Piezomimetic ceramic production for bone biomaterial development

Abstract

Bone is a composite tissue that contains various materials, including proteins, crystalline molecules, ions and cells. This complex structure is under study to gain a better understanding of bone properties, especially for the development of bone-like biomaterials that can be used for various purposes. The composite nature of the bone makes it difficult to mimic all of its properties in bone biomaterials. Bone biomaterials can be used as in-vivo implants or for in-vitro tissue development. Various types of materials, such as polymers, ceramics, metals and biological molecules, are used for these purposes. In addition to the structural properties of bone, it has been observed that the surface characteristics of the bone play a crucial role in the growth and differentiation of certain bone cells. This has opened the area for bone topography mimicking materials. Some studies focus on formation of irregular surfaces through physical or chemical methods, while some other studies focus on direct imprinting of the bone surface topography onto certain materials like polymers. In addition to its physical and chemical properties, bone also has electrical properties, which has significant importance. Bone generates electrical currents and electric fields under mechanical stress. In terms of this property, bone shows the nature of a piezoelectric material. The piezoelectric effect observed in bone is produced by collagen molecules and has very important effects on the metabolism of bone growth. Charged ions, such as calcium, intercellular fluids, cytoskeleton proteins, voltage-dependent channels and macromolecules with electrical charges such as growth factors, all contribute to bone healing as they are influenced by the electrical fields generated in the bone. By mimicking the electrical properties of bone, it is possible to develop more effective bone biomaterials. With advancements in material science, biocompatible piezoelectric materials have been developed to mimic the electrical properties of bone. These piezoelectric materials can produce electrical currents and fields to in response to externally applied mechanical forces. In the light of this information, it is clear that both topography and piezoelectricity play a crucial role in bone tissue engineering. In this thesis, these properties were combined to develop new types of materials. First, a novel method was developed to transfer the topography of bone surfaces onto hard metal surfaces. Using these metals with surfaces as master molds, ceramics were produced to have both bone surface mimicking characteristics and piezoelectric properties. Cortical bones were used as bone templates for the bone mimicking materials production. Bones were decellularized with a developed method utilizing di-sodium tetraborate as a cross-linker, to preserve surface hardness of the bones. To produce bone mimicking metal surfaces, the positive bone surfaces were molded in silicone rubber under pressure and heat. These rubber molds were used for transferring bone surface structure to special wax. The molded wax materials were then used to create a negative copy of bone topography in plaster molds by using heated furnace. In the last step, the plasters were filled with brass metal under vacuum to imprint the negative bone topography onto metal as positive structures. After this step, the metal molds were used as master molds in a hydraulic press for transferring the surface topography of metals onto ball milled KNN (potassium sodium niobate) ceramic powder to form green bodies. The green bodies were sintered under temperature controlled conditions to produce intact ceramic materials. The piezoelectric properties of sintered ceramics were gained under a strong electric field. Stereo light microscopy, AFM (atomic force microscopy), SEM (scanning electron microscopy), XRD (X-Ray diffraction) analysis, charge counting and compression tests were used to analyze the results. The analysis has demonstrated the successful production of piezoelectric KNN ceramics with bone surface topography characteristics. By combining topographic and piezoelectric properties, a new term, «piezotopography» is proposed for these type of materials. «Piezotopography» refers to functional topographic structures which have piezoelectric properties. «Piezotopograhic material» refers to materials that possess both functional surface structures and piezoelectric properties. Additionally, by combining mimetic surface feature and piezoelectric properties, a new term, «piezomimetic» is proposed. «Piezomimetic material» refers to materials having functional mimetic surface which have piezoelectric properties. In summary, a new method has been developed to produce ceramics with both bone surface mimicry and piezoelectricity properties.