Improving Biocompatibility for Next Generation of Metallic Implants.

The increasing need for joint replacement surgeries, musculoskeletal repairs, and orthodontics worldwide prompts emerging technologies to evolve with healthcare's changing landscape. Metallic orthopaedic materials have a shared application history with the aerospace industry, making them only partly efficient in the biomedical domain. However, suitability of metallic materials in bone tissue replacements and regenerative therapies remains unchallenged due to their superior mechanical properties, eventhough they are not perfectly biocompatible. Therefore, exploring ways to improve biocompatibility is the most critical step toward designing the next generation of metallic biomaterials. This review discusses methods of improving biocompatibility of metals used in biomedical devices using surface modification, bulk modification, and incorporation of biologics. Our investigation spans multiple length scales, from bulk metals to the effect of microporosities, surface nanoarchitecture, and biomolecules such as DNA incorporation for enhanced biological response in metallic materials. We examine recent technologies such as 3D printing in alloy design and storing surface charge on nanoarchitecture surfaces, metal-on-metal, and ceramic-on-metal coatings to present a coherent and comprehensive understanding of the subject. Finally, we consider the advantages and challenges of metallic biomaterials and identify future directions.

3D Printing in alloy design to improve biocompatibility in metallic implants.

3D Printing (3DP) or additive manufacturing (AM) enables parts with complex shapes, design flexibility, and customization opportunities for defect specific patient-matched implants. 3DP or AM also offers a design platform that can be used to innovate novel alloys for application-specific compositional modifications. In medical applications, the biological response from a host tissue depends on a biomaterial's structural and compositional properties in the physiological environment. Application of 3DP can pave the way towards manufacturing innovative metallic implants, combining structural variations at different length scales and tailored compositions designed for specific biological responses. This study shows how 3DP can be used to design metallic alloys for orthopedic and dental applications with improved biocompatibility using in vitro and in vivo studies. Titanium (Ti) and its alloys are used extensively in biomedical devices due to excellent fatigue and corrosion resistance and good strength to weight ratio. However, Ti alloys' in vivo biological response is poor due to its bioinert surface. Different coatings and surface modification techniques are currently being used to improve the biocompatibility of Ti implants. We focused our efforts on improving Ti's biocompatibility via a combination of tantalum (Ta) chemistry in Ti, the addition of designed micro-porosity, and nanoscale surface modification to enhance both in vitro cytocompatibility and early stage in vivo osseointegration, which was studied in rat and rabbit distal femur models.

3D Printing for Bone Regeneration.

PURPOSE OF REVIEW: The purpose of this review is to illustrate the current state of 3D printing (3DP) technology used in biomedical industry towards bone regeneration. We have focused our efforts towards correlating materials and structural design aspects of 3DP with biological response from host tissue upon implantation. The primary question that we have tried to address is-can 3DP be a viable technology platform for bone regeneration devices? RECENT FINDINGS: Recent findings show that 3DP is a versatile technology platform for numerous materials for mass customizable bone regeneration devices that are also getting approval from different regulatory bodies worldwide. After a brief introduction of different 3DP technologies, this review elaborates 3DP of different materials and devices for bone regeneration. From cell-based bioprinting to acellular patient-matched metallic or ceramic devices, 3DP has tremendous potential to improve the quality of human life through bone regeneration among patients of all ages.

Enzyme-delivery Metal-organic Framework Composite Coatings for Restoration of Hyperglycemia-damaged Osteoblast Differentiation.

