Table 4.
Gradient-Based Approaches Highlighting Current Trends of Application in Implant Design
| Application | Gradient type (transition type) | Fabrication method | Geometry/gradient direction | Implant/coating thickness scale | Thickness scale of the gradient region or number of step transitionsa | Materials usedb | References |
|---|---|---|---|---|---|---|---|
| Bioceramic materials with bimodal pore structure for bone replacement | Porosity and pore size (step gradients) | Differential impregnation–heat sintering | Cylindrical/axial | ≥mm | One transition | HA and a cellulosic sponge | 124 |
| Dip casting–vacuum impregnation–stitching/press fitting | Cylindrical/radial | ≥mm | One transition | HA/TCP and polymeric foams | 112 | ||
| Multiple slip-casting–heat sintering | Cylindrical/radial | ≥mm | One transition | HA, PVC, and Li3PO4 | 125 | ||
| Multiple tape-casting–sintering–heat-induced removal of porogens | Bar or disc/axial | ≥mm | Four transitions | HA, PBMA | 126 | ||
| Graded orthopaedic replacement to reduce stress-shielding | Porosity and stiffness (step gradients) | Liquid phase sintering | Bar/longitudinal | ≥mm | Two transitions | Ti, Si powder | 123 |
| UHMWPE wear resistance (for total joint arthroplasty, etc.) | Crosslinking density (continuous gradients) | Irradiation (low-energy electron beam) | Finished acetabular liner/radial | ≥mm | mm | UHMWPE | 121 |
| Melt-irradiation (low-energy electron beam) | Hemispherical/radial | ≥mm | mm | UHMWPE | 120 | ||
| Material composition [diffusion-based IPNc] (continuous gradient) | Swelling at elevated temperature with ultrasonics | – | >monolayer | – | UHMWPE, PLL | 122 | |
| Bioceramic coatings on metallic implants | Material composition (step gradients) | Plasma spray | – | 660 μm (coating thickness) | Three transitions | HA, Ti-6Al-4V, TCP/TiO2 | 114 |
| Plasma spray | – | – | Three transitions | HA, Ti | 115 | ||
| Material composition (continuous gradients) | Plasma spray | – | 100 μm | 40 μm | HA, Ti | 110 | |
| Bioceramic polymer–based skull implant (e.g., calvarial defect repair) | Material composition and porosity (step gradients) | Hot pressing–gas foaming | Custom (CAD/CAM assisted) | ∼cm | Four transitions | PLA, CaCO3, Ca3(PO4)2 | 127 |
| Bioceramic polymer intervertebral disc prosthesis | Material composition (continuous gradient) | Centrifugation | Cylindrical/radial | ≥mm | Across the sample (100%) | HA, polycarbonate-urethane | 128 |
| Polymeric coatings on metallic implants | Material composition (step gradients) | Dip coating | – | 1 mm (coating thickness) | Two transitions | Ti, PLA, HA, CaCO3 | 116 |
| Gradient surface treatment of metallic implant to improve biocompatibility/osteoconductivity | Material composition (sodium titanate and apatite) | Sodium hydroxide treatment–heat treatment | – | ≥mm | Nanoscale | Ti-6Al-4V, sodium titanate | 117 |
| (Continuous gradient) material composition (apatite) (step gradient) | Sol-gel and slurry dip coating–drying and sintering | – | <100 μm | Three transitions | ZrO2, HA, fluorapatite | 118 | |
| Degree of oxidation (porous outer layer–dense inner layer) (continuous gradient) | Electrochemical oxidation (preanodic oxidation–microarc oxidation) | – | 16 μm (coating thickness) | <1 μm (nanoscale) | Ti, TiO2 | 119 | |
| As a fast-screening tool for biocompatibility assessment of the metallic implants | Material composition (step and continuous gradient) | Sedimentation/powder packing–sintering (furnace/high-frequency induction heating/spark plasma sintering) | Cylindrical/axial | ≥mm | (See the articles for details) | Ti, HA (and others) | 130, 131 |
| Material composition (step gradients) | Powder packing–hot isotactic pressing | Bar | ≥cm | Nine transitions | Ti, Ni | 129 |
For continuous gradient approaches, % thickness of the gradient region compared to the overall thickness of the structure is indicated. To eliminate/reduce interfacial stresses that may cause delamination, a higher % value of the thickness of the gradient region compared to the overall thickness of the implant/coating can be selected. However, one must also take the envisioned application into consideration. Some of the studies utilized step gradients, where dual/multiple layers were present having sharp (discontinuous) interfaces in between. The diffusive effects may have led to a blurred region at the interface; however, we do not treat it as a continuous gradient unless measured. Studies, where multilayered structures were created and the overall gradient region comprised of several step transitions that spanned the entire implant/coating, are marked by the number of such transitions.
HA: hydroxyapatite; TCP: tricalcium phosphate; PVC: polyvinyl polyacrylate; PBMA: poly(butylmethacrylate); UHMWPE: ultra-high-molecular-weight polyethylene; PLA: poly(lactic acid).
IPN: interpenetrating network.