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compared with similar nanocomposite films.26 A sequence of Al2O3/chitosan films produced with a bottom-up spin-coating technique produced more ductility and flaw tolerance than prior works, with observed ultimate tensile strains up to 21 percent.21 An alternative approach used ecofriendly vacuum filtration, similar to papermaking, to fabricate high-strength and high-stiffness MTM/PVA composites with varying optical transparencies, as well as gas barrier and fire-resistant properties.27 (See Table 1 for property data.) Thin films have outstanding mechanical properties and unique functionalities, but they are, in fact, thin—less than 1 mm. Although they are impractical for many structural applications, films can be used as hard coatings, displays, sensors, and optical equipment. Porous scaffolds Porous scaffolds that mimic bone are ideal for tissue engineering applications, such as load-bearing bone implants that promote tissue ingrowth or other applications requiring high porosity and reasonable mechanical strength. Researchers report many different methods to emulate the trabecular architecture of cancellous bone including direct foaming28 and polymer sponge replication. 29 However, mimicking cancellous bone is not ideal for load-bearing applications because of its high porosity, near isotropic structure, and poor mechanical properties. Instead, highly anisotropic scaffolds with unidirectionally aligned pores have shown great potential for load-bearing applications.30,31 For example, researchers made porous scaffolds by dipping polyurethane foams repeatedly into ZrO2 or hydroxyapatite slurries, drying, and then burning out the foam. The ZrO2 scaffold achieved high compressive strength (35 MPa), whereas the strength of the hydroxyapatite scaffold was ~4 MPa.29 Another interesting method uses the unidirectional porosity of wood as a template to fabricate anisotropic hydroxyapatite scaffolds, following a series of chemical treatments.32 The method achieved optimal pore-size distribution (100–300 μm) required for the migration and proliferation of osteoblasts (bone synthesizing cells). However, the mechanical properties were poor compared with other unidirectionally aligned porous scaffolds (Table 1). The advent of additive manufacturing enables fabrication of a variety of porous scaffolds with designer architectures. This technique has gained tremendous attention in the medical industry as an efficient means to customize scaffolds for biomedical implants. Direct ink–write assembly has been used to form bioactive glass scaffolds with regular pore spacing of 500 μm and cell walls 100 μm thick.31 High compressive strength and modulus were reached (136 MPa and 2 MPa) with a porosity of 60 percent, which is within the range of cancellous bone (Table 1). The scaffolds promoted nucleation of hydroxyapatite after soaking in simulated body fluid for two weeks, indicating that the material is ideal for osseointegration. However, the mechanical properties of 3D-printed parts depend on the formation Figure 3. Natural and artificial osteonal and trabecular bone structures. Credit: (a–d, f) Porter; UCSD, (e) Deville; LBL (a-b) Schematic osteonal and trabecular bone architectures; (c) natural osteons in cortical bone; (d) natural trabecular struts in cancellous bone; (e) aligned freeze cast microstructure mimics osteonal bone architecture.38 (f) artificial scaffold mimics trabecular bone architecture. Scale bars: (c) 100 μm; (d) 500 μm; (e) 500 μm; (f) 25 μm. and resolution of the layers (highest resolutions are ~10 μm). Ultimately, the interface between layers is the weakest point of the structure and may lead to catastrophic crack initiation, propagation, and subsequent failure. Because of its high cost, high energy consumption, extended fabrication times, limited material availability, restricted workspace, and poor material properties, 3D printing is not the most economical means to develop highperformance scaffolds. Until additive manufacturing technologies improve, other methods to fabricate high-strength, porous scaffolds are preferable. Currently, one of the best methods for forming aligned porous scaffolds is freeze casting from a (usually) aqueous-based slurry. Adding surfactants and binders improves particle dispersion and as-cast strength. With this method, the slurry is poured into a mold set on a freezing surface. The freezing surface is controlled, such that a thermal gradient leads to the directional solidification of the slurry. Constitutional supercooling sets up instabilities on the liquid–solid interface as the freezing front advances in the slurry. These perturbations (instabilities) crystallize into ice dendrites that shoot out into the liquid. The ice crystals expel particles between dendrites, thereby forming a lamellar structure. Finally, the frozen slurry is freezedried and sintered to form a structurally robust scaffold. Processing variables include volume fraction of solid powder, cooling rate, and liquid properties, such as viscosity. A polymer or metal can be infiltrated into these scaffolds to form lamellar composites. Bulk composites Because of their large macrostructures and scalability, bulk ceramic-based composites are, quite possibly, the most versatile and high performance bioin- American Ceramic Society Bulletin, Vol. 93, No. 5 | www.ceramics.org 21


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