Ceramics

Ceramics are usually made up of two or more elements, containing metallic and non-metallic elements, and they are typically crystalline solid, prepared by applying heat and pressure to a powder. Based on X-ray diffraction characteristics, ceramics can be divided into crystalline ceramics, which have long-range order, and glassy ceramics, which in turn possess only short-range order. In general, ceramic materials have the high melting temperature, elastic modulus, compressive strength, and wear resistant, but low thermal conductivity and density. All of these properties

Table 2.3 Four different types of ceramic materials and usages in biomedical applications

Type

Characteristics

Examples

Applications

Nonporous, nearly inert ceramics

Nonporous, inert, strong and stiff

Alumina and Zirconia

Orthopedics (bone screws and plates); dental restorations

Pyrolytic carbon

Coating material; blood contacting applications; joint components

Porous ceramics

Porous, inert, lower strong, coating for metallic implants

Calcium

hydroxy-apatite

(HA)

Primary structural component of bone; bone fillers; coating

Bioactive

ceramics

Nonporous, bioactive, establish bonds with bone tissue

Silicate glass Soda-lime glass Borosilicate glass

Vertebral prostheses; vertebral spacers; Iliac crest prostheses; bone grafting

Resorbable

ceramics

Resorbable, porous/ nonporous, degrade with time

Calcium phosphate P-Tricalcium phosphate (TCP)

Repair material for bone damaged; space filling material for bone loss Orthopedic applications

make ceramics be prospecting biomaterials in biomedical applications. In recent years, ceramics have developed into an important subset of biomaterials because of their great biocompatibility in the human body, which varies from inert ceramic oxides to the other extreme of resorbable materials. Hence bioceramics are particularly used for dental and orthopedic tissue replacements. For example, joint substitutes are commonly coated with bioceramic materials to reduce wear and inflammatory response. Other medical instruments like pacemakers, kidney dialysis machines and respirators also have a big demand on surgical cermets. Ceramics have broad applications in many aspects from medical or research intentions, and Table 2.3 above shows the four different types of ceramic materials and general usages in biomedical applications.

For example, Calcium phosphate (CaP), as a major constituent of bone, has been extensively studied. And the representative CaPs, hydroxyapatite (HA), tricalcium phosphate (TCP) and the mixture of HA and TCP, known as biphasic calcium phosphate (BCP), have been successfully used in the clinic as the bone powder filler and coatings on the metal surface to improve the implant integration with bone. It is also one of the matrix materials for the fiber- or tube- reinforced scaffolds. Both HA and TCP have similar chemical composition, structural and elastic modulus to the mineral phase of native bone. From the biocompatibility perspective, they are excellent candidates as bone tissue engineering scaffolds (see Fig. 2.3). Teixeira et al. [22] prepared HA and BCP scaffolds with 70% interconnected porosity (68% pores are 400 ^m and 2% are 0.7 ^m in diameter), which successfully supported new bone formation in immune deficient male mice and were more prominent in the case of HA scaffolds. Woodard et al. [23] observed the lamellar and woven bone formation in a multi-scale porous HA scaffolds containing macroporosity (250-350 ^m) and microporosity (2-8 ^m). High compressive strength of 30.2 ± 6.0 MPa was reported

(a) P-TCP scaffolds prepared using polymethylmethacrylate balls with 500-600 pm in diameter [27];

Fig. 2.3 (a) P-TCP scaffolds prepared using polymethylmethacrylate balls with 500-600 pm in diameter [27]; (b) 3-D printing of calcium sulfate cylindrical scaffolds with the sample length 12 mm [28]; (c) commercially available HA cylindrical scaffolds with different porosity produced by sponge replica method [29] (Adapted with permission from Refs. [27-29]. Copyright 2009, 2015, 2015 Elsevier Ltd)

with 70% porosity and the pore size at 50-600 pm. The macroporous HA scaffolds also allowed the cell adhesion, proliferation, and more importantly, internal migration, which will benefit the HA scaffolds to form a secure bonding with bone tissue [24]. HA and TCP scaffolds with internal controlled architectures have been prepared by direct 3D printing [25, 26]. Combined with microwave sintering, the maximum compressive strength was improved to 10.95 ± 1.28 MPa for TCP scaffolds with 42% porosity and the pore size at 400 pm. The TCP scaffolds with >60% porosity increased osteoblast activity and facilitated the new bone formation in the pores [25].

The calcium phosphate cement (CPC) is approved by FDA for repairing craniofacial defects. However, the CPC applications are limited to the non-stress-bearing bone because of their brittle and weak nature, which is characterized by a sudden fracture without any significant preceding plastic deformation [30]. To overcome the brittle fracture behavior of conventional CPCA, a large number of materials have been used to reinforce CPCs. The reinforcements include technical reinforcements (E-glass, carbon fibers/nano-tubes, SiC whiskers or polypropylene, aramid and polyamide fibers), calcium silicates (wollastonite whiskers), phosphates (hydroxyapatite whiskers) or carbonates (aragonite whiskers), and degradable polymers (chitosan, PLGA and PCL) [31]. In all the composite approaches, the CPC reinforced by fibers is one of the most successful techniques, particularly using long continuous fibers [32]. Contributions to the macroscopic behavior come from strength and stiffness of both fiber and cementitious matrix. Fiber reinforced CPCs are an emerging class of biomedical materials to overcome the brittle fracture behavior of conventional CPC [33, 34]. The application of fiber reinforcements in medical applications is not feasible now, for there are many important questions need to be solved.

However, ceramics show limited clinical applications for tissue engineering because of the brittle characteristic and difficulty of shaping for implantation [35]. Janas et al. [36] provided a hard tissue scaffold comprising a resorbable ceramic fiber. The scaffold was formed by first creating unfired (green) bioresorbable ceramic fibers via the viscous suspension spinning process (VSSP) and successfully used the ceramic fibers to reinforce ceramic tissue scaffolds with sacrificing the intrinsic mechanical properties. Similarly, CaPs are commonly used materials for the restoration of bone defects with excellent biocompatibility and bioactivity. Even through, brittleness and low flexural/tensile strength so far limit their application to non-load bearing areas. Researchers now use collagen fibers to enhance the CPCs, which greatly improve their strength and toughness and have been a major strategy to overcome the current mechanical limitations of CPCs. Despite the fact that there is not so much research on the fiber or tube reinforced tissue scaffolds. In the near future, the fiber or tube reinforcement will become a hot point on ceramic tissue scaffolds.

 
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