Characterization of silica-based and borate-based, titanium-containing bioactive glasses for coating metallic implants
Introduction
In the field of prosthetics, two technologies for attaching the residual limb and the prosthetic implant are widely utilized: socket attachment and direct skeletal (or bone-anchored) attachment [1]. Socket attachment is the most common method [2], with designs already established for the different applications, e.g. below, through or above-knee amputations [3], [4], [5], [6]. In general, socket attachment consists of wrapping the prosthetic limb around the residual limb, where the prosthesis serves as the socket for the residual limb, with quadrilateral and ischial containment sockets being the most noteworthy technologies [7]. Compared to socket attachment, direct skeletal attachment (DSA) is a relatively new technology, where an implant is attached directly to the patient's bone at the residual limb. Upon healing, the implant in DSA serves as the attachment mechanism between the prosthesis and the body [1]. In achieving osseointegration, the implant is permanently connected to the bone, resulting in high force and moment interaction between the prosthesis and the body [8]. DSA technology offers the advantage over socket technology via a reduction in skin-related complications and residual limb constraints within the socket, which is due to the limited direct contact between the prosthetic implant and the skin [9].
Titanium is regularly used in prosthetics due to its ability to create a permanent bond to bone, via osseointegration [10], [11], a condition achieved when there is no relative motion between the implant and the bone with which it is in direct contact [12]. It is this characteristic that has also made DSA devices more favorable than socket attachment for prosthetics. Nonetheless, there are concerns regarding DSA that include potential infection, skin irritation and breakdown, implant failure and risk of a broken bone in the residual limb [13], [14], [15], [16], [17]. Addressing these concerns will aid in shifting the current paradigm from socket attachment towards DSA.
It is important to understand the overall mechanics of the DSA system, as loads that may negatively affect the residual limb bone may occur in this situation [17]. This places the patients at risk of requiring additional treatment if the bone weakens or fractures due to incomplete osseointegration or due to detrimental bone remodeling induced by stress shielding [18]. Different approaches have been taken towards improving the patient's experience with regards to DSA, including modifying the implant surface by chemical etching with hydrochloric and sulfuric acid, sandblasting, titanium plasma-spraying, hydroxylapatite (HA) plasma-spraying, coating the implant with a titanium dioxide (TiO2) layer through anodic oxidation, and with bioactive glass [9], [19], [20], [21]. Among these methods, HA coating has been used for over 20 years, exploiting its ability to promote bone ingrowth [22], [23], [24]; yet there are concerns with HA use as it has no mechanism to retard bacterial or biofilm colonization at the implant site. Coatings have also been produced based on chlorhexidine and silicone with ammonia couplings [25], [26], but these have little clinical applicability due to erosion of the compounds as they migrate to the surface. Of these approaches, bioactive glasses have showed encouraging results [19].
The use of bioactive materials has proliferated since the development of Hench's 45S5 Bioglass® in the 1960s [27] due to its favorable interaction with living tissue. Bioglass was the first synthetic to chemically adhere to both hard and soft tissue [27]. While Hench acknowledged that Bioglass® is unsuitable as a coating [28], he developed criteria for an optimal bone replacement material [29], which included that “the material should resorb at the same rate that bone is regenerated, with byproducts that are beneficial and easily excreted by the body so that bone will restore to a healthy state”. In-situ degradation of these materials makes them desirable for clinical applications owing to the release of beneficial ions to the surrounding tissues promoting antibacterial behavior, bone formation and growth, tissue healing, etc. [30], [31], [32]. Bioactive glasses have been employed for coating metals [33], [34], [35], yet some of these proposed compositions contain aluminum [33], [35], which has been associated with defective bone mineralization alongside concerns over its neurotoxicity [36]. Other compositions have been deficient in zinc [34], [35], an antibacterial component [32], [37], [38] to aid in the healing process, also known to inhibit the growth of caries-related bacterial such as Streptococcus mutans [39]. Although virtually all materials facilitate biofilm formation which may lead to bacterial infection, bacteria attach less readily to glass [40], providing a rationale for a glass-based solution. As bioactive glasses influence genetic expression, differentiation and cell proliferation by the release of ions [31], [41], [42], [43], engineering control of the biological response via dissolution products creates an opportunity for innovation. The proposed compositions in this work are expected to provide superior performance as they are expected to inhibit bacterial growth due to the addition of zinc, while the absence of aluminum minimizes the possibility of the coating causing toxicity in surrounding tissues. Furthermore, incorporating titanium in the glass compositions is expected enhance osseointegration [10], [11], [12].
This study outlines the characterization of two novel bioactive glass series, a silica-based glass series and a borate-based glass series that contain increasing amounts of titanium oxide (TiO2). Titanium is employed to exploit its osseointegrative capability at the interface of the metallic implant and the bone. TiO2 will be added in increments of 5 mol% up to 20 mol%. Characterization techniques included energy dispersive spectroscopy (EDS), X-ray diffraction (XRD) differential scanning calorimetry (DSC), Fourier transform infrared (FTIR) spectroscopy, particle size analysis (PSA) and magic-angle spinning-nuclear magnetic resonance (MAS-NMR).
Section snippets
Glass preparation
Silica-based and borate-based glasses were formulated for this study. The glass compositions, as well as the nomenclature, are reported in Table 1. TiO2 was added at the expense of SiO2 for the SRT series and at the expense of B2O3 for the BRT series. The glasses were prepared by weighing out appropriate amounts of analytical grade reagents (Fisher Scientific, Ottawa, ON, Canada & Sigma-Aldrich, Oakville, ON, Canada), firing (1400–1500 °C for 1 h for the silica-based glasses, 1200 °C for 1 h for
Network connectivity (NC)
Table 2 lists the network connectivity calculations for the fired glass formulations. For both glass series, the addition of TiO2 contributing to BO did not alter significantly the network connectivity regardless of ZnO contribution of BO or NBO. In considering the contribution of TiO2 of NBO in the form of TiO62 −, network connectivity decreased as TiO2 is increased, with lower connectivity achieved as ZnO contributed to NBO as network modifier Zn2 +.
X-ray diffraction (XRD)
Crystallinity of the fired glasses was
Discussion
With respect to the silica-based series, amorphous glasses were achieved for glass SRT3, which contains 15 mol% of TiO2, evidenced by the amorphous hump found in the XRD traces; whereas partial crystallinity (i.e. the amorphous hump remained visible in the background of the XRD traces, as exhibited by SRT3) was found for glasses SRT0, SRT1, SRT2 and SRT4, containing 0 mol%, 5 mol%, 10 mol% and 20 mol% TiO2, respectively. XRD traces were compared to the ICDD database, and identified as Sodium Calcium
Conclusions
Incorporation of TiO2 to silica-based and borate-based glasses was achieved through a standard glass-firing process and characterization techniques were employed to evaluate the intrinsic features of these glasses. MAS-NMR proved the role of P2O5 as a network modifier for both glass series by evidencing only Q0 structures (and Q1 structures for the silica-based glasses with crystal structures), whereas FTIR proved the role of TiO2 as a network modifier by lack of peaks assignable to titanium
Acknowledgments
The authors would like to thank the Collaborative Health Research Project fund (#315694-DAN) for financing this research and the Ryerson University Strategic Hire program for early assistance with Mr. Rodriguez's stipend. Additionally, the authors would like to thank Tim Keenan, Yiming Li, Ehsan Zeimaran, Qiang Li, Glenn Facey, Satoshi Hayakawa, Simon Chon and the Malaysian Office of Higher Education High Impact Research program (HIR3/EP8) for assisting with aspects of data collection. Marcello
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