Developing atom probe tomography for unique nanoscale insights into biomaterials
Keywords: bone
atom probe tomography
biomaterials
bioactive glass

Bone provides structure and support for vertebrates, and it is the largest ion exchanger in the body to maintain homeostasis. Bone is a complex and heterogeneous composite material mainly composed of inorganic phases (mineral), organic phases (collagen, non-collagenous proteins), and water. Understanding the spatial structure and chemical composition of bone across different length scales is of great significance for elucidating its biomineralization mechanism, mechanical support, bone pathological treatment, and bone scaffold development. However, the simultaneous characterization of structure and chemical information of bone at the nanoscale presents many limitations, especially the exploration of 3D spatial structure and the mapping of low atomic mass elements.
Among the many synthetic bone substitutes, bioactive glasses (BG) are an attractive candidate with applications in critical bone damage repair, as they stimulate biological responses that favor bone formation and angiogenesis. However, previously it has been difficult to develop an amorphous BG that combines a 3D porous structure with a strong biological activity because conventional BG easily crystallizes during processing. The crystallization of bioactive glass limits the dissolution rate of the material and therefore slows down the surface reactivity, leading to a decrease in scaffold bioactivity and bone regeneration capabilities. In recent years, a new amorphous 3D strontium-containing BG (pSrBG) scaffold was developed by adding strontium to increase the bioactivity of BG. Prior characterization of this material showed a scaffold with a near-perfect bone contact without fibrous tissue coverage, and that it supports nearly exclusively lamellar bone repair, similar to normal and functional bone. Besides, strontium was detected in new form bone and plasma after 21 days of transplantation in vivo, indicating that strontium successfully diffused as the material dissolved. However, due to the detection limits of conventional characterization techniques, it was not possible to specify the precise locality of released ions, and hence, it is not clear whether the locally achievable strontium concentration at the interface exceeds the medically acceptable range. Furthermore, the mechanism of Sr uptake into the bone and bone repair remains inconclusive.
Atom probe tomography (APT) is a 3D microscopy characterization technique with a unique combination of high spatial and chemical resolution, which can be used to characterize this new type of biomaterial, animal bone, and the interface between material and bone.
The work in this thesis presents the enabling preliminary development of APT techniques prior to this interface analysis. The synthetic bone material and porcine trabecular bone were investigated and characterised using APT. The challenges and corresponding countermeasures of biomaterials for APT sample preparation and experiments are outlined. The influence of various experimental parameters such as temperature, detection rate, laser pulse energy, and pulse frequency on the data quality by the LEAP-5000XR is explored and discussed. To this end, optimal operating conditions of APT were investigated and selected for two strontium-containing bioactive glass particles, the pSrBG scaffold, and porcine trabecular bone. The structure, including the spatial distribution of collagen and mineral phases, and their chemical composition were analyzed at the atomic level within the porcine trabecular bone. The challenges and limitations of APT in reconstruction analysis and quantitative chemical composition measurements of biomaterials are addressed. This study demonstrates that APT has the unique capacity to identify and characterize significant compositional variations in nanoscale volumes within individual bone phases that may provide new insights into the further development and demonstration of the potential of APT in exploring the spatial structure and chemical composition of bones. It also provides the basis for advancing knowledge in APT research at the interface of pSrBG and bone.