Solid Freeform Technique Research Articles

Nanomaterials for 3D Printing of Polymers via Stereolithography: Concept, Technologies, and Applications

AbstractStereolithography (SLA) is an additive manufacturing method with one of the highest accuracies (down to 100 nm) of all solid freeform techniques and has been used in various areas, such as medicine, automotive, aerospace, electronics, and others. However, most resins available nowadays are derived from petroleum. Its toxicity, low biocompatibility, and growing environmental concerns are limiting its application. This review discusses the development of biobased and biocompatible materials for different SLA processes as well as the usage of nanocomposites to increase their applicability. A comprehensive overview of the SLA technologies, photopolymerization chemistry, and resin properties are also provided. Finally, various examples using different types of materials are explored, to show the current and future capabilities of the SLA technique.

Current advances in solid free-form techniques for osteochondral tissue engineering

Osteochondral (OC) lesions are characterized by defects in two different zones, the cartilage region and subchondral bone region. These lesions are frequently associated with mechanical instability, as well as osteoarthritic degenerative changes in the knee. The lack of spontaneous healing and the drawbacks of the current treatments have increased the attention from the scientific community to this issue. Different tissue engineering approaches have been attempted using different polymers and different scaffolds’ processing. However, the current conventional techniques do not allow the full control over scaffold fabrication, and in this type of approaches, the tuning ability is the key to success in tissue regeneration. In this sense, the researchers have placed their efforts in the development of solid free-form (SFF) techniques. These techniques allow tuning different properties at the micro–macro scale, creating scaffolds with appropriate features for OC tissue engineering. In this review, it is discussed the current SFF techniques used in OC tissue engineering and presented their promising results and current challenges.

Solid Freeform Techniques Application in Bone Tissue Engineering for Scaffold Fabrication.

Solid freeform techniques are revolutionising technology with great potential to fabricate highly organized biodegradable scaffolds for damaged tissues and organs. Scaffolds fabricated via Solid freeform (SFF) techniques have more pronounced effect in bone tissue engineering. SFF techniques produce various types of scaffolds from different biomaterials with specific pore size, geometries, orientation, interconnectivity and anatomical shapes. Scaffolds needs to be designed from such biomaterials which can attach directly to natural tissues and mimic its properties, so ideally mechanical properties of scaffolds should be same as that of regenerating tissues for best results. The scaffolds designed without optimized mechanical properties would lead to the reduced nutrition diffusion within tissue engineered constructs (TECs) causing tissue necrosis. These scaffolds are mainly processed from ceramics and polymers like calcium phosphate, polydioxane, €-polycaprolactone, polylactic and polyglycolic acids etc. While, hydrogel scaffolds provide bridge for encapsulated cells and tissues to integrate with natural ECM. Likewise, 2D images from radiography were not sufficient for the prediction of the brain structure, cranial nerves, vessel and architecture of base of the skull and bones, which became possible using the 3D prototyping technologies. Any misrepresentation can lead to fatal outcomes. Biomodelling from these techniques for spinal surgery and preoperative planning are making its way toward successful treatment of several spinal deformities and spinal tumor. In this review we explored laser based and printing SFF techniques following its methodologies, principles and most recent areas of application with its achievements and possible challenges faced during its applications.

Effect of layer thickness and printing orientation on mechanical properties and dimensional accuracy of 3D printed porous samples for bone tissue engineering.

Powder-based inkjet 3D printing method is one of the most attractive solid free form techniques. It involves a sequential layering process through which 3D porous scaffolds can be directly produced from computer-generated models. 3D printed products' quality are controlled by the optimal build parameters. In this study, Calcium Sulfate based powders were used for porous scaffolds fabrication. The printed scaffolds of 0.8 mm pore size, with different layer thickness and printing orientation, were subjected to the depowdering step. The effects of four layer thicknesses and printing orientations, (parallel to X, Y and Z), on the physical and mechanical properties of printed scaffolds were investigated. It was observed that the compressive strength, toughness and Young's modulus of samples with 0.1125 and 0.125 mm layer thickness were more than others. Furthermore, the results of SEM and μCT analyses showed that samples with 0.1125 mm layer thickness printed in X direction have more dimensional accuracy and significantly close to CAD software based designs with predefined pore size, porosity and pore interconnectivity.

