牙釉質是一種兼具高硬度、高耐磨性及高韌性的生物複合材料。本論文旨在探討草食性動物牙釉質的力學性能和微觀結構特徵，以了解生物礦物化材料具有一般工程材料無法比擬之既堅硬又富有韌性之特性的原因。草食性動物利用臼齒相互摩擦的研磨作用來將食物磨碎，其臼齒相較於雜食性動物及肉食性動物，更需要堅固且耐磨之機械性質來維持進食以滿足牠們的生存需求。本研究針對草食性動物牙釉質的表面硬度及牙釉質表面至EDJ的硬度梯度特徵進行探討，牙釉質的表面硬度可代表其在研磨過程中抵抗磨損的能力，從牙釉質表面至EDJ的硬度梯度特徵更提供了吸收能量的能力以防止牙齒受外力傷害造成的損傷，並由牙釉質中損失模數的分布探討礦物質含量對硬度梯度的調控。
透過與臺北市立動物園的合作，取得馬、雙峰駱駝、長頸鹿、臺灣野山羊、北非髯羊、斑哥羚羊、臺灣山羌、臺灣水鹿及臺灣梅花鹿等九種草食性動物的臼齒樣本。對臼齒樣本依序進行切割、鑲埋、研磨及拋光後，經由深度感測壓痕硬度量測技術的方法及機械物理性質係數圖來探索生物礦物化材料之物理機械的精緻微觀架構。並使用同步輻射的光源來檢測牙釉質的元素成分分布及含量，建立物理機械性質與成份含量之間的關係。
本研究詳細探討了在草食性動物中，不一樣的食物類型對臼齒特性帶來影響。結果顯示，擁有高冠齒的草食性動物通常咀嚼較堅硬的食物且具有較高的表面硬度，例如牛科動物；而擁有低冠齒的草食性動物通常咀嚼較柔軟的食物且具有較低的表面硬度，例如鹿科動物。草食性動物的壽命愈長會具有更厚的牙釉質，以足夠的牙釉質厚度堆疊來承擔研磨作用下臼齒的摩擦損耗。具有較高表面硬度及牙釉質厚度的草食性動物有相應較高之功能梯度臼齒硬度斜率，可有效轉移所受外力避免臼齒破損，而臼齒縱切面硬度斜率是受依有機物及無機物的比例分布調控之損失模數所控制。牙釉質中釉桿柱呈現交錯排列，並由蛋白質包裹在其周圍，由釉桿柱承受外部應力後再由蛋白質將其轉移消散，單位面積內釉桿柱數量愈少者其表面硬度愈小，因釉桿柱數量較少導致黏彈性質較不佳，故表面硬度不可太高，否則容易碎裂。草食性動物之牙釉質表面硬度、牙釉質至EDJ的硬度梯度及牙釉質厚度與牠們的飲食和壽命之間的相關性，相較於本實驗室先前對草食性動物、雜食性動物及肉食性動物牙釉質的研究，於統計學意義上更具顯著意義，此現象可歸因於草食性動物進食所依賴的臼齒研磨作用導致之牙釉質機械性質的演變。本研究闡明了物種如何在演化過程中發展其臼齒之磨潤功能以適應飲食及壽命的需求，並為前瞻工程材料的開發提供指引。

Tooth enamel is a hard biomaterial that exhibits both high wear resistance and high toughness, a unique combination that is distinct from engineering material. This dissertation aims to investigate the mechanical properties and microstructure characteristics of the enamel of various herbivores. Herbivores use the rubbing action of the molars to grind food. Consequently, the enamel of their molars needs high hardness to provide the wear-resistance needed for feeding comparing with omnivores and carnivores. Specifically, this study investigated the surface hardness of the enamel and the gradient feature of the hardness from the outer surface of the enamel to the EDJ (Enamel-Dentin Junction). Enamel hardness represents the ability to resist wear during the grinding action of feeding. The gradient feature of the hardness from the outer surface of the enamel to the EDJ provides the capability to absorb energy for preventing damage under dental trauma. And the loss modulus represents the regulation of mineral content on the hardness gradient.
