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Biomimetic organic-inorganic hybrid structural materials

Final Report Summary - BIOHYMAT (Biomimetic organic-inorganic hybrid structural materials)

One of the most sophisticated natural hybrid structures is bone. Bone is both a material in which organic and inorganic phases are carefully integrated at the nanoscale and a complex hierarchical structure that exhibits features at multiple length scales up to the macro-level. As a result this natural tissue combines excellent strength and toughness. This project is inspired by the design of natural bone to develop bottom-up approaches to create organic-inorganic hybrid materials with specific structural and biological properties. Their design is based on the control of the composition of the different phases, the analysis and engineering of their interfaces, and structural manipulation from the nano to the macro levels. There are three main objectives each of them related to one of these main areas. The first one is the synthesis and characterization of hydrogels in order to develop organic matrices. The second is to develop effective approaches to analyse and engineer organic-inorganic interfaces. The last is the fabrication of three-dimensional structures with bioinspired designs.

Hydroxyapatite/Chitosan composites

A key metric that defines the mechanical properties of bone is the bone mineral density (BMD), that is a parameter correlated with the degree and effects of numerous bone diseases. One existing problem in the study of these diseases is the lack of simple techniques for the measurement of BMD with the spatial resolution required to create accurate mineral density maps. Hydroxyapatite is a calcium phosphate with similar composition to the mineral component of bone while chitosan is an organic matrix that has attracted much interest lately for bone engineering applications. I have developed an effective protocol to synthesize monolithic hydroxyapatite/chitosan composite materials with a density comparable to natural bone. These materials can be used in combination with quantitative back-scattered Scanning Electron Microscopy (qbSEM) to develop an approach to accurately determine BMD. Composite hydroxyapatite/chitosan hybrids with different mineral contents can be used as calibration standards with a spectrum of mineral densities that cover the typical range of bone mineral densities. These standards have been used, in collaboration with the Department of Bioengineering of Imperial College, to determine bone density in a variety of healthy, aged, and diseased mice models (Figure 1). This approach can be used to study different diseases that can affect the mineral density of bone, like osteopetrosis (BMD increase resulting in denser bones) or Paget’s disease (opposite effect). Also, a reduction in BMD has been directly linked to an increased risk of fracture in osteoporotic bone. A variation in the organic/inorganic ratio of bone also has an effect on the BMD, with an increase in inorganic content resulting in an increase in density. These composite materials open new opportunities to study the effect of different pathologies on bone mineral content and correlate the measurements with local analysis of mechanical properties.

Organic/inorganic interfaces

The second part of the work has been correlated with the basic analysis of organic/inorganic interfaces similar to those present in hybrid materials. Adhesion at the organic/inorganic interface determines to great extent the mechanical response of natural and synthetic composites and one of the key issues in hybrid design is the lack of the quantitative data on the adhesion of biopolymers (both natural and synthetic) and bioceramics needed to guide materials selection.

The study has been carried out using natural (collagen and chitosan) and synthetic polymers (poly(lactic-co-glycolic acid), PLGA) and calcium phosphates (hydroxyapatite (HA) and tricalcium phosphate (TCP)) typically used in biomedical applications. One of the main issues has been to set up a reliable test to assess the mechanical strength of the organic/ceramic interface that can be subsequently extended to the study of other organic-inorganic systems. Some of the tests evaluated have been the double cantilever bean (DCB) and the modified four-point bending and shear test (Figure 2). The samples have been prepared by bonding two ceramic or glass substrates coated with thin films of a polymer layer. I have developed different approaches based on techniques such as spin coating or tape casting to fabricate biopolymer films with controlled thickness (30-500 nm) and prepare organic/inorganic bonds. Reproducible results have been obtained using shear testing. This test has been used to systematically study the effect of factors like the environment (humidity) and coupling agents (γ-MPS) on the interfacial strength. In particular it has been possible to quantify the detrimental effect of humidity in the different systems. Finally, mechanical adhesion has been compared with the thermodynamic work of adhesion measured using the sessile drop method. As a result of this work we have established a systematic methodology to study adhesion at the organic/inorganic interfaces and its relation with chemistry and environment.

Hierarchical structures

One of key aspect of natural materials that has been extremely difficult to reproduce synthetically is their hierarchical structure that exhibits characteristic features at multiple length scales from the nano to the macro levels. The last objective of the project is the development of complex organic/inorganic hierarchical structures with micro to macroscopic features designed to provide different properties.
The work has been focussed on the freeze casting technique. This process consists on the controlled freezing of the water-based particle suspensions. As the ice grows it expels the particles that accumulate in the space between the ice crystals. After sublimating the water a scaffold that is a negative of the ice remains. We have been used this process to fabricate ceramic scaffolds that can then be filled with a second organic phase to create a hybrid. The freezing conditions have been controlled to manipulate the ice structure and form scaffolds with different architectures. The work has focussed on the control of the processing variables (particle concentration in the suspensions, freezing speeds, use of addtivites...) to manipulate structure. For example, two of the most interesting bioinspired structures obtained are shown in Figure 3, where by modifying the additives (organics) and the processing conditions it has been possible to obtain three-dimensional structures with honeycomb and lamellar structures microporosity. This work opens new opportunities to control the architecture of freeze casted materials in the fabrication of porous ceramics and hybrid composites for a wide range of applications beyond biomaterials, from filters to catalyst support.

Other carbon based hybrid composites

In addition to the previously described work, a different kind of carbon hybrid materials has been studied during the project. I have added different carbon phases to a ceramic phase (zirconia) and processed using the Spark Plasma Sintering (SPS) technique. These studies have been motivated by the growth of interest on some of those phases, such as graphene oxide (GO), carbon nanotubes (CNT) and amorphous carbon, because of their numerous technological applications. The study has been focused in the influence of the different carbon structures in the sintering and grain growth of the ceramic phase. In the case of the GO the material was synthesized in the laboratory through the exfoliation of graphite following a modified Hummers method. The SPS technique has allowed a rapid high temperature processing without degrading the carbon phase and minimizing the grain growth. It has been confirmed by Raman spectroscopy that with this kind of processing it is possible the reduction of the GO to graphene in vacuum atmosphere. The sintering processes were studied in detail calculating the sintering activation energies and correlating them with the sintering mechanism that took place in different stages of the process. The results of this study shown how it is possible to modify the activation energy of the sintering process depending on the carbon phase added into the composite. The data obtained from this study give valuable information for the processing of these carbon composites controlling the sintering mechanism and helping to design the final structure and properties of the material. In figure 4 the fracture microstructure of one of the grapheme oxide composites after the processing is shown.

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