Permeability analysis of scaffolds for bone tissue engineering

Porous artificial bone substitutes, especially bone scaffolds coupled with osteobiologics, have been developed as an alternative to the traditional bone grafts. The bone scaffold should have a set of properties to provide mechanical support and simultaneously promote tissue regeneration. Among these properties, scaffold permeability is a determinant factor as it plays a major role in the ability for cells to penetrate the porous media and for nutrients to diffuse. Thus, the aim of this work is to characterize the permeability of the scaffold microstructure, using both computational and experimental methods. Computationally, permeability was estimated by homogenization methods applied to the problem of a fluid flow through a porous media. These homogenized permeability properties are compared with those obtained experimentally. For this purpose a simple experimental setup was used to test scaffolds built using Solid Free Form techniques. The obtained results show a linear correlation between the computational and the experimental permeability. Also, this study showed that permeability encompasses the influence of both porosity and pore size on mass transport, thus indicating its importance as a design parameter. This work indicates that the mathematical approach used to determine permeability may be useful as a scaffold design tool.

Laser shock processing on microstructure and hardness of polycrystalline cubic boron nitride tools with and without nanodiamond powders

High amplitude, short duration shock waves created by a 1064nm, 10ns Q-switched Nd:YAG laser were used to increase the hardness as well as build successive layers of nanodiamond on sintered polycrystalline cubic boron nitride (PcBN) tools. Multiple scans of laser shocking were applied. Scanning electron microscopy, Raman spectroscopy, Tukon microhardness tester, and optical surface profilometer were used to evaluate the microstructure, phase change, Vicker’s microhardness and surface roughness. Results indicated that laser shock processing of plain PcBN changed the binder concentration, caused phase transition from cubic to hexagonal form, increased the hardness, and almost unaffected surface roughness. Laser shock wave sintering of nanodiamond powders on PcBN resulted in deagglomeration and layer-by-layer build-up of nanoparticles for a thickness of 30μm inferring that a novel solid freeform technique designated as “shock wave induced freeform technique (SWIFT)” is being discovered for making micro-tools. Depending on the number of multiple laser shocks, the hardness of nanodiamond compact was lower or higher than that of PcBN. It is hypothesized that nanodiamond particles could serve as crack deflectors, increasing the fracture toughness of PcBN.

Finite element predictions compared to experimental results for the effective modulus of bone tissue engineering scaffolds fabricated by selective laser sintering

A current challenge in bone tissue engineering is to create scaffolds with suitable mechanical properties, high porosity, full interconnectivity and suitable pore size. In this paper, polyamide and polycaprolactone scaffolds were fabricated using a solid free form technique known as selective laser sintering. These scaffolds had fully interconnected pores, minimized strut thickness, and a porosity of approximately 55%. Tensile and compression tests as well as finite element analysis were carried out on these scaffolds. It was found that the values predicted for the effective modulus by the FE model were much higher than the actual values obtained from experimental results. One possible explanation for this discrepancy, viz. the surface roughness of the scaffold and the presence of micropores in the scaffold struts, was investigated with a view to making recommendations on improving FE model configurations for accurate effective property predictions.

Novel re-entrant porous composite structure: a potential for orthopaedic applications

It was hypothesized that the nonlinear load-displacement relationship displayed by bone could be conferred on an implant by tailoring its structure, yielding an enhanced mechanical stimulation of the tissues. Composite structures would feature piezoelectric properties that could also stimulate osteogenesis. Preliminary mechanical and electromechanical investigations of such porous structures are presented. Initial trial bowtie specimens with various aspect ratii were made from Nickel powder via a solid free form process and from stainless steel shim stocks. Poled Barium Titanate plates were sandwiched between stainless steel bowtie cells to create composite structures.Results: Under quasi-static compression, the Nickel structures displayed a nonlinear mechanical behavior at small strains and an overall strain-stress relationship similar to bone. Under cyclic compressive tests to 0.6 percent strain, all structures presented a repeatable nonlinear strain-stress behavior. The curves were fitted by a second-order polynomial whose coefficients are function of the relative density of the structure to a power n. Composite stainless steel/BaTiO3 bowtie structures confirmed that their electromechanical properties can be tailored.Discussion: Certain patients present metabolic degeneration that hamper bone healing. A ductile and tough structural material with piezoelectric properties such as the new composite structures in development presents the potential to overcome those limitations. They could have the advantages of existing devices without some of the drawbacks. Those porous implants may reduce the needs, costs, and risks linked to the additional use and implementation of an electrical stimulator and BMPs. Furthermore, the solid free form technique gives control over the mechanical properties of the structure. Thus, the mechanotransduction activity of biologic cells can be fully exploited to trigger a faster implant-tissue bonding, which could lead to reduction of surgical cost and time.

