Characterization of silica-based and borate-based, titanium-containing bioactive glasses for coating metallic implants

Highlights

• Incorporation of TiO₂ into silica and borate-based glasses.

• Greater processing window of borate over silica-based glasses favorable in coating.

• Glass transition temperature of borate-based glasses not influenced by TiO₂ addition.

• Role of P₂O₅ as network modifier verified through MAS-NMR and FTIR for both series.

Abstract

Bioactive glasses have found applications in diverse fields, including orthopedics and dentistry, where they have been utilized for the fixation of bone and teeth and as scaffolds for drug delivery. The present work outlines the characterization of two novel titanium-containing glass series, one silica-based and one borate-based. For the silica-based series, titanium is added at the expense of silicon dioxide whereas for the borate-based series, it is added at the expense of boron oxide as confirmed by Energy Dispersive Spectroscopy. Amorphous structures are obtained for silica-based glass at 15 mol% TiO₂ and for borate-based glasses at 0 mol% and 5 mol%, with low crystal peak intensities exhibited within the remaining glasses. MAS-NMR proves 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 proves that Ti acted as a network modifier in the glass as there was an absence of peaks assignable to titanium bonding. This implies that the two glass series will degrade in-situ and release ions at the site of implantation. Additionally, thermal data sourced from these glasses indicate processing windows which make them suitable for enameling onto implants, with the borate-based series exhibiting greater processing windows over the silica-based series, hence making the borate glasses more suitable for coating metallic implants compared to their silica-based counterparts.

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).