Stefan Lohfeld

Selective laser sintering of hydroxyapatite/poly-ε-caprolactone scaffolds☆

Selective laser sintering (SLS) enables the fabrication of complex geometries with the intricate and controllable internal architecture required in the field of tissue engineering. In this study hydroxyapatite and poly-epsilon-caprolactone, considered suitable for hard tissue engineering purposes, were used in a weight ratio of 30:70. The quality of the fabricated parts is influenced by various process parameters. Among them Four parameters, namely laser fill power, outline laser power, scan spacing and part orientation, were identified as important. These parameters were investigated according to a central composite design and a model of the effects of these parameters on the accuracy and mechanical properties of the fabricated parts was developed. The dimensions of the fabricated parts were strongly dependent on the manufacturing direction and scan spacing. Repeatability analysis shows that the fabricated features can be well reproduced. However, there were deviations from the nominal dimensions, with the features being larger than those designed. The compressive modulus and yield strength of the fabricated microstructures with a designed relative density of 0.33 varied between 0.6 and 2.3 and 0.1 and 0.6 MPa, respectively. The mechanical behavior was strongly dependent on the manufacturing direction.

Fabrication, mechanical and in vivo performance of polycaprolactone/tricalcium phosphate composite scaffolds.

This paper explores the use of selective laser sintering (SLS) for the generation of bone tissue engineering scaffolds from polycaprolactone (PCL) and PCL/tricalcium phosphate (TCP). Different scaffold designs are generated, and assessed from the point of view of manufacturability, porosity and mechanical performance. Large scaffold specimens are produced, with a preferred design, and are assessed through an in vivo study of the critical size bone defect in sheep tibia with subsequent microscopic, histological and mechanical evaluation. Further explorations are performed to generate scaffolds with increasing TCP content. Scaffold fabrication from PCL and PCL/TCP mixtures with up to 50 mass% TCP is shown to be possible. With increasing macroporosity the stiffness of the scaffolds is seen to drop; however, the stiffness can be increased by minor geometrical changes, such as the addition of a cage around the scaffold. In the animal study the selected scaffold for implantation did not perform as well as the TCP control in terms of new bone formation and the resulting mechanical performance of the defect area. A possible cause for this is presented.

Biomodels of bone: a review

In this paper, a definition of a biomodel is presented, based on which different specific types of biomodels are identified, viz., virtual biomodels, computational biomodels, and physical biomodels. The paper then focuses on both physical and virtual biomodels of bone, and presents a review of model generation methodologies, giving examples of typical biomodel applications. The use of macroscale biomodels for such issues as the design and preclinical testing of surgical implants and preoperative planning is discussed. At the microscale, biomodels of trabecular bone are examined and the link with scaffolds for tissue engineering is established. Conclusions are drawn on the state of the art, and the major developments necessary for the continued expansion of the field are identified. Finally, arguments are given on the benefits of integrating the use of the different types of biomodels reviewed in this paper, for the benefit of future research in biomechanics and biomaterials.

Investigation of the mechanical interaction of the trabecular core with an external shell using rapid prototype and finite element models.

The mechanical properties of vertebral bone have been widely studied with the ultimate goal of improving fracture risk prediction. However, the mechanical interaction between the cortical shell and the trabecular core is not well understood. The objective of this study was to investigate this interaction and to determine what effect it has on the ultimate strength of the whole bone. This objective was achieved by compression testing rapid prototype (RP) models of cylindrical trabecular bone cores, with and without an integral surrounding shell and incorporating increasing levels of artificially induced bone loss. Corresponding finite element (FE) models were generated and the load sharing of the shell and trabecular core was analysed under linear elastic loading conditions. The results of the physical RP model tests and corresponding FE analyses indicated that there was a reinforcing effect between the cortical shell and the trabecular core for all models tested and that the reinforcing effect became relatively more important to the ultimate strength of the whole bone as the bone volume fraction of the trabecular core decreased. It was found that two mechanisms contributed to the reinforcing effect: (i) load transfer from the highly stressed shell into the connecting outer trabeculae of the core for the shelled model. This did not occur for the un-shelled model where the load dropped off at the outer unsupported trabeculae; (ii) the stiffening effect on the shell due to the support provided by the connecting struts of the trabecular core, which serves to inhibit bending and buckling behaviour in the shell under compression loading. It was found that the stiffening on the shell was the more dominant contributor to the overall reinforcing effect between the shell and the trabecular core.

