Lattice Scaffolds Research Articles

Surface Tension-Assisted Additive Manufacturing of Tubular, Multicomponent Biomaterials.

The fabrication of functional biomaterials for organ replacement and tissue repair remains a major goal of biomedical engineering. Advances in additive manufacturing (AM) technologies and computer-aided design (CAD) are advancing the tools available for the production of these devices. Ideally, these constructs should be matched to the geometry and mechanical properties of the tissue at the needed implant site. To generate geometrically defined and structurally supported multicomponent and cell-laden biomaterials, we have developed a method to integrate hydrogels with 3D-printed lattice scaffolds leveraging surface tension-assisted AM.

In-situ re-melting and re-solidification treatment of selective laser sintered polycaprolactone lattice scaffolds for improved filament quality and mechanical properties

Selective laser sintering (SLS) is a promising additive manufacturing technique that produces biodegradable tissue-engineered scaffolds with highly porous architectures without additional supporting. However, SLS process inherently results in partially melted microstructures which significantly impair the mechanical properties of the resultant scaffolds for potential applications in tissue engineering and regenerative medicine. Here, a novel post-treatment strategy was developed to endow the SLS-fabricated polycaprolactone (PCL) scaffolds with dense morphology and enhanced mechanical properties by embedding them in dense NaCl microparticles for in-situ re-melting and re-solidification. The effects of re-melting temperature and dwelling time on the microstructures of the SLS-fabricated filaments were studied. The results demonstrated that the minimum requirements of re-melting temperature and dwelling time for sufficient treatment were 65 °C and 5 min respectively and the size of the SLS-fabricated filaments was reduced from 683.3 ± 28.0 μm to 601.6 ± 17.4 μm. This method was also highly effective in treating three-dimensional (3D) PCL lattice scaffolds, which showed improved filament quality and mechanical properties after post-treatment. The treated PCL scaffolds with an initial compressive modulus and strength of 3027.8 ± 204.2 kPa and 208.8 ± 14.5 kPa can maintain their original shapes after implantation in vivo for 24 weeks. Extensive newly-grown tissues were found to gradually penetrate into the porous regions along the PCL filaments. Although degradation occurred, the mechanical properties of the implanted constructs stably maintained. The presented method provides an innovative, green and general post-treatment strategy to improve both the filament quality and mechanical properties of SLS-fabricated PCL scaffolds for various tissue engineering applications.

Additively-manufactured poly-ether-ether-ketone (PEEK) lattice scaffolds with uniform microporous architectures for enhanced cellular response and soft tissue adhesion

Additively-manufactured PEEK orthopedic implants have recently gained extensive attention due to their prominent characteristics such as good biocompatibility, low radiographic artifacts and similar elastic modulus to native bones. However, the inherent drawback associated with PEEK implants was their biologically inert surface which caused unsatisfactory cellular response and poor adhesion between the implants and surrounding soft tissues. Here we developed a sulfonation-treatment strategy to create microporous architectures onto the filaments of the additively-manufactured PEEK lattice scaffolds. The sulfonation time in the range of 30–45 s was found to facilitate the formation of uniform microscale pores throughout the printed PEEK lattice scaffolds and simultaneously have slight effect on their composition and mechanical properties. Biological results showed that the presence of microscale pores on the additively-manufactured PEEK lattice scaffolds significantly improved the spreading, proliferation and calcium deposition of bone-specific cells in comparison with the untreated PEEK lattice scaffolds. In vivo experiments demonstrated that the sulfonation-treated micropores facilitated the adhesion of newly-regenerated soft tissues to form tight implant-tissue bonding interfaces. The presented method provides a promising approach to improve the surface bioactivity of additively-manufactured PEEK lattice scaffolds for enhanced cellular response and soft tissue adhesion.

Finite element evaluations of the mechanical properties of polycaprolactone/hydroxyapatite scaffolds by direct ink writing: Effects of pore geometry

Osteochondral (OC) defects usually involve the damage of both the cartilage and its underneath subchondral bone. In recent years, tissue engineering (TE) has become the most promising method that combines scaffolds, growth factors, and cells for the repair of OC defects. An ideal OC scaffold should have a gradient structure to match the hierarchical mechanical properties of natural OC tissue. To satisfy such requirements, 3D printing, e.g., direct ink writing (DIW), has emerged as a technology for precise and customized scaffold fabrication with optimized structures and mechanical properties. In this study, finite element simulations were applied to investigate the effects of pore geometry on the mechanical properties of 3D printed scaffolds. Scaffold specimens with different lay-down angles, filament diameters, inter-filament spacing, and layer overlaps were simulated in compressive loading conditions. The results showed that Young's moduli of scaffolds decreased linearly with increasing scaffold porosity. The orthotropic characteristics increased as the lay-down angle decreased from 90° to 15°. Moreover, gradient transitions within a wide range of strain magnitudes were achieved in a single construct by assembling layers with different lay-down angles. The results provide quantitative relationships between pore geometry and mechanical properties of lattice scaffolds, and demonstrate that the hierarchical mechanical properties of natural OC tissue can be mimicked by tuning the porosity and local lay-down angles in 3D printed scaffolds.