There is a significant clinical need to develop effective treatments for bone defects in patients with diabetes mellitus (DM), as they are at higher risk of fractures and impaired healing. Guided bone tissue engineering using biocompatible and biodegradable polymers is a promising approach. However, current diabetic bone regenerative therapies often fail due to the accumulation of advanced glycation products, which can affect the integration of traditional tissue engineering scaffolds with native bone. Therefore, novel approaches are needed to improve the efficacy of diabetic bone regeneration. This study presents a proof-of-concept development of a multifunctional polymer composite coating tailored towards restoring diabetes-related damage in osteoblast differentiation. Our composite system involves 3D-printed poly(caprolactone fumarate) (PCLF) and poly(caprolactone) (PCL) blend scaffolds coated with multifunctional chitosan methacrylate (chiMA). The chiMA coating is embedded with a sustained-release formulation of glucose oxidase (GOx) from MIL-127 metal-organic frameworks making the coating a stimuli-responsive biomolecule delivery system. The multifunctional coating is designed for the sustained release of GOx and sodium pyruvate for in vitro glucose modulation and oxidative stress reduction, respectively. We propose that sustained release of GOx from MIL-127 embedded chiMA coatings can modulate the high glucose (HG) cellular milieu towards normal glucose (NG), enhancing osteoblast (OB) differentiation via downstream effects. Our results show successful synthesis of MIL-127, encapsulation of GOx, and fabrication of composite coating on the PCLF/PCL scaffolds with effective enzyme activity measured as a function of lowering glucose concentration in HG media for 144 h to normal levels. In vitro evaluation of OB viability, attachment, proliferation, and differentiation showed an overall decrease in cellular activity in HG conditions, which was restored through the glucose-modulating functionality of the GOx-releasing MIL-127 coatings. Our results also presented preliminary evidence of a statistical correlation between DM-related gene markers and osteogenic markers in vitro that requires further exploration. Although this proof-of-concept study holds promise for advancing precision biomaterials development for diabetic tissue engineering and meeting the unmet clinical need for effective treatments and warrants future in vivo evaluation of the composite coating and molecular biology understanding of correlations between DM and osteogenic markers.

Biodegradable poly(caprolactone fumarate) 3D printed scaffolds for segmental bone defects within the Masquelet technique.

Segmental bone defects, often clinically treated with nondegradable poly(methylmethacrylate) (PMMA) in multistage surgeries, present a significant clinical challenge. Our study investigated the efficacy of 3D printed biodegradable polycaprolactone fumarate (PCLF)/PCL spacers in a one-stage surgical intervention for these defects, focusing on early bone regeneration influenced by spacer porosities. We compared nonporous PCLF/PCL and PMMA spacers, conventionally molded into cylinders, with porous PCLF/PCL spacers, 3D printed to structurally mimic segmental defects in rat femurs for a 4-week implantation study. Histological analysis, including tissue staining and immunohistochemistry with bone-specific antibodies, was conducted for histomorphometry evaluation. The PCLF/PCL spacers demonstrated compressive properties within 6 ± 0.5 MPa (strength) and 140 ± 15 MPa (modulus). Both porous PCLF/PCL and Nonporous PMMA formed collagen-rich membranes (PCLF/PCL: 92% ± 1.3%, PMMA: 86% ± 1.5%) similar to those induced in the Masquelet technique, indicating PCLF/PCL's potential for one-stage healing. Immunohistochemistry confirmed biomarkers for tissue regeneration, underscoring PCLF/PCL's regenerative capabilities. This research highlights PCLF/PCL scaffolds' ability to induce membrane formation in critical-sized segmental bone defects, supporting their use in one-stage surgery. Both solid and porous PCLF/PCL spacers showed adequate compressive properties, with the porous variants exhibiting BMP-2 expression and woven bone formation, akin to clinical standard PMMA. Notably, the early ossification of the membrane into the pores of porous scaffolds suggests potential for bone interlocking and regeneration, potentially eliminating the need for a second surgery required for PMMA spacers. The biocompatibility and biodegradability of PCLF/PCL make them promising alternatives for treating critical bone defects, especially in vulnerable patient groups.

Porous metal implants: processing, properties, and challenges.

