在漫長的進化過程中，生物體演化出獨特的結構來增強其機械性質。異向性結構是自然界中一種常見的微結構特徵，在橫向和縱向上提供了不同的機械特性。管狀結構在自然界中存在於牙齒、竹子、樹木和牛角中，具有很強的抗衝擊性或韌性。另一種常見的結構是蛋白質和礦物質組成的複合材料。透過使用冷凍鑄造法和聚合物搭配形成仿生多孔複合材料已被廣泛研究。
在第一項研究中，我們創建並設計一種溶膠凝膠/冷凍鑄造複合方法，用以改進典型冷凍鑄造法的缺點。此外，這種複合法不需在高溫環境燒結成塊，也不需在低溫低壓的環境下昇華乾燥冰晶。此製程過程可在溫和條件下進行，對有機物的使用具有優勢及價值。四乙氧基矽烷(TEOS)為此多孔材的無機成分，並透過添加聚合物形成多孔複合材料。也可於初始無機漿料中混入有機聚合物，即可簡單地形成具有複雜形狀及結構的多孔複合材料。此多孔材料具有3D樹枝狀的微結構，其形貌、孔隙率及機械性質受冷卻速率、水含量和聚合物含量影響。此外，添加聚合物於初始漿料有助於保持形狀並提供穩定的機械性質，減少裂紋的生成並進一步提升孔隙率，可達到95%以上。與無機多孔材料的生坯相比，聚合物和無機混合製成的複合多孔材料的抗壓強度和韌性顯著提高。事實上，這種新的混合方法可以在溫和的條件下合成複合材料；因此，它可以拓寬製造方法的多樣性並擴大潛在的應用。
在第二項研究中，發展了一種聚合物冷凍鑄造法以製備具有管狀結構的多孔材料。此研究中，聚合物必須具有水溶性和可交聯的官能基，對於實驗的成敗是至關重要的，而聚乙烯醇(PVA)是少數滿足這些要求的材料之一，其交聯溫度低於453K，且聚合物不需要像典型的冷凍鑄造法一樣在高溫下燒結成型。此PVA多孔材料具有管狀結構，其孔徑和機械性質可以透過冷卻速率、固含量和交聯過程進行調整。PVA多孔材料的機械性質會受到不同濕度的環境影響，在0%、53%、75%的相對濕度下，測得的極限抗壓強度分別為22MPa、13MPa、11MPa。此PVA多孔材料具有結構異向性，使得在軸向與輻射向展現出不同的壓應力特性。在縱向上，應力應變曲線顯示出延展性，並在降伏強度之後顯示出一系列鋸齒狀斷裂直至緻密化。在橫向上，應變小於60%時展現出可壓縮性，並且可透過溶脹/消溶脹的過程使其展現出結構可恢復性。由於PVA多孔材料為異向性的管狀結構，軸向的熱傳導率高於輻射向的熱傳導率，輻射向的值為0.116W mk-1，被認為是良好的絕熱材料。

During a long period of evolution, organisms have evolved distinctive structures to strengthen the mechanical properties. One of the common elements is anisotropy in the microstructure formation in nature, providing different mechanical properties in the transversal and the longitudinal directions. The tubular structure in nature exists in teeth, bamboo, trees, and horns, which possess great impact resistance or toughness. Another common strategy is to utilize composite materials based on the constituents of proteins and minerals. Bio-inspired porous composite scaffolds have been extensively studied by using the freeze-casting method and varying polymer infiltration methods.
In this first study, a sol-gel/freeze-casting hybrid method is created and designed to modify the typical freeze-casting method and subsequently applied in forming scaffold. Furthermore, this hybrid method does not require sintering at high temperature nor drying under low pressure and low temperature. In terms of organics, these advantages are excellently valuable to prepare scaffolds under mild conditions. Tetraethyl orthosilicate (TEOS) is used as the inorganic constituent of the scaffold, which is further infiltrated with polymers to form composite scaffolds. The complex shapes and architectures of scaffold can be formed and molded easily by mixing organic polymers and inorganic ceramics as initial slurry. Composite scaffolds with 3D dendritic structures and porosities can be affected by altering the cooling rate, the water content and polymer infiltrated parameters. Moreover, initial slurry with additive polymer can assist in maintaining the shape and provide a stable mechanical property, which can help to reduce the number of cracks and further increase the porosity, reaching over 95%. The compressive strength and toughness of composite scaffold fabricated by mixing organic and inorganic constituents are significantly enhanced compared to the green body of inorganic scaffold. As a matter of fact, this novel hybrid method can synthesize scaffolds or composites under mild conditions; therefore, it can broaden the diversity of fabrication approaches and expand potential applications.
In the second research, a polymeric freeze-casting method is developed to fabricate scaffolds with tubular microstructure. Polymers with water-soluble and cross-linkable functional groups are essential for material selection and polyvinyl alcohol (PVA) is one of the few materials that meet these requirements. The crosslinking temperature is below 453K, which does not require sintering at high temperature as for typical freeze-casting method. The pore sizes and mechanical properties of tubular structures in PVA scaffold can be tuned by using various cooling rates, solid loadings, and crosslinking treatments. The porosity of the PVA scaffold is around 66%. The ultimate compressive strengths of PVA scaffolds are measured to be 22MPa, 13MPa, 11MPa under relative humidities of 0%, 53%, 75%, respectively. Anisotropic structure shows different mechanical behavior and characteristics in two directions. In the axial direction, the stress-strain curve shows ductility and exhibits series of jagged breaks after the yield strength until densification. In the radial direction, the scaffold shows compressibility below the strain of 60%, and exhibits complete recoverability during the swelling/deswelling process. The thermal conductivity in the axial direction is higher than that in the radial direction due to the anisotropic tubular structure along the scaffold. The value in the radial direction is 0.116 W mk-1, which is considered as a good thermal insulator.

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