The mechanical properties of human hard tissues are closely linked to their hierarchical organization, with nano- and microstructures at various length scales contributing uniquely to overall performance. Biological materials such as teeth are naturally evolved biocomposites characterized by structural and mechanical heterogeneity across multiple scales. They have evolved complex hierarchical structures, making them the most resilient components in the human body capable of withstanding masticatory forces exceeding 1000 N and enduring millions of loading cycles over the lifetime [1]. Dentine, which is tough, elastic, and comprised of 70 wt% hydroxyapatite (HAp), and a stiff, wear-resistant, and highly mineralized (96 wt% HAp) layer known as enamel, connected by the dentinoenamel junction [2], [3], [4], form the main bulk of the tooth.

When it becomes necessary to replace the dental tissue with a dental crown, it is imperative to use materials that match the material properties of dental tissue to ensure their longevity and prevent damage to the surrounding healthier teeth. Unfortunately, this challenge has yet to be fully overcome. Current dental crowns are made entirely of ceramic (all-ceramic crowns) due to their aesthetics similar to dental tissue, with zirconia (ZrO2) crowns most commonly used because their porcelain veneer provides a wide range of color customization [5], [6], [7]. However, porcelain veneers are prone to fracture, with a reported 3-year chipping rate of approximately 5.4 % [2] which is a significant setback. This has led to the adoption of monolithic ceramic crowns, which can cause excessive wear to opposing teeth due to being far harder and stiffer than dental tissue [8], [9].

Ceramics, such as alumina (Al2O3) and ZrO2, inherently exhibit limited dislocation mobility due to their strong ionic and covalent bonds, resulting in negligible plastic deformation at ambient temperatures [5], [10], [11], [12], [13]. Conversely, different modes of inelastic deformation, such as in situ phase transformation, can provide limited alternative deformation mechanisms. Significantly, ceramics are brittle and have a severe sensitivity to flaws, meaning intrinsic toughening is conventionally unattainable. If extrinsic shielding mechanisms are deficient, fracture occurs catastrophically by bond breaking at the crack tip, resulting in critically low intrinsic toughness [14], [15], [16], [17]. Adhesive cementation is commonly utilized in clinical settings to mitigate this issue, but it is ineffective for long-term maintenance. Thus, extrinsic toughening of dental ceramics remains the only currently viable method, although it is not the most desirable, as is true for most brittle materials [18].

Moreover, dental materials can be made softer and less stiff by combining ceramics with a compliant polymer phase. As such, polymer-infiltrated ceramic networks which possess elastic moduli and hardness values similar to dental tissue have been created [19]. However, these materials display an isotropic microstructure and far lower fracture toughness compared to other dental ceramics. A possible solution is to implement a bioinspired architecture at the microscale that mimics the structure of dental tissue, but this complex structure is considerably beyond the capabilities of current manufacturing techniques. Nonetheless, inspiration can be taken from nacre found in mollusk shells as their material properties are comparable to those of human teeth [20]. It is a highly mineralized material consisting of ∼95 vol% minerals and ∼5 vol% biopolymers and proteins. The mineral content exists in the form of aragonite platelets stacked and held together by biopolymers and proteins, forming what is called a “brick-and-mortar” structure [21]. The brick-and-mortar architecture of nacre promotes crack deflection and progressive toughening, resulting in significantly higher fracture toughness than its individual mineral and organic components, and exhibits characteristic R-curve behavior. Moreover, it demonstrates rising R-curve behavior, wherein the material’s fracture resistance increases with crack extension due to progressive toughening mechanisms such as crack bridging, platelet pull-out, and microcracking that activate and intensify as the crack propagates.

Freeze-casting has become a popular method for producing porous ceramic scaffolds due to its versatility and simplicity [22], [23], [24], [25]. The process operates on the basic principle that a colloidal ceramic suspension is solidified under conditions in which the solidifying dispersant will displace the suspended ceramic particles away from the advancing solid front. The frozen dispersant is then sublimated, and the resulting green body is sintered to form a porous ceramic scaffold. This technique has typically been performed unidirectionally, where the colloidal suspension is subject to a single temperature gradient. The result generates a scaffold with pores that are continuous parallel to the temperature gradient, but randomly organized in the plane perpendicular to the gradient [26]. With the advent of bi-directional freeze-casting, where the colloidal suspension is subject to two perpendicular temperature gradients, it has become possible to create highly aligned lamellar structures that can be used for manufacturing nacre-inspired ceramic composites [27], [28], [29], [30], [31].

The objective of this study is to identify phase-dependent fracture toughening mechanisms of bioinspired composites. Eight different Al2O3-based composites were investigated with different polymers, including polymethyl methacrylate (PMMA), epoxy, polyurethane (PU) and urethane dimethacrylate/triethylene glycol dimethacrylate (UDMA-TEGDMA). In situ micromechanical fracture toughness testing was performed and the R-curve behavior of the composites were explored to determine an optimum material combination for dental crown applications. Thus, greater insight is provided into their fracture behavior, allowing the development of next-generation bioinspired materials that are both affordable and satisfactory to the clinical demand for increased fracture toughness.