Electrospun Coated Metal Implants

Metal implants are often used in cases where mechanical support is required. Various metals, in particular those with mechanical properties closer to bone, have been investigated for use as implant material. Titanium and its alloy are commonly used as orthopedic or dental implants due to its biocompatibility, good strength and excellent chemical stability. Magnesium alloy is also a potential candidate as its mechanical properties are near to natural bone and it is biodegradable which avoids the need for a second surgical procedure. Gradual degradation of the magnesium alloy will also aid in the smooth transfer of stress to the newly formed bones and facilitate long term recovery of the bone. As with most implants, it is important to foster good integration of the metal implants with the host tissue.

Metal is a commonly used material as collector for electrospun fibers, thus it is relatively straight forward to coat a metal implant with electrospun nanofibers. This has been demonstrated on titanium implant [Ravichandran 2009] and electrospun fibers have also been deposited on magnesium alloy sheet [Soujanya et al 2014] and stainless steel [Khosravi et al 2020]. To enhance integration, mineralization has been carried out on the nanofibers to form polymer-hydroxyapatite composite. Collagen is often used as one of the components in the nanofibers to enhance mineralization [Iafisco et al 2012, Ravichandran 2009]. Ravichandran (2009) showed that adhesion of mesenchymal stem cells (MSC) on titanium plate and alloy is significantly enhanced with nanofiber coating even with man-made polymer such as poly(lactic acid)-co-poly(glycolic acid) (PLGA). Incorporation of the PLGA nanofiber with collagen and hydroxyapatite (HA) further increases the adhesion of MSCs on the implant surface. Titanium alloy coated with PLGA/collagen and HA nanofibers showed better proliferation and exhibited significantly greater alkaline phosphatase activity and mineral secretion than untreated titanium alloy. The relative ease of blending solution for electrospinning has allowed researchers to use multiple additives with selected polymer matrices to create a coating conducive to the intended implant environment. Khosravi et al (2020) investigated the use of electrospun fiber as a coating for 316L stainless steel implants. Poly(ε-caprolactone) (PCL) was selected as the base material to be electrospun due to its known biocompatibility. However, PCL is also known to be hydrophobic hence gelatin was added to reduce its hydrophobicity and to improve cell adhesion. Since this coating is for a bone implant, forsterite (Mg₂SiO₄) has been added to create a better osteoconductive environment. Forsterite has been known to release Mg ions after implantation which enhances cell proliferation and bone regeneration. In vitro test by immersion of the electrospun PCL/gelatin/forsterite fibers in simulated body fluid (SBF) solution has shown significantly greater hydroxyapatite (HA) formation on the scaffold containing 3 wt% forsterite nanoparticles compared to that with 1 wt% forsterite nanoparticles in both 3 and 7 days. Li et al (2012) went further to load nanofiber with anti-bacterial gentamicin for coating on the titanium implant. The resultant coated implant demonstrated antibacterial efficacy for 1 week against Staphylococcus aureus with significant reduction in adhesion compared with bare titanium implants. The gentamicin-loaded nanofibers did not show any cytotoxicity on osteoblasts. Zhang et al (2014) demonstrated in vitro and in vivo efficacy of vancomycin-loaded poly(lactic-co-glycolic acid) (PLGA) in preventing infection. In vitro showed a drug release profile of an initial burst release on day 1 followed by slow and controlled release over 28 days without any observable cytotoxicity. The drug loaded nanofibrous coating on titanium implant was shown to prevent infection in an animal fracture model where Staphylococci aureus was deliberately introduced to the implant site of the rabbit. In contrast tibia without vancomycin-coated implant showed osteolysis and serious soft-tissue swelling in all animals.

Scanning electron microscopy (SEM) images and energy dispersive X-rays (EDAX) analysis of collagen-hydroxyapatite composite on titanium plate. [Iafisco et al 2012. Bioinorganic Chemistry and Applications, vol. 2012, Article ID 123953, 8 pages, 2012. doi.org/10.1155/2012/123953. This work is licensed under a Creative Commons Attribution 3.0 Unported License.]