Synthesis and Evaluation of Novel Biodegradable Hydrogels Based on Poly(ethylene glycol) and Sebacic Acid as Tissue Engineering Scaffolds

Novel biodegradable poly(ethylene glycol) (PEG) based hydrogels, namely, PEG sebacate diacrylate (PEGSDA) were synthesized, and their properties were evaluated. Chemical structures of these polymers were confirmed by Fourier transform infrared and proton nuclear magnetic resonance (1H NMR) spectroscopy. After photopolymerization, the dynamic shear modulus of the hydrogels was up to 0.2 MPa for 50% PEGSDA hydrogel, significantly higher than conventional hydrogels such as PEG diacrylate (PEGDA). The swelling ratios of these macromers were significantly lower than PEGDA. The in vitro degradation study demonstrated that these hydrogels were biodegradable with weight losses about 66% and 32% for 25% and 50% PEGSDA after 8 weeks of incubation in phosphate-buffered saline at 37 degrees C. In vitro biocompatibility was assessed using cultured rat bone marrow stromal cells (MSCs) in the presence of unreacted monomers or degradation products. Unlike conventional PEGDA hydrogels, PEGSDA hydrogel without RGD peptide modification induced MSC cell adhesion similar to tissue culture polystyrene. Finally, complex three-dimensional structures of PEGSDA hydrogels using solid free form technique were fabricated and their structure integrity was better maintained than PEGDA hydrogels. These hydrogels may find use as scaffolds for tissue engineering applications.

Defects and prevention in ceramic components fabricated by inkjet printing

Inkjet printing has been used as a solid freeform technique to fabricate a wider rage of miniature ceramic components with structures difficult to obtain using traditional ceramic fabrication techniques. Quality control of fabricated components is crucial to avoid macroscopic and microscopic defects. Macroscopic defects are those due to lack of accuracy of deposition, which affect both the precision of size and shape definitions. Microscopic defects are internal voids or flaws due to deposition or subsequent treatments. This paper reports the work in the identification of different types of defects, their causes and prevention in terms of ink properties and printing conditions.

Solid freeform fabrication of three-dimensional scaffolds for engineering replacement tissues and organs

Most tissue engineering (TE) strategies for creating functional replacement tissues or organs rely on the application of temporary three-dimensional scaffolds to guide the proliferation and spread of seeded cells in vitro and in vivo. The characteristics of TE scaffolds are major concerns in the quest to fabricate ideal scaffolds. This paper identifies essential structural characteristics and the pre-requisites for fabrication techniques that can yield scaffolds that are capable of directing healthy and homogeneous tissue development. Emphasis is given to solid freeform (SFF), also known as rapid prototyping, technologies which are fast becoming the techniques of choice for scaffold fabrication with the potential to overcome the limitations of conventional manual-based fabrication techniques. SFF-fabricated scaffolds have been found to be able to address most, if not all the macro- and micro-architectural requirements for TE applications. This paper reviews the application/potential application of state-of-the-art SFF fabrication techniques in creating TE scaffolds. The advantages and limitations of the SFF techniques are compared. Related research carried out worldwide by different institutions, including the authors’ research are discussed.

Using SALDVI and SALD with multi-material structures

Selective-area laser deposition and selective-area laser-deposition vapor infiltration are two gas-phase solid-freeform techniques capable of the direct fabrication of arbitrary structures. The wide range of available gas precursors allows unique combinations of materials to be achieved in the final shape. Tailoring of the local microstructure can be achieved by carefully controlling processing temperature, gas partial pressure, and other variables. The versatility of the two techniques can be seen in the fabrication of a structure comprising multiple materials.