Samples of a molar of nine herbivores of Horse, Bactrian Camel, Giraffe, Formosan Serow, Barbary Sheep, Bongo, Formosan Reeves’s Muntjac, Formosan Sambar, and Formosan Sika were obtained through cooperation with Taipei Zoo. After sequentially cutting, embedding, grinding, and polishing the molar samples, the method of depth-sensing indentation and Dynamic Mechanical Analysis are used to explore the mechanical properties of these biomineralized materials. And use the synchrotron radiation light source to detect the distribution and content of the element composition of enamel, and establish the relationship between the physical and mechanical properties and the content of the composition. Finally, this study characterized the microstructure of the prism arrangement of the enamel of various herbivores using scanning probe microscopy.
The results show that herbivores with hypsodont usually chew harder foods and thus higher molar surface hardness, such as Bovidae; whereas herbivores with brachydont usually chew softer foods and thus lower molar surface hardness, such as Cervidae. The herbivores with longer longevity possess higher thickness to endure the wear loss under grinding action for nutrition intake in a longer life span. The herbivores with higher surface hardness or enamel thickness have a correspondingly higher functional gradient molar hardness slope to effectively absorb the impact energy under dental trauma. The hardness slope of the longitudinal section of the molar is controlled by the loss modulus which represents the ratio of organic matter and inorganic matter. The enamel rods in enamel are arranged in a staggered arrangement and are surrounded by proteins. The enamel rods bear external stress and then are transferred and dissipated by the protein. The smaller the number of enamel rods per unit area, the lower the surface hardness. Because the smaller number of enamel rods results in poor viscoelastic properties, the surface hardness should not be too high or it will be brittle. The relationship between the surface hardness of enamel, the hardness gradient from enamel to EDJ, and the thickness of enamel with their diet and longevity is more statistically significant than previous studies of our group on the enamel of different species across herbivores, omnivores, and carnivores. This phenomenon can be attributed to the dominant grinding action during the feeding of the herbivores results in a more direct consequence between the evolution of the mechanical property of the enamel and diet. This study shed light on how evolution develops the tribological capability of each species to adapt to its diet and longevity and provides a guideline for the development of cutting-edge engineering material.

[1] L. Cheng, A. Thomas, J. L. Glancey, and A. M. Karlsson, “Mechanical behavior of bio-inspired laminated composites,” Compos. Part Appl. Sci. Manuf., vol. 42, no. 2, pp. 211–220, Feb. 2011, doi: 10.1016/j.compositesa.2010.11.009.
[2] A. Barani, A. J. Keown, M. B. Bush, J. J.-W. Lee, H. Chai, and B. R. Lawn, “Mechanics of longitudinal cracks in tooth enamel,” Acta Biomater., vol. 7, no. 5, pp. 2285–2292, May 2011, doi: 10.1016/j.actbio.2011.01.038.
[3] V. Imbeni, J. Kruzic, G. Marshall, and R. Ritchie, “The Dentin–Enamel Junction and the Fracture of Human Teeth,” Nat. Mater., vol. 4, pp. 229–32, Apr. 2005, doi: 10.1038/nmat1323.
[4] C. Sanchez, H. Arribart, and M. M. Giraud Guille, “Biomimetism and bioinspiration as tools for the design of innovative materials and systems,” Nat. Mater., vol. 4, no. 4, Art. no. 4, Apr. 2005, doi: 10.1038/nmat1339.
[5] J. D. Currey and P. M. Sheppard, “Mechanical properties of mother of pearl in tension,” Proc. R. Soc. Lond. B Biol. Sci., vol. 196, no. 1125, pp. 443–463, Apr. 1977, doi: 10.1098/rspb.1977.0050.
[6] A. P. Jackson, J. F. V. Vincent, R. M. Turner, and R. M. Alexander, “The mechanical design of nacre,” Proc. R. Soc. Lond. B Biol. Sci., vol. 234, no. 1277, pp. 415–440, Sep. 1988, doi: 10.1098/rspb.1988.0056.
[7] B. Ji and H. Gao, “Mechanical properties of nanostructure of biological materials,” J. Mech. Phys. Solids, vol. 52, no. 9, pp. 1963–1990, Sep. 2004, doi: 10.1016/j.jmps.2004.03.006.
