth structure, (7) high strength, (8) fracture toughness, (9) biocompatibility, (10) high affinity for bone tissue, (11) noncarcinogenic property, (12) absence of an oncogenic effect, (13) exhibiting minimal ion release compared with metallic implants, and (14) zirconia grain serves as a nucleation site for development of calcium-based minerals.

A material surface is known to be uniquely reactive, with properties different from the bulk. The purpose of surface modification is to alter these surface properties to enhance the biological performance of the surface, without changing the bulk properties of the material. Many approaches have been made to improve surface properties of an implant material. Two key approaches are (1) optimization of roughness and (2) application of bioactive coating. The aim of this article is to discuss various surface modification techniques for zirconia-based implant bioceramics.

SANDBLASTING

Sandblasting, also known as airborne particle abrasion, produces a surface with micro-roughness. Several parameters affect the roughness on the implant surface—size, shape, kinetic energy of the particles, etc. During the sandblasting process, compressed air pressure creates impulse to eject the particles. Thus the kinetic energy obtained by the particles depends on the density, volume, and velocity of the particles. The main advantage of sandblasting is we get a homogenous and gentle anisotropic abrasion on hard materials such as ceramic, glass, and silicon. Alumina particles are usually preferred due to their low cost, hardness, and needle-like shape. The disadvantage of using sandblasting is that it could slightly alter the surface chemistry due to inevitable alumina contamination. This drawback can be overcome by acid etching treatment, which has been proven to remove alumina residues resulting from sandblasting. Many studies have proved that sandblasted zirconia surfaces show slight enhancement in cell attachment but its metabolic activity is less compared to etched zirconia surfaces.[1]

ACID ETCHING

Acid etching can be performed by hydrofluoric acid, nitric acid, or sulfuric acid. Heat treatment following acid etching helps to smoothen the sharp edges caused by etching process.[2] Advantages of acid etching is the homogenous roughening of the material regardless of the materials size and shape.[3] In addition to it, this method imposes no risk of delamination of material as it does not exert stress on the material.[4] However, disadvantage of acid etching is that it might cause chemical changes that are undesirable.[5] The topography formed by acid etching depends on prior treatment, composition of acid mixture, temperature, and treatment time.[3] Acid etching is used to fabricate a microscale surface texture with the ability to achieve interlocking between implant and bone. Recent studies have combined sandblasting and acid etching, which enhances the degree of micro-roughness to zirconia as well.[2,6] Such a combination has been proposed to optimize micro-roughness, which would provide an improved material surface for osteoblast cell attachment and proliferation.

SELECTIVE INFILTRATION ETCHING

This can be performed by coating the material surface with a specific infiltration glass and heating it above its glass transition temperature. Infiltration of molten glass then occurs between the material grains. This technique is limited only to a certain extent, because only the surface grains joined with the infiltration glass are involved and this allows a control over the area required to be treated. Traces of infiltration agent can be removed by immersion in 5% hydrofluoric acid, with a nanoscale produced on washing with water.[7] Selective infiltration etching has been applied to create a nano-porous surface on zirconia implant.[8] The advantage of this technique is that the surface chemistry of material remains unchanged and the nanoscale surface roughness can be enhanced without losing material or changing the microscopic surface roughness.

POLISHING

Polishing gives a smooth surface when compared to acid etching and sandblasting. It is known that the epithelial cells are more likely to adhere to the rough surface (acid etching and sandblasting) compared to smooth (polishing) surface, whereas fibroblast adhere well onto both rough and smooth surface.[9] Polishing of a zirconia surface is performed using silicon carbide polishing paper and diamond or silica suspension with a polishing machine.[8] Mechanical surface treatment including polishing permits a change to the surface topography without modifying the surface chemistry. The average surface roughness of polished zirconia biomaterial ranges from 8 to 200nm.[8,10] Apart from giving a smooth texture, polishing also serves to clean the implant surface to a certain extent.

LASER TREATMENT

Unlike blasting and etching, laser treatment exerts no risk of surface contamination as there is no contact between the laser and the biomaterial.[11] Also laser surface treatments improve material wettability, by altering the surface properties, which in turn plays a key role in cell adhesion.[12] Wettability test can be conducted by putting a drop of liquid on a flat solid material surface and the contact angle is used to represent the final shape of the drop. Higher contact angle indicates a lower wettability of material.[12] The influence of wettability affects protein adsorption and cell adhesion. Protein adsorption relies on the nature of the protein-bearing aqueous solution, and cells are believed to behave differently in response to different organization of adsorbed protein layers.[13]