Evaluating the effect of increasing ceramic content on the mechanical properties, material microstructure and degradation of selective laser sintered polycaprolactone/β-tricalcium phosphate materials

Orthopaedic scaffold materials were fabricated from polycaprolactone (PCL) and composite PCL-β-tricalcium phosphate (PCL/β-TCP) powders using selective laser sintering (SLS). Incorporating β-TCP particles is desirable to promote osteogenesis. The effects of increasing β-TCP content on the materials mechanical properties and microstructure were evaluated. The wt% of β-TCP and PCL particle sizes were found to influence material microstructure and mechanical properties, with increasing ceramic content causing a small but significant increase in stiffness but significant reductions in strength. Degradation of materials was achieved using accelerated ageing methods. The influence of β-TCP content on degradation at 7 weeks was evaluated through changes in mechanical properties and microstructure, and the ceramic particles were found to reduce elastic modulus and increase strength. The results of this study highlight the influence of ceramic content on mechanical properties and degradation behaviour of PCL/β-TCP SLS materials, and indicate that these changes must be considered in the design of scaffolds for critical-sized defects.

Computational modelling of ovine critical-sized tibial defects with implanted scaffolds and prediction of the safety of fixator removal

Computational model geometries of tibial defects with two types of implanted tissue engineering scaffolds, β-tricalcium phosphate (β-TCP) and poly-ε-caprolactone (PCL)/β-TCP, are constructed from µ-CT scan images of the real in vivo defects. Simulations of each defect under four-point bending and under simulated in vivo axial compressive loading are performed. The mechanical stability of each defect is analysed using stress distribution analysis. The results of this analysis highlights the influence of callus volume, and both scaffold volume and stiffness, on the load-bearing abilities of these defects. Clinically-used image-based methods to predict the safety of removing external fixation are evaluated for each defect. Comparison of these measures with the results of computational analyses indicates that care must be taken in the interpretation of these measures.

Laser Sintering for the Fabrication of Tissue Engineering Scaffolds

Laser sintering (LS) utilises a laser to sinter powder particles. A volumetric model is sliced and processed cross section by cross section to create a physical part. In theory, all powdered materials are suitable for sintering; however, only few have been tested successfully. For tissue engineering (TE) applications of this rapid prototyping technology it is an advantage that no toxic solvents or binders are necessary. This chapter reviews the direct and indirect use of LS to fabricate scaffolds for TE from single and multiphase materials.

Bridging the osteochondral gap in mandibular condyle reconstruction with multiphasic 3D printing

This is an accompanying abstract of a poster presented at 4th TERMIS World Congress Boston, Massachusetts September 8–11, 2015. Final publication is available from Mary Ann Liebert, Inc., publishers https://www.liebertpub.com/doi/pdf/10.1089/ten.tea.2015.5000.abstracts

3D Printing of Microspheres for Tissue Engineering Scaffolds

This is an accompanying abstract of a poster presented at 4th TERMIS World Congress Boston, Massachusetts September 8–11, 2015. Final publication is available from Mary Ann Liebert, Inc., publishers https://www.liebertpub.com/doi/pdf/10.1089/ten.tea.2015.5000.abstracts

A route for digital design and manufacturing of customised maxillofacial implants

Rapid Prototyping (RP) and more recently Rapid Manufacturing (RM) as a part of Engineering Assisted SurgeryTM enables manufacturing of customised implants and prostheses prior to surgical procedures. Beginning with CT or MRI scans, implants can be customised designed for a named patient and optimised for shape and mechanical requirements using digital data only, disregarding all physical models except for the final implant. In this paper a route to digitise the design of an existing implant is described, consisting of transfer of CT data, design of the prostheses, FE analysis, rapid manufacturing from titanium, and quality control.