[8] J. C. Weaver et al., “Analysis of an ultra hard magnetic biomineral in chiton radular teeth,” Mater. Today, vol. 13, no. 1, pp. 42–52, Jan. 2010, doi: 10.1016/S1369-7021(10)70016-X.
[9] S. P. Ho, M. Balooch, H. E. Goodis, G. W. Marshall, and S. J. Marshall, “Ultrastructure and nanomechanical properties of cementum dentin junction,” J. Biomed. Mater. Res. A, vol. 68A, no. 2, pp. 343–351, 2004, doi: https://doi.org/10.1002/jbm.a.20061.
[10] B. An, R. Wang, and D. Zhang, “Role of crystal arrangement on the mechanical performance of enamel,” Acta Biomater., vol. 8, no. 10, pp. 3784–3793, Oct. 2012, doi: 10.1016/j.actbio.2012.06.026.
[11] M. A. McCollum and P. T. Sharpe, “Developmental genetics and early hominid craniodental evolution,” BioEssays, vol. 23, no. 6, pp. 481–493, 2001, doi: https://doi.org/10.1002/bies.1068.
[12] L. H. He and M. V. Swain, “Understanding the mechanical behaviour of human enamel from its structural and compositional characteristics,” J. Mech. Behav. Biomed. Mater., vol. 1, no. 1, pp. 18–29, Jan. 2008, doi: 10.1016/j.jmbbm.2007.05.001.
[13] Y.-R. Jeng, T.-T. Lin, H.-M. Hsu, H.-J. Chang, and D.-B. Shieh, “Human enamel rod presents anisotropic nanotribological properties,” J. Mech. Behav. Biomed. Mater., vol. 4, no. 4, pp. 515–522, May 2011, doi: 10.1016/j.jmbbm.2010.12.002.
[14] N. Lin, S.-J. Dong, T.-T. Kong, R. Wang, R. Zou, and Q. Liu, “Heat Transfer Behavior across the Dentino-Enamel Junction in the Human Tooth,” PLoS ONE, vol. 11, Sep. 2016, doi: 10.1371/journal.pone.0158233.
[15] M. A. Garrido, I. Giráldez, L. Ceballos, M. T. G. del Río, and J. Rodríguez, “Nanotribological behaviour of tooth enamel rod affected by bleaching treatment,” Wear, vol. 271, no. 9, pp. 2334–2339, Jul. 2011, doi: 10.1016/j.wear.2011.01.059.
[16] A. Barani, M. B. Bush, and B. R. Lawn, “Effect of property gradients on enamel fracture in human molar teeth,” J. Mech. Behav. Biomed. Mater., vol. 15, pp. 121–130, Nov. 2012, doi: 10.1016/j.jmbbm.2012.06.014.
[17] D. Raabe, C. Sachs, and P. Romano, “The crustacean exoskeleton as an example of a structurally and mechanically graded biological nanocomposite material,” Acta Mater., vol. 53, no. 15, pp. 4281–4292, Sep. 2005, doi: 10.1016/j.actamat.2005.05.027.
[18] B. J. F. Bruet, J. Song, M. C. Boyce, and C. Ortiz, “Materials design principles of ancient fish armour,” Nat. Mater., vol. 7, no. 9, Art. no. 9, Sep. 2008, doi: 10.1038/nmat2231.
[19] L.-H. He, Z.-H. Yin, L. Jansen van Vuuren, E. A. Carter, and X.-W. Liang, “A natural functionally graded biocomposite coating – Human enamel,” Acta Biomater., vol. 9, no. 5, pp. 6330–6337, May 2013, doi: 10.1016/j.actbio.2012.12.029.
[20] L. Tombolato, E. E. Novitskaya, P.-Y. Chen, F. A. Sheppard, and J. McKittrick, “Microstructure, elastic properties and deformation mechanisms of horn keratin,” Acta Biomater., vol. 6, no. 2, pp. 319–330, Feb. 2010, doi: 10.1016/j.actbio.2009.06.033.
[21] D. Raabe, A. Al-Sawalmih, S. B. Yi, and H. Fabritius, “Preferred crystallographic texture of α-chitin as a microscopic and macroscopic design principle of the exoskeleton of the lobster Homarus americanus,” Acta Biomater., vol. 3, no. 6, pp. 882–895, Nov. 2007, doi: 10.1016/j.actbio.2007.04.006.