Al-Khatee A et al (2023) used electrospinning to coat the surface of metallic implant with polycaprolactone (PCL)/chitosan/CaTiO₃ (CA1) fibers and PCL/chitosan/BaTiO₃. The solution for electrospinning was prepared by blending BaTiO₃ nanoparticles or CaTiO₃ nanoparticles into PCL and chitosan solution. The prepared solution was electrospun onto Ti-25Zr discs and MC3T3-E1 cells were cultured on coated and uncoated discs. Their study found much higher cell viability after seven days on PCL/Chitosan/CaTiO3 (CA1) coated discs at 272% compared to uncoated discs and ALP activity at 5 fold that of uncoated discs. Cell culture on PCL/Chitosan/BaTiO3 (BA1) coated disc compared to uncoated discs had 182% better cell viability and 4 fold increase in ALP activity. The coated discs showed inhibitory activities against Staphylococcus aureus (S. aureus) and Streptococcus mutans (Sterp. mutans) compared to uncoated discs. These antibacterial properties may be due to the presence of chitosan in the fiber. Therefore the nanocomposite fiber coating was able to demonstrate antibacterial properties and encourage cell proliferation and differentiation on metallic implants.

Having a nanofiber coating on the metal implant has been shown to improve cell adhesion and proliferation on the implant surface and this may potentially foster better integration between the implant and the host tissue. Beside better cell adhesion and proliferation, there are other important requirements such as the ability of the nanofibers to adhere on the surface of the implant without peeling off in vivo. For biodegradable nanofibers, the initial presence of nanofibers may encourage cell adhesion on the implant. However, integration between the host tissue and the implant needs to be investigated once the nanofibers are fully resorbed by the body.

Unlike titanium, magnesium alloys will degrade in physiological condition due to the presence of chloride ions which leads to the formation of magnesium chloride. Without any protection, this will lead to a rapid corrosion and degradation of the magnesium alloy. Electrospinning a layer of polycaprolactone fibers on the magnesium alloy has been shown to significantly reduce its degradation rate by up to five times lower than unprotected samples after 7 days immersion in simulated body fluid [Soujanya et al 2014]. Water contact angle test showed that magnesium alloy is hydrophilic while the alloy sheet with the polycaprolactone coating is hydrophobic. It is likely that the hydrophobicity of the fiber coating prevented or reduces the contact between the fluid and the alloy surface and therefore reduces the corrosion rate. However, based on corrosion protection alone, a PCL dip coating on Mg which fully cover the surface of the metal offered better corrosion resistance than electrospun nanofibrous coating with corrosion rate at 0.3 ng per second for the former and 0.9 ng per second for the latter [Lee et al 2014]. Since electrospun nanofibrous coating is porous, water may still come into contact with the surface of Mg and this will certainly initiate corrosion while the same material completely covered by PCL will only start to corrode after PCL dissolved. For an implant, corrosion resistance is not the only consideration. More tests need to be carried out to determine the effect on cell biocompatibility on the fiber coated magnesium alloy and its impact on corrosion rate.

A potential application of electrospun coated metal is the introduction of infection mitigation property post-surgery. During an open surgery, the wound may be exposed to bacteria which may lead to post-surgery infection. This may be mitigated by having nanofibrous coating on the implant which exhibits antibacterial properties. Aboelzahab et al (2012) electrospun Fe- and Ag-Doped TiO₂ nanofibers and test it against Staphylococcus aureus which is a common cause of infection in healthcare settings. Upon exposure of the TiO₂ nanofibers and 1 wt% Ag-doped TiO₂ nanofibers to infra-red, the survival rate of Staphylococcus aureus was reduced to less than 13% after just 30 s of exposure.

Kiran et al (2018) electrospun polycaprolactone (PCL) nanofibers incorporated with TiO₂ nanoparticles on titanium plate surface to increase its bioactivity and imbue antibacterial properties. Their results showed that TiO₂ nanoparticles concentration of 7 wt% in PCL nanofibers were toxic to human fetal osteoblastic cell (hFOB). However, at lower concentration of 5 wt% TiO₂, proliferation of hFOB cells were enhanced. Antibacterial properties at this concentration was effective against S.aureus with a significant reduction of bacteria to almost 76% under UV light. However, at lower concentration of 2 wt% TiO₂, there was insignificant reduction in bacteria.

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