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A D V A N C E D M A T E R I A L S & P R O C E S S E S | O C T O B E R 2 0 1 8 3 2 improved material conformity and de- veloping composite UHMWPE-hydroxy- apatite graded materials, which can further reduce wear [9] . Synthetic hydro- xyapatite replicates the naturally occur- ring mineral component of bone. Use of UHMWPE is still considered the gold standard, and the novel use of fillers that enhance the mechanical and lubri- cation properties of UHMWPE are an in- teresting area of research. Mixing UHMWPE with nanofiller materials, which increase the surface area-to-volume ratio, is reported to lead to rapid interaction and improvement in the chemical and physical properties of UHMWPE. Nanofiller materials with good tribological characteristics in- clude carbon nanofibers (CNF), carbon nanotubes (CNT), and graphene [10,11] . Although some results show that the UHMWPE-filler composite is less bio- compatible than traditional UHMWPE, other studies demonstrate a signifi- cant improvement in terms of mechan- ical performance and wear resistance, and in the case of CNT, enhanced bone growth and improved antioxidant prop- erties [12] . Lahiri et al. [11] show that the addition of 1 wt% graphene to UHM- WPE improves wear resistance by 4.5 times. However, Baena et al. [12] re- port that the efficacy of using carbon nanofillers depends on homogeneous dispersion throughout the polymer ma- trix, and further work is needed to op- timize their incorporation within the complex, highly viscous UHMWPE poly- mer matrix. In addition to CNFs and CNTs, par- ticle reinforcement using other hard particles is also a very promising and novel area of study. Incorporating tita- nium particles, zirconium, platinum- zirconium quasicrystals, natural coral particles, and quartz reduces UHMWPE wear by 36% to 60% [13,14] . A possible mechanism could be that the hard par- ticles bear the load and protect the polymer matrix. However, the detach- ment and migration of hard particles into the joint space where they can serve as a third body and accelerate wear within the bearing is a concern [12] . The use of ion-beam surface mod- ification as a technique to enhance the performance of UHMWPE in joint arthroplasty was recently investigat- ed. Boampong et al. [15] reports that ion beam texturing hardens, stiffens, and increases UHMWPE wettability, result- ing in a reduction in wear compared with the control material. They also note that nitrogen ion implantation im- proves tribology and enhances surface mechanical properties. In addition to surface texturing, novel surface coating techniques that deposit a thin film onto the UHMWPE surface have been investigated. A nylon coating was demonstrated to achieve lower cytotoxicity, less wear debris-in- duced osteolysis, and superior mechan- ical properties compared with neat UHMWPE [16] . Results of studies of zirco- nium carbon nitride (ZrC x N 1–x ) coatings with embedded silver nanoparticles for both its antimicrobial properties and corrosion resistance show im- proved mechanical and tribological performance compared with the con- trol material. Calderon et al. [17] report that a hydrogenated diamond-like car- bon (DLCH) coating on UHMWPE im- proves wear resistance, hardness, and biocompatibility. BACTERIA AND BIOFILMS Bacterial infection and biofilm for- mation are major complications associ- ated with implants and are exceedingly difficult to treat. There is still concern about antibiotic-resistant strains and the difficulties of treating bone infec- tion with systemically administered antibiotics. Infection can arise both during the short term (after surgical placement) and long term (throughout the implant lifetime). Development of drug-eluting joint implants that can sus- tain delivery of a drug and maintain the necessary mechanical strength to with- stand loading has remained elusive. However, Suhardi et al. [18] report devel- opment of a novel antibiotic-releasing UHMWPE to treat prosthetic joint in- fection. The polymer contains irregu- larly shaped antibiotic clusters, which enable release of effective drug doses over an extended time without compro- mising material strength. Results ob- tained from a lupine animal model of prosthetic joint infection show that the promising antibiotic-releasing polymer successfully eliminated infection, while implantation of a drug-release bone ce- ment spacer was not effective. In addition, many new approach- es to developing antimicrobial polymer coatings for a wide variety of implant- ed medical devices, including ortho- pedic implants, are being studied. Various antimicrobial agents includ- ing antibiotics (e.g., gentamicin), silver compounds, antimicrobial peptides, and nitric oxide, as well as strategies to inhibit bacterial adhesion (e.g., an- tifouling polymer brushes, surface to- pographies) have been incorporated into polymers, which can be applied to the implant as well as PMMA bone ce- ment to allow for localized antimicro- bial effects [19] . The use of bioresorbable polymers such as poly(lactic-co-glycolic acid), poly(glycolic acid), poly (lactic acid), and natural antimicrobial poly- mers (e.g., chitosan) enable controlled delivery of the antimicrobial agents. A nanofiber-based electrospun composite coating consisting of poly (lactic-coglycolic acid) (PLGA) nano- fibers embedded in a poly( ε -capro- lactone) (PCL) film was developed to locally co-deliver combinatorial antibi- otics from the implant surface. Using a preclinical animal model, Ashbaugh et al. [20] demonstrated that applica- tion of the coating resulted in complete bacterial clearance from the implant and surrounding bone and joint tissue while promoting osseointegration. Oth- er concerns when applying polymer- ic coatings to orthopedic implants are the associated cell-biomaterial interac- tions. Osseointegration of the implant is critical to ensure optimal fixation and improved implant longevity. There- fore, any coatings applied to the surface should also serve to optimize bony inte- gration (Fig. 3). SUMMARY All joints in the human body are susceptible to wear. To attain the level of efficacy seen in the natural synovial joint, artificial joint replacement must balance the elements of lubrication, friction, and wear. Generation of wear