[22] P.-Y. Chen, A. Y.-M. Lin, J. McKittrick, and M. A. Meyers, “Structure and mechanical properties of crab exoskeletons,” Acta Biomater., vol. 4, no. 3, pp. 587–596, May 2008, doi: 10.1016/j.actbio.2007.12.010.
[23] Z.-L. Zhao, T. Shu, and X.-Q. Feng, “Study of biomechanical, anatomical, and physiological properties of scorpion stingers for developing biomimetic materials,” Mater. Sci. Eng. C, vol. 58, pp. 1112–1121, Jan. 2016, doi: 10.1016/j.msec.2015.09.082.
[24] B. Wang, W. Yang, J. McKittrick, and M. A. Meyers, “Keratin: Structure, mechanical properties, occurrence in biological organisms, and efforts at bioinspiration,” Prog. Mater. Sci., vol. 76, pp. 229–318, Mar. 2016, doi: 10.1016/j.pmatsci.2015.06.001.
[25] Z. Q. Liu, D. Jiao, Z. Y. Weng, and Z. F. Zhang, “Structure and mechanical behaviors of protective armored pangolin scales and effects of hydration and orientation,” J. Mech. Behav. Biomed. Mater., vol. 56, pp. 165–174, Mar. 2016, doi: 10.1016/j.jmbbm.2015.11.013.
[26] S. Bentov et al., “Enamel-like apatite crown covering amorphous mineral in a crayfish mandible,” Nat. Commun., vol. 3, no. 1, Art. no. 1, May 2012, doi: 10.1038/ncomms1839.
[27] Z. Liu, Y. Zhu, D. Jiao, Z. Weng, Z. Zhang, and R. O. Ritchie, “Enhanced protective role in materials with gradient structural orientations: Lessons from Nature,” Acta Biomater., vol. 44, pp. 31–40, Oct. 2016, doi: 10.1016/j.actbio.2016.08.005.
[28] S. Suresh, “Graded Materials for Resistance to Contact Deformation and Damage,” Science, vol. 292, no. 5526, pp. 2447–2451, Jun. 2001, doi: 10.1126/science.1059716.
[29] I. Jäger and P. Fratzl, “Mineralized Collagen Fibrils: A Mechanical Model with a Staggered Arrangement of Mineral Particles,” Biophys. J., vol. 79, no. 4, pp. 1737–1746, Oct. 2000, doi: 10.1016/S0006-3495(00)76426-5.
[30] S. Kamat, X. Su, R. Ballarini, and A. H. Heuer, “Structural basis for the fracture toughness of the shell of the conch Strombus gigas,” Nature, vol. 405, no. 6790, Art. no. 6790, Jun. 2000, doi: 10.1038/35016535.
[31] R. Menig, M. H. Meyers, M. A. Meyers, and K. S. Vecchio, “Quasi-static and dynamic mechanical response of Haliotis rufescens (abalone) shells,” Acta Mater., vol. 48, no. 9, pp. 2383–2398, May 2000, doi: 10.1016/S1359-6454(99)00443-7.
[32] R. Menig, M. H. Meyers, M. A. Meyers, and K. S. Vecchio, “Quasi-static and dynamic mechanical response of Strombus gigas (conch) shells,” Mater. Sci. Eng. A, vol. 297, no. 1, pp. 203–211, Jan. 2001, doi: 10.1016/S0921-5093(00)01228-4.
[33] T. L. Norman, D. Vashishth, and D. B. Burr, “Fracture toughness of human bone under tension,” J. Biomech., vol. 28, no. 3, pp. 309–320, Mar. 1995, doi: 10.1016/0021-9290(94)00069-G.
[34] P. Fratzl et al., “Mineral crystals in calcified tissues: A comparative study by SAXS,” J. Bone Miner. Res., vol. 7, no. 3, pp. 329–334, 1992, doi: https://doi.org/10.1002/jbmr.5650070313.
