Abstract
Polymers are widely used in many applications in the field of biomedical engineering. Among eclectic selections of polymers, those with low melting temperature (Tm < 200 °C), such as poly(methyl methacrylate), poly(lactic-co-glycolic acid), or polyethylene, are often used in bone, dental, maxillofacial, and corneal tissue engineering as substrates or scaffolds. These polymers, however, are bioinert, have a lack of reactive surface functional groups, and have poor wettability, affecting their ability to promote cellular functions and biointegration with the surrounding tissue. Improving the biointegration can be achieved by depositing hydroxyapatite (HAp) on the polymeric substrates. Conventional thermal spray and vapor phase coating, including the Food and Drug Administration (FDA)-approved plasma spray technique, is not suitable for application on the low Tm polymers due to the high processing temperature, reaching more than 1000 °C. Two non-thermal HAp coating approaches have been described in the literature, namely, the biomimetic deposition and direct nanoparticle immobilization techniques. In the current review, we elaborate on the unique features of each technique, followed by discussing the advantages and disadvantages of each technique to help readers decide on which method is more suitable for their intended applications. Finally, the future perspectives of the non-thermal HAp coating are given in the conclusion.
Highlights
The uses of polymers are ubiquitous in today’s world due to their low synthesis cost, tunable mechanical properties to suit the intended application, non-toxic degradation products, and ease of manufacturing [1,2,3,4,5,6]
Depending on the technique used to knock the ions off the target, Physical vapor deposition (PVD) methods are identified as pulsed electron deposition if the ions are pulled out from the target through collisions with electrons [50], or as pulsed laser deposition if the method uses a high-power laser beam to bombard the target, resulting in a gaseous phase that consists of atoms and ions, which propel towards the substrate as a plasma plume [51]
By carrying out a 3-point bending test on the poly(methyl methacrylate) (PMMA) sheets, we found that the ultimate stress (p = 0.481) and strain at break (p = 0.279) were similar to the pristine PMMA (Figure 4A–C)
Summary
The uses of polymers are ubiquitous in today’s world due to their low synthesis cost, tunable mechanical properties to suit the intended application, non-toxic degradation products, and ease of manufacturing [1,2,3,4,5,6]. A common feature of the abovementioned techniques is high processing and/or annealing temperature that can reach a temperature above 1000 ◦C This obviously limits their application for biomaterials with relatively low melting temperature (Tm), such as poly(methyl methacrylate) (PMMA; Tm = 160 ◦C), poly(ethylene glycol) (PEG; Tm = 60 ◦C), polylactic acid (PLA; Tm = 160 ◦C), and poly(ε-caprolactone) (PCL; Tm = 60 ◦C), to name a few [37]. Methods, such as thermal spraying and sputter coating, can only be applied on surfaces that are in the line of sight and, are not amenable for coating devices with complex dimensions or with pores [38]. We end the review with a summary and future perspective of non-thermal HAp coating
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