Modelling the Mechanical Properties of Hydroxyapatite Scaffolds Produced by Digital Light Processing-Based Vat Photopolymerization

Porosity is a key feature in dictating the overall performance of biomedical scaffolds, with special relevance to mechanical properties. Usually, compressive strength and elastic modulus are the main parameters used to determine the potential mechanical suitability of porous scaffolds for bone repair. However, their assessment may not be so easy from an experimental viewpoint and, especially if the porosity is high, so reliable for brittle bioceramic foams. Hence, assessing the relationship between porosity and mechanical properties based only on the constitutive parameters of the solid material is a challenging and important task to predict the scaffold performance for optimization during design. In this work, a set of equations was used to predict the compressive strength and elastic modulus of bone-like hydroxyapatite scaffolds produced by digital light processing-based vat photopolymerization (total porosity about 80 vol.%). The compressive strength was found to depend on total porosity, following a power-law approximation. The relationship between porosity and elastic modulus was well fitted by second-order power law, with relative density and computational models obtained by numerical simulations

Computational models for the simulation of the elastic and fracture properties of highly porous 3D-printed hydroxyapatite scaffolds

Bone scaffolding is a promising approach for the treatment of critical-size bone defects. Hydroxyapatite can be used to produce highly porous scaffolds as it mimics the mineralized part of bone tissue, but its intrinsic brittleness limits its usage. Among 3D printing techniques, vat photopolymerization allows for the best printing resolution for ceramic materials. In this study, we implemented a Computed micro-Tomography based Finite Element Model of a hydroxyapatite porous scaffold fabricated by vat photopolymerization. We used the model in order to predict the elastic and fracture properties of the scaffold. From the stress–strain diagram of a simulated compression test, we computed the stiffness and the strength of the scaffolds. We found that three morphometric features substantially affect the crack pattern. In particular, the crack propagation is not only dependent on the trabecular thickness but also depends on the slenderness and orientation of the trabeculae with respect to the load. The results found in this study can be used for the design of ceramic scaffolds with heterogeneous pore distribution in order to tailor and predict the compressive strength

Digital light processing stereolithography of hydroxyapatite scaffolds with bone-like architecture, permeability, and mechanical properties

This work deals with the additive manufacturing and characterization of hydroxyapatite scaffolds mimicking the trabecular architecture of cancellous bone. A novel approach was proposed relying on stereolithographic technology, which builds foam-like ceramic scaffolds by using three-dimensional (3D) micro-tomographic reconstructions of polymeric sponges as virtual templates for the manufacturing process. The layer-by-layer fabrication process involves the selective polymerization of a photocurable resin in which hydroxyapatite particles are homogeneously dispersed. Irradiation is performed by a dynamic mask that projects blue light onto the slurry. After sintering, highly-porous hydroxyapatite scaffolds (total porosity ~0.80, pore size 100-800 µm) replicating the 3D open-cell architecture of the polymeric template as well as spongy bone were obtained. Intrinsic permeability of scaffolds was determined by measuring laminar airflow alternating pressure wave drops and was found to be within 0.75-1.74 × 10−9m2, which is comparable to the range of human cancellous bone. Compressive tests were also carried out in order to determine the strength (~1.60 MPa), elastic modulus (~513 MPa) and Weibull modulus (m = 2.2) of the scaffolds. Overall, the fabrication strategy used to print hydroxyapatite scaffolds (tomographic imaging combined with digital mirror device [DMD]-based stereolithography) shows great promise for the development of porous bioceramics with bone-like architecture and mass transport properties

Application of high resolution DLP stereolithography for fabrication of tricalcium phosphate scaffolds for bone regeneration

Bone regeneration requires porous and mechanically stable scaffolds to support tissue integration and angiogenesis, which is essential for bone tissue regeneration. With the advent of additive manufacturing process, production of complex porous architecture has become feasible. However, a balance has to be sorted between porous architecture and mechanical stability which facilitates bone regeneration for load bearing applications. 
 Current study evaluates used of high resolution digital light processing (DLP) -based additive manufacturing to produce complex but mechanical stable scaffolds based on β-tricalcium phosphate (β-TCP) for bone regeneration. 
 Four different geometries, a rectilinear Grid, hexagonal Kagome, schwart primitive and hollow Schwarz are designed with 400 µm pores and 75 or 50 vol.% porosity. However after initial screening for design stability and mechanical properties, only a rectilinear Grid structure, a hexagonal Kagome structure are found to be reproducible and showed higher mechanical properties. 
 Micro computed tomography (µ-CT) analysis shows < 2 vol.% error in porosity and < 6 % relative deviation of average pore sizes for the Grid structures. At 50 vol.% porosity, this architecture also has the highest compressive strength of 44.7 MPa (Weibull modulus is 5.28), while bulk specimens reach 235 ± 37 MPa. 
 To evaluate suitability of 3D scaffolds produce by DLP methods for bone regeneration, scaffolds were cultured with murine preosteoblastic MC3T3-E1 cells. Short term study showed cells growth over 14 days, with more than two-fold increase of alkaline phosphatase (ALP) activity compared to cells on 2D tissue culture plastic. Collagen deposition was increased by a factor of 1.5 – 2 when compared to the 2D controls. This confirm retention of biocompatible and osteo-inductive properties of β-TCP following DLP process. 
 This study has implications for designing of the high resolution porous scaffolds for bone regenerative applications and contributes to understanding of DLP based additive manufacturing process for medical applications

Multiscale ceramic components from preceramic polymers by hybridization of vat polymerization-based technologies

A novel approach to fabricate ceramic structures at multiple scales in a single component, based on the hybridization of additive manufacturing technologies, was developed by combining 3D macro-stereolithography (Digital Light Processing, DLP) with two-photon lithography (2PL), to produce cm-sized sample geometries with sub-\u3bcm surface features. The preceramic structures in the sub-\u3bcm scale were realized by 2PL directly on easily manageable DLP macro-sized samples of the same ceramic composition. In this way, preceramic structures presenting both features typical of DLP printers (with a minimum size of around 50 \u3bcm) and features well below their resolution limit were realized. We report here, for the first time, the realization of polymer-derived ceramic SiOC ceramic components structured in 3D across several length scales (with micron and mesoscale 3D features), produced by pyrolysis at 1000 \ub0C of preceramic parts, without shape distortion during the pyrolysis step

High-reliability data processing and calculation of microstructural parameters in hydroxyapatite scaffolds produced by vat photopolymerization

The accurate determination of mass transport and microstructural properties within highly-porous trabecular bone specimens and substitutes still represents a challenge due to the complex arrangement in the three-dimensional space, where adjacent pores can be hardly identified due to the open-cell disordered structure resulting from the reciprocal alternation of struts and voids. In the present study, the complete set of mass transport properties of hydroxyapatite (HA) scaffolds produced by digital light processing (DLP)-based vat photopolymerization was determined by applying the recent Ergun-Wu resistance model. Input data include the intrinsic permeability of the scaffolds, obtained by acoustic experimental measurements, and the equivalent pore diameter, calculated as a function of total porosity and average trabecular size from accurate micro-computed tomography (mu-CT) scans. The results, corroborated by an accurate and robust statistical analysis, were compared with previous literature data and confirmed a feasible and concrete application of DLP-derived HA scaffolds in clinical practice