[35] P. Fratzl, S. Schreiber, and K. Klaushofer, “Bone Mineralization as Studied by Small-Angle X-Ray Scattering,” Connect. Tissue Res., vol. 34, no. 4, pp. 247–254, Jan. 1996, doi: 10.3109/03008209609005268.
[36] P. Fratzl, O. Paris, K. Klaushofer, and W. J. Landis, “Bone mineralization in an osteogenesis imperfecta mouse model studied by small-angle x-ray scattering.,” J. Clin. Invest., vol. 97, no. 2, pp. 396–402, Jan. 1996, doi: 10.1172/JCI118428.
[37] P. Fratzl, H. F. Jakob, S. Rinnerthaler, P. Roschger, and K. Klaushofer, “Position-Resolved Small-Angle X-ray Scattering of Complex Biological Materials,” J. Appl. Crystallogr., vol. 30, no. 5, Art. no. ARRAY(0x977edb4), Oct. 1997, doi: 10.1107/S0021889897001775.
[38] S. Rinnerthaler, P. Roschger, H. F. Jakob, A. Nader, K. Klaushofer, and P. Fratzl, “Scanning Small Angle X-ray Scattering Analysis of Human Bone Sections,” Calcif. Tissue Int., vol. 64, no. 5, pp. 422–429, May 1999, doi: 10.1007/PL00005824.
[39] H. Gao, B. Ji, I. L. Jäger, E. Arzt, and P. Fratzl, “Materials become insensitive to flaws at nanoscale: Lessons from nature,” Proc. Natl. Acad. Sci., vol. 100, no. 10, pp. 5597–5600, May 2003, doi: 10.1073/pnas.0631609100.
[40] T. Mori and K. Tanaka, “Average stress in matrix and average elastic energy of materials with misfitting inclusions,” Acta Metall., vol. 21, no. 5, pp. 571–574, May 1973, doi: 10.1016/0001-6160(73)90064-3.
[41] P. G. de Gennes, “Soft Adhesives,” Langmuir, vol. 12, no. 19, pp. 4497–4500, Jan. 1996, doi: 10.1021/la950886y.
[42] C. Hui, D. Xu, and E. J. Kramer, “A fracture model for a weak interface in a viscoelastic material (small scale yielding analysis),” J. Appl. Phys., vol. 72, no. 8, pp. 3294–3304, Oct. 1992, doi: 10.1063/1.351451.
[43] P. Rahulkumar, A. Jagota, S. J. Bennison, and S. Saigal, “Cohesive element modeling of viscoelastic fracture: application to peel testing of polymers,” Int. J. Solids Struct., vol. 37, no. 13, pp. 1873–1897, Mar. 2000, doi: 10.1016/S0020-7683(98)00339-4.
[44] A. Crompton and K. Hiiemae, “How mammalian molar teeth work,” Discov. N. Hav., vol. 5, pp. 23–34, Jan. 1969.
[45] A. Herrel and P. Aerts, “Biomechanical Studies of Food and Diet Selection,” in eLS, American Cancer Society, 2004. doi: 10.1038/npg.els.0003213.
[46] B. Bushan, Handbook of Micro/Nano Tribology. CRC Press, 2020.
[47] M. B. Daia et al., “Nanoindentation investigation of Ti/TiN multilayers films,” J. Appl. Phys., vol. 87, no. 11, pp. 7753–7757, May 2000, doi: 10.1063/1.373450.
[48] Q. Ma and D. Clarke, “Size dependent hardness of silver single crystals,” J. Mater. Res., vol. 10, pp. 853–863, Mar. 1995, doi: 10.1557/JMR.1995.0853.
[49] S. P. Baker, “The Analysis of Depth-Sensing Indentation Data,” MRS Online Proc. Libr., vol. 308, no. 1, pp. 209–216, Dec. 1993, doi: 10.1557/PROC-308-209.
[50] J. B. Pethicai, R. Hutchings, and W. C. Oliver, “Hardness measurement at penetration depths as small as 20 nm,” Philos. Mag. A, vol. 48, no. 4, pp. 593–606, Apr. 1983, doi: 10.1080/01418618308234914.
[51] G. M. Pharr and W. C. Oliver, “Nanoindentation of silver-relations between hardness and dislocation structure,” J. Mater. Res., vol. 4, no. 1, pp. 94–101, Jan. 1989, doi: 10.1557/JMR.1989.0094.
