Bioinspired and Multifunctional Tribological Materials for Sliding, Erosive, Machining, and Energy-Absorbing Conditions: A Review.

Friction, wear, and the consequent energy dissipation pose significant challenges in systems with moving components, spanning various domains, including nanoelectromechanical systems (NEMS/MEMS) and bio-MEMS (microrobots), hip prostheses (biomaterials), offshore wind and hydro turbines, space vehicles, solar mirrors for photovoltaics, triboelectric generators, etc. Nature-inspired bionic surfaces offer valuable examples of effective texturing strategies, encompassing various geometric and topological approaches tailored to mitigate frictional effects and related functionalities in various scenarios. By employing biomimetic surface modifications, for example, roughness tailoring, multifunctionality of the system can be generated to efficiently reduce friction and wear, enhance load-bearing capacity, improve self-adaptiveness in different environments, improve chemical interactions, facilitate biological interactions, etc. However, the full potential of bioinspired texturing remains untapped due to the limited mechanistic understanding of functional aspects in tribological/biotribological settings. The current review extends to surface engineering and provides a comprehensive and critical assessment of bioinspired texturing that exhibits sustainable synergy between tribology and biology. The successful evolving examples from nature for surface/tribological solutions that can efficiently solve complex tribological problems in both dry and lubricated contact situations are comprehensively discussed. The review encompasses four major wear conditions: sliding, solid-particle erosion, machining or cutting, and impact (energy absorbing). Furthermore, it explores how topographies and their design parameters can provide tailored responses (multifunctionality) under specified tribological conditions. Additionally, an interdisciplinary perspective on the future potential of bioinspired materials and structures with enhanced wear resistance is presented.

Biomimetic design of implants for long bone critical-sized defects.

This computational study addresses new biomimetic load-bearing implants designed to treat long bone critical-sized defects in a proximal diaphysis region. The design encompasses two strategies: a Haversian bone-mimicking approach for cortical bone and lattices based on triply periodic minimal surfaces (TPMS) for trabecular bone. Compression tests are modeled computationally via a non-linear finite element analysis with Ti6Al4V alloy as a base material. Nine topologies resembling cortical bone are generated as hollow cylinders with different channel arrangements simulating Haversian (longitudinal) and Volkmann (transverse) canals to achieve properties like those of a human cortical bone (Strategy I). Then, the selected optimal structure from Strategy I is merged with the trabecular bone part represented by four types of TPMS-based lattices (Diamond, Primitive, Split-P, and Gyroid) with the same relative density to imitate the whole bone structure. The Strategy I resulted in finding a hollow cylinder including Haversian and Volkmann canals, optimized in canals number, shape, and orientation to achieve mechanical behavior close to human cortical bone. The surface area and volume created by such canals have the maximum values among all studied combinations of transverse and longitudinal channels. Strategy II reveals the effect of interior design on the load-bearing capacity of the whole component. Between four types of selected TPMS, Diamond-based lattice and Split-P have more uniform stress distribution, resulting in a superior load-bearing efficiency than Gyroid and Primitive-based design showing less uniformity. This work offers a new design of the bone-mimicking implant, with cortical and trabecular bone components, to repair long bone critical-sized defects.

Bioactive Ceramic Scaffolds for Bone Tissue Engineering by Powder Bed Selective Laser Processing: A Review.

The implementation of a powder bed selective laser processing (PBSLP) technique for bioactive ceramics, including selective laser sintering and melting (SLM/SLS), a laser powder bed fusion (L-PBF) approach is far more challenging when compared to its metallic and polymeric counterparts for the fabrication of biomedical materials. Direct PBSLP can offer binder-free fabrication of bioactive scaffolds without involving postprocessing techniques. This review explicitly focuses on the PBSLP technique for bioactive ceramics and encompasses a detailed overview of the PBSLP process and the general requirements and properties of the bioactive scaffolds for bone tissue growth. The bioactive ceramics enclosing calcium phosphate (CaP) and calcium silicates (CS) and their respective composite scaffolds processed through PBSLP are also extensively discussed. This review paper also categorizes the bone regeneration strategies of the bioactive scaffolds processed through PBSLP with the various modes of functionalization through the incorporation of drugs, stem cells, and growth factors to ameliorate critical-sized bone defects based on the fracture site length for personalized medicine.

Selective laser sintered bio-inspired silicon-wollastonite scaffolds for bone tissue engineering.

The scaffolds, which morphologically and physiologically mimic natural features of the bone, are of high demand for regenerative medicine. To address this challenge, we have developed innovative bioactive porous silicon- wollastonite substrates for bone tissue engineering. Additive manufacturing through selective laser melting approach has been exploited to fabricate scaffolds of different architecture. Unique material combining osteoinductivity, osteoconductivity and bioactive elements allows flexibility in design. As the porous structure is required for the ingrowth of the bone tissue, the CAD designed scaffolds with pore size of 400 µm and hierarchical gradient of pore size from 50 µm to 350 µm have been 3D printed and tested in vitro. The scaffolds have demonstrated not only the enhanced viability and differential patterning of human mesenchymal cells (hMSC) guided by the biomimetic design onto extra and intra scaffold space but also promoted the osteogenic differentiation in vitro. Both homogeneous and gradient scaffolds have shown the differential expression of primary transcription factors (RUNX2, OSX), anti-inflammatory factors and cytokines, which are important for the regulation of ossification. The effective elastic modulus and compressive strength of scaffolds have been calculated as 1.1 ± 0.9 GPa and 37 ± 13.5 MPa with progressive failure for homogeneous structured scaffold; and 1.8 ± 0.9 GPa and 71 ± 9.5 MPa for gradient-structured scaffold with saw-tooth fracture mode and sudden incognito failure zones. The finite element analysis reveals more bulk stress onto the gradient scaffolds when compared to the homogeneous counterpart. The findings demonstrate that as-produced composite ceramic scaffolds can pave the way for treating specific orthopaedic defects by tailoring the design through additive manufacturing.