ULTRAVIOLET LIGHT TREATMENT

Several studies have revealed that bone implant contact of implants treated with ultraviolet (UV) light was highly enhanced due to the effect of superhydrophilicity.[14,15] A material is described as superhydrophilic when the water droplet contact angle is less than 5°. Hydrophilicity is one of the key factors involved with the initial interaction with proteins and cells, which is beneficial for the early phases of wound healing and osseointegration.[16] Hydroxyl (−OH) and oxygen (−O₂) groups are formed on the outermost layer when the hydrophilic oxide surface binds to water.[17] In the tissue fluid, the formation of hydroxylated oxide surface would improve the surface reactivity with the surrounding ions, amino acids, and proteins.[18] Osteoblasts cultured on hydrophilic surfaces have been shown to exhibit higher level of differentiation markers, including alkaline phosphatase and osteocalcin, compared to hydrophobic surfaces.[18] The effect of hydrophilic surfaces on osseointegration can also be supported by improvements in the bone implant contact and bone anchorage during bone healing in the early stages.[19] Other than hydrophilicity, the atomic percentage of hydrocarbon has also been studied and it has been shown to change after UV treatment.[20]

COATING

Yttrium stabilized zirconia (YSZ) reinforced hydroxyapatite (HA) coating has been fabricated to enhance its coating stability and adhesive strength.[21] Calcium phosphate (CP)–based coatings are commonly fabricated by plasma-spraying technique in industry due to its versatility.[22] In spite of its several flaws such as variation in coating/implant bond strength and modification of HA structure, this technique provides a high deposition rate and low cost. New techniques for depositing CP-based coatings are constantly being developed to address the issues associated with plasma spraying.[23] Pardun et al.,[24] synthesized a YSZ/HA coating by wet powder spraying (WPS) using a double action airbrush spray for an in vitro study. The advantage of WPS is its versatility in coating curved surfaces with different thickness. But the long-term stability of these coating has not yet been determined.

Electrophoretic deposition (EPD) has been proposed as a better alternative to traditional techniques due to its simplicity and high versatility.[25] A novel approach that combines both plasma electrolytic oxidation and EPD has been used to fabricate and coat zirconia/HA film on zirconium. Good biocompatibility, corrosion resistance, and bioactivity obtained through this process broaden its potential applications.[26] Apart from HA, silica is also widely used as a coating for zirconia. The use of RKKP, a well-known bioactive silica–based glass showed positive cell response and osseointegration. RKKP coating was fabricated using either an enameling and firing technique or thermal treatment.[27,28] Another silica bioactive glass, AP40, was determined to be comparable with RKKP in terms of reactivity.[28] Frandsen et al.[29] sputter coated a commercial zirconia implant with a film of titanium and conducted anodization to produce a coating of TiO₂ nanotubes on the surface. Besides enhanced osteoblast behavior, superhydrophilicity with a zero contact angle was observed on the treated implant.[29]

BIOFUNCTIONALIZATION

Biofunctionalization, also called as biomimetic surface modification, involves immobilization of biomolecules on the surface to alter their biochemical properties and biological responses.[15] Biofunctionalization allows anchorage of organic components such as proteins, enzymes, and peptides on the implant surface to take control of the implant tissue interface,[30] thereby determines the type of tissue that develops.[31] Arginine–glycine–aspartate (RGD) is widely utilized as an adhesive peptide. Many adhesive proteins are found to possess RGD as their cell recognition site, including fibronectin, fibrinogen, and collagen. These RGD sequences are identified by at least one of the integrins. Adhesion proteins and integrins make up a pair that provides cell anchorage, differentiation, and growth signal. RGD peptide has proven to be successfully immobilized on the YTZP with this method and thus enhances the biocompatibility of the material.[32]

SELF-ASSEMBLY

Self-assembly is defined as an autonomous process by which components organize into patterns or structures without external intervention.[33] Self-assembled monolayers (SAMs) are spontaneously formed by solution deposition through the immersion of a proper substrate into a solution of an active surfactant in an appropriate solvent (e.g., organic or aqueous) or by aerosol spraying or vapor deposition of the active organic compound onto the solid surface. The driving force for self-assembly is usually the specific interaction between the head group of the surfactant and the surface of the substrate. Most surfactants consist of three distinctive parts: (1) the surface active head group that binds strongly to the surface, (2) the terminal group that is located at the monolayer surface shows the interfacial properties of the assembly, and (3) the alkane chain serves as a linker between the head and the terminal groups and facilitates the packing of the molecules in the monolayer with the Van der Waals interactions between adjacent methylene groups that orient and stabilize the monolayer.[34,35,36,37] By, therefore, carefully composing the mixture of substrate, SAM solution, and subsequent terminal functionalization, a multitude of subsequent molecule adhesion is feasible.

CONCLUSION

The scope of surface modification techniques on bioceramic, to improve cellular response in implantation, is still ongoing, but promising. In time, surface modification will be the way to improve the reliability of implantation for humans.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.