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3D printed COLLagen type I-Hydroxyapatite prostheses for the middle EAR

Periodic Reporting for period 2 - COLLHEAR (3D printed COLLagen type I-Hydroxyapatite prostheses for the middle EAR)

Reporting period: 2020-07-17 to 2021-07-16

Conductive hearing loss, due to traumas or pathologies of the middle ear, affects more than 5% of the population worldwide and more than 15% of the elderly. COLLHEAR aims at giving a contribution by developing a new generation of biocompatible middle ear micro-prostheses through an interdisciplinary approach involving material science, engineering and tissue engineering. This bioengineering and clinical subject is strongly related to the current European Community priorities: indeed it is a fact that by living in a world basing upon communication, any hearing impairments can be considered a relevant obstacle in human relations that can lead to isolation and depression.
The Overall Objective of this proposal is to provide new biocompatible bulk micro-prostheses through an optimisation of the current 3D printing technology for biomaterials. This main goal is a topical subject that is strongly related to the 2014-2020 priorities of the European Community. Specifically, it will represent a remarkable contribution to the promotion of the social inclusion improving the quality of life for people (Priority 9), through a strengthening of the research and innovation in this scientific field (Priority 1), helping deaf people to reinstate their position in a world dominated by communication.
COLLHEAR was conceived to meet the following sub-objectives:
-) O1: Multiscale modeling of the Collagen (COL)/Hydroxyapatite (HA) composite for acousto-mechanical purposes. The objective seeks to answer the questions related to the role of COL/HA in transmitting and dissipate transient loads.
-) O2: Optimization of the 3D printing technology to manufacture the micro-prostheses. Within this goal, the Experienced Researcher (ER) wants to study an optimization of 3D printing equipment in order to efficiently fabricate micro-prostheses.
-) O3: Acousto-mechanical characterization of the micro-prostheses. The fabricated micro-prostheses undergo mechano-acoustic tests to assess their mechanical properties in view of the clinical application.
-) O4: Biological studies for cell adhesion and growth. The fabricated micro-prostheses are studied from a biological standpoint to assess the biocompatibility in view of the eventual implantation (not expected within the project).
WP1 aims at developing a multiscale model of COL/HA composite to assess the capability of the material of transmitting and dissipating transient loads.
A first study was carried on wave propagation and energy dissipation in COL peptides. The results showed a strong dependency on the viscoelastic behavior based on loading direction and hydration state. This work suggested distinct roles of collagen in terms of wave transmission in macro-tissues with different percentages of water, such as tendon and eardrum.
In a second study, the ER investigated wave transmission and energy dissipation along dry/hydrated mineralized fibrils, studying the influence of the mineral percentage, water content, and input velocity on the mechanical response of the material. Results showed decreasing trends for both wave speeds and Young’s Moduli over input velocity with a marked strengthening effect in the region where HA is accumulated. In contrast, the dissipative behavior is not affected by either loading conditions or mineral percentage, showing a stronger damping effect upon faster inputs compatible with the bone behavior at the macroscale.
A third study concerned the optimization of middle ear prostheses by maximizing the global stiffness and using the smallest possible volume constraint that ensured material continuity. This investigation optimized the prosthesis topology in response to static displacement loads with amplitudes that normally occur during sound stimulation. Following earlier studies, the ER discussed how the presence and arrangement of holes on the surface of the prosthesis plate in contact with the umbo affect the overall geometry. Finally, the achieved designs were validated through a finite-element model, in which the prosthesis performance was assessed upon dynamic sound pressure loads by considering four different constitutive materials: titanium, cortical bone, silk, and COL/HA.
WP2 concerns the strategies for printing micro-prostheses. The activities started with an extended review of the methodologies currently employed to 3D print HA-reinforced materials.
After the training sessions, the ER followed two parallel research avenues: • Optimization study on the topology of the 3D printing nozzle. This activity exploits the synergies of finite-element models with machine learning (ML) algorithms and the mechanical characterization of materials. The results showed the capability of the developed algorithm to predict the shear stress on the ink based on the geometry of the nozzle. Moreover, it was found that conical nozzles with angles less than 10° and diameters smaller than 400 microns are suitable for 3D-printing prostheses. • Printing the micro-prostheses. The ER was able to 3D print the micro-prostheses made of HA, Silk/HA, and COL/HA with a composition equal to 10/90 (w/w).
WP3 relates to the acousto-mechanical characterization of the micro-prostheses. The ER worked on experimental measurements on human temporal bones and tested the micro-prostheses against acoustic inputs in the acoustic range. The results showed the good suitability of the prostheses to transmit the acoustic energy towards the inner ear.
WP4 relates to the biological assessment of the prostheses. Two main studies were carried out. The first concerned the culture of epithelial cells on the prostheses while, in contrast, the second related to a dynamic study with a dedicated bioreactor to assess the capability of hMSCs to migrate on the prostheses when loaded by a simultaneous mechanical and acoustic input. In both cases the prostheses were successfully covered by cells, demonstrating the capability of the device of being integrated into the host tissues.
The activities carried out during the fellowship gave the opportunity to describe for the first time the behavior of collagen-based materials at the nanoscale by highlighting the viscoelastic properties of such tissues upon transient loads. This will benefit not only a better understanding of materials' physiology but also give the opportunity to design new bioinspired biomaterials for bone replacements and, focusing on COLLHEAR, for middle ear prostheses, which characteristics will be optimized to transfer mechanical energy and dissipate potentially hurtful inputs. From the manufacturing standpoint, the activities gave the possibility to develop new strategies to fabricate micro-component with biocompatible materials with impacts on both materials science (e.g. a study on material printability, mechanical characterization) and tissue engineering (i.e. development of micro-scaffolds for cell culture to be employed as tissue replacements).
Finally, the acousto-mechanical and biological characterization of the prostheses presented the final validation of the approach, demonstrating how these new devices may represent a real breakthrough in the commercial scenario, leading to a significant reduction in the costs for the replacements, and the development of more-biocompatible and customized prostheses less prone to body rejection with improved acousto-mechanical performance.