[52] T. F. Page, W. C. Oliver, and C. J. McHargue, “The deformation behavior of ceramic crystals subjected to very low load (nano)indentations,” J. Mater. Res., vol. 7, no. 2, pp. 450–473, Feb. 1992, doi: 10.1557/JMR.1992.0450.
[53] D. M. Turley and L. E. Samuels, “The nature of mechanically polished surfaces of copper,” Metallography, vol. 14, no. 4, pp. 275–294, Dec. 1981, doi: 10.1016/0026-0800(81)90001-X.
[54] J. B. Pethica and D. Tabor, “Contact of characterised metal surfaces at very low loads: Deformation and adhesion,” Surf. Sci., vol. 89, no. 1, pp. 182–190, Jan. 1979, doi: 10.1016/0039-6028(79)90606-X.
[55] P. J. Burnett and T. F. Page, “Surface softening in silicon by ion implantation,” J. Mater. Sci., vol. 19, no. 3, pp. 845–860, 1984.
[56] W. D. Nix and H. Gao, “Indentation size effects in crystalline materials: A law for strain gradient plasticity,” J. Mech. Phys. Solids, vol. 46, no. 3, pp. 411–425, Mar. 1998, doi: 10.1016/S0022-5096(97)00086-0.
[57] B. Bhushan, Springer Handbook of Nanotechnology. Springer, 2017.
[58] K. W. McElhaney, J. J. Vlassak, and W. D. Nix, “Determination of indenter tip geometry and indentation contact area for depth-sensing indentation experiments,” J. Mater. Res., vol. 13, no. 5, pp. 1300–1306, May 1998, doi: 10.1557/JMR.1998.0185.
[59] A. Bolshakov and G. M. Pharr, “Influences of pileup on the measurement of mechanical properties by load and depth sensing indentation techniques,” J. Mater. Res., vol. 13, no. 4, pp. 1049–1058, Apr. 1998, doi: 10.1557/JMR.1998.0146.
[60] A. E. Giannakopoulos and S. Suresh, “DETERMINATION OF ELASTOPLASTIC PROPERTIES BY INSTRUMENTED SHARP INDENTATION,” vol. 40, no. 10, p. 8.
[61] W. C. Oliver and G. M. Pharr, “An improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments,” J. Mater. Res., vol. 7, no. 6, pp. 1564–1583, Jun. 1992, doi: 10.1557/JMR.1992.1564.
[62] M. Sullivan and B. C. Prorok, “New Insight into Pile-up in Thin Film Indentation,” in MEMS and Nanotechnology, Volume 5, Cham, 2014, pp. 89–95. doi: 10.1007/978-3-319-00780-9_11.
[63] W.-C. Chiang, H.-H. Lin, T.-S. Wu, and C.-F. Chen, “Bulding a cloud service for medical image processing based on service-orient archtecture,” in 2011 4th International Conference on Biomedical Engineering and Informatics (BMEI), Oct. 2011, vol. 3, pp. 1459–1465. doi: 10.1109/BMEI.2011.6098638.
[64] J. J. N. HEYWOOD, “Explaining patterns in modern ruminant diversity: contingency or constraint?,” Biol. J. Linn. Soc., vol. 99, no. 4, pp. 657–672, Apr. 2010, doi: 10.1111/j.1095-8312.2010.01436.x.
[65] N. S. Heckeberg, “The systematics of the Cervidae: a total evidence approach,” PeerJ, vol. 8, p. e8114, Feb. 2020, doi: 10.7717/peerj.8114.
[66] T. J. Hackmann and J. N. Spain, “Invited review: Ruminant ecology and evolution: Perspectives useful to ruminant livestock research and production,” J. Dairy Sci., vol. 93, no. 4, pp. 1320–1334, Apr. 2010, doi: 10.3168/jds.2009-2071.
[67] J. L. Cantalapiedra et al., “Dietary innovations spurred the diversification of ruminants during the Caenozoic,” Proc. R. Soc. B Biol. Sci., vol. 281, no. 1776, p. 20132746, Feb. 2014, doi: 10.1098/rspb.2013.2746.