Porous and functionally graded materials have seen extensive applications in modern biomedical devices-allowing for improved site-specific performance; their appreciable mechanical, corrosive, and biocompatible properties are highly sought after for lightweight and high-strength load-bearing orthopedic and dental implants. Examples of such porous materials are metals, ceramics, and polymers. Although, easy to manufacture and lightweight, porous polymers do not inherently exhibit the required mechanical strength for hard tissue repair or replacement. Alternatively, porous ceramics are brittle and do not possess the required fatigue resistance. On the other hand, porous biocompatible metals have shown tailorable strength, fatigue resistance, and toughness. Thereby, a significant interest in investigating the manufacturing challenges of porous metals has taken place in recent years. Past research has shown that once the advantages of porous metallic structures in the orthopedic implant industry have been realized, their biological and biomechanical compatibility-with the host bone-has been followed up with extensive methodical research. Various manufacturing methods for porous or functionally graded metals are discussed and compared in this review, specifically, how the manufacturing process influences microstructure, graded composition, porosity, biocompatibility, and mechanical properties. Most of the studies discussed in this review are related to porous structures for bone implant applications; however, the understanding of these investigations may also be extended to other devices beyond the biomedical field.

Direct comparison of additively manufactured porous titanium and tantalum implants towards in vivo osseointegration.

Material properties of implants such as volume porosity and nanoscale surface modification have been shown to enhance cell-material interactions in vitro and osseointegration in vivo. Porous tantalum (Ta) and titanium (Ti) coatings are widely used for non-cemented implants, which are fabricated using different processing routes. In recent years, some of those implants are being manufactured using additive manufacturing. However, limited knowledge is available on direct comparison of additively manufactured porous Ta and Ti structures towards early stage osseointegration. In this study, we have fabricated porous Ta and Ti6Al4V (Ti64) implants using laser engineered net shaping (LENS™) with similar volume fraction porosity to compare the influence of surface characteristics and material chemistry on in vivo response using a rat distal femur model for 5 and 12 weeks. We have also assessed whether surface modification on Ti64 can elicit similar in vivo response as porous Ta in a rat distal femur model for 5 and 12 weeks. The harvested implants were histologically analyzed for osteoid surface per bone surface. Field emission scanning electron microscopy (FESEM) was done to assess the bone-implant interface. The results presented here indicate comparable performance of porous Ta and surface modified porous Ti64 implants towards early stage osseointegration at 5 weeks post implantation through seamless bone-material interlocking. However, a continued and extended efficacy of porous Ta is found in terms of higher osteoid formation at 12 weeks post-surgery.

Electrically polarized TiO2 nanotubes on Ti implants to enhance early-stage osseointegration.

Ti is characteristically bioinert and is supplemented with modifications in surface topography and chemistry to find use in biomedical applications. The aim of this study is to understand the effects of surface charge on TiO2 nanotubes (TNT) on Ti implants towards early stage osseointegration. We hypothesize that charge storage on TNT will improve bioactivity and enhance early-stage osseointegration in vivo. Commercially pure Ti surface was altered by growing TNT via anodic oxidation followed by the introduction of surface charge through electrothermal polarization to form bioelectret. Our results indicate a stored charge of 37.15 ±â€¯14 mC/cm2 for TNT surfaces. The polarized TNT (TNT-Ps) samples did not show any charge leakage up to 18 months, and improved wettability with a measured contact angle less than 1°. No cellular toxicity through osteoblast proliferation and differentiation in vitro were shown by the TNT-Ps. Enhanced new bone formation at 5 weeks post-implantation for the TNT-Ps in contrast to TNTs was observed in vivo. Histomorphometric analyses show ∼40% increase in mineralized bone formation around the TNT-P implants than the TNTs at 5 weeks, which is indicative of accelerated bone remodeling cycle. These results show that stored surface charge on TiO2 nanotubes helped to accelerate bone healing due to early-stage osseointegration in vivo. STATEMENT OF SIGNIFICANCE: To improve surface bioactivity of metallic biomaterials, various approaches have been proposed and implemented. Among them, stored surface charge has been explored to enhance biological responses for hydroxyapatite ceramics where charged surfaces show favorable bone tissue ingrowth. However, surface charge effects have not yet been explored as a way to mitigate bio-inertness of titanium. This study intends to understand novel integration of bioactive titania-nanotubes and charge storage as surface modification for titanium implants. Our results show excellent biological response due to surface charge on titania-nanotubes offering possibilities of faster healing particularly for patients with compromised bone health.