A Novel Three-Dimensional Desalination System Utilizing Honeycomb-Shaped Lattice Structures for Flow-Electrode Capacitive Deionization

Capacitive deionization (CDI) technology has attracted a great deal of attention over the past few decades due to its high energy efficiency (less than 1 kWh/m-3) compared to conventionally utilized reverse osmosis (2–4 kWh/m-3) and distillation based (50–80 kWh/m-3) desalination technologies. However, due to a couple of issues including cost and requirement for a disruptive discharging step, it has been regarded as a challenging task to perform large-scale desalination of seawater by the CDI process. Herein, we report a novel design for a 3D desalination system utilizing porous lattice scaffolds. It can desalinate water as salty as sea water (35 g/L) with desalting efficiency comparable with conventional flow-electrode-based CDI (FCDI). By coating of ion exchange membranes and a graphene layer inside each channel of the lattice structures, honeycomb-shaped desalination cells could be realized. First, the desalination performance of the new design was investigated in batch mode. It exhibited comparable desalting performance while maintaining typical advantages of FCDI systems, such as low energy consumption and no need of a discharging step. Furthermore, since the porous structures involve channels for ion transportation and act as a structural scaffold, the cell architecture is remarkably compact and it can be readily scaled-up by varying the number of unit cells and its configuration, allowing great increase in the salt removal capacity. We scaled-up the desalination system to a 3 × 3 cell, which showed desalination efficiency four times higher than that of a 1 × 3 cell. Such facile build-up of the unit cell structure and versatile design of cell configuration provide strong potential for further scalability. We also determined that this system can be operated successfully in continuous mode, indicating that neither a distinct discharging step nor substitution of inflow and effluent are required. Because there is opportunity for much more improvement of desalting performance in terms of the electrode materials, resistance at the interface, and conductivity of the current collecting layer of this system, we expect that our approach has opened a new door in the field of desalination research.

Fabrication of multiple-layered gradient cellular metal scaffold via electron beam melting for segmental bone reconstruction

The triple- and double-layered mesh Ti-based alloy scaffolds were successfully fabricated using electron beam melting (EBM). In this study Ti-based alloy cylindrical scaffolds with different 3D architectures intended for the segmental bone defect treatment were systematically compared. All lattice-like scaffolds were additively manufactured using EBM technology from Ti6Al4V to mimic the structures of human trabecular bone. Cylindrically-shaped lattice scaffolds (outer diameter of 15mm and length of 35mm) of five different types were designed and manufactured. Four types were tubular with inner hole diameter of 5mm and two lattice layers of different density. Fifth type was cylindrical with three lattice layers of different density. In all samples outer lattice layer was most dense, and inner layers- least dense. Mechanical properties of scaffolds were determined by conducting uniaxial compression testing. The strain-stress curves for all samples with gradient porosities showed considerable ductility.

Uncertainty quantification and validation of 3D lattice scaffolds for computer-aided biomedical applications

A methodology is proposed for uncertainty quantification and validation to accurately predict the mechanical response of lattice structures used in the design of scaffolds. Effective structural properties of the scaffolds are characterized using a developed multi-level stochastic upscaling process that propagates the quantified uncertainties at strut level to the lattice structure level. To obtain realistic simulation models for the stochastic upscaling process and minimize the experimental cost, high-resolution finite element models of individual struts were reconstructed from the micro-CT scan images of lattice structures which are fabricated by selective laser melting. The upscaling method facilitates the process of determining homogenized strut properties to reduce the computational cost of the detailed simulation model for the scaffold. Bayesian Information Criterion is utilized to quantify the uncertainties with parametric distributions based on the statistical data obtained from the reconstructed strut models. A systematic validation approach that can minimize the experimental cost is also developed to assess the predictive capability of the stochastic upscaling method used at the strut level and lattice structure level. In comparison with physical compression test results, the proposed methodology of linking the uncertainty quantification with the multi-level stochastic upscaling method enabled an accurate prediction of the elastic behavior of the lattice structure with minimal experimental cost by accounting for the uncertainties induced by the additive manufacturing process.

A novel three-dimensional desalination system utilizing honeycomb-shaped lattice structures for flow-electrode capacitive deionization

A highly compact and scalable three-dimensional desalination cell was realized by utilizing honeycomb-shaped porous lattice scaffolds.

Three-dimensional CaP/gelatin lattice scaffolds with integrated osteoinductive surface topographies for bone tissue engineering

Surface topography is known to influence stem cells and has been widely used as physical stimuli to modulate cellular behaviour including adhesion, proliferation and differentiation on 2D surfaces. Integration of well-defined surface topography into three-dimensional (3D) scaffolds for tissue engineering would be useful to direct the cell fate for intended applications. Technical challenges are remaining as how to fabricate such 3D scaffolds with controlled surface topography from a range of biodegradable and biocompatible materials. In this paper, a novel fabrication process using computer numerically controlled machining and lamination is reported to make 3D calcium phosphate/gelatin composite scaffolds with integrated surface micropatterns that are introduced by embossing prior to machining. Geometric analysis shows that this method is versatile and can be used to make a wide range of lattices with porosities that meet the basic requirements for bone tissue engineering. Both in vitro and in vivo studies show that micropatterned composite scaffolds with surfaces comprising 40 μm pits and 50 μm grooves were optimal for improved osteogenesis. The results have demonstrated the potential of a novel fabrication process for producing cell-instructive scaffolds with designed surface topographies to induce specific tissue regeneration.

Minimal perfusion flow for osteogenic growth of mesenchymal stem cells on lattice scaffolds

A modeling approach to identify sets of culture conditions to promote homogeneous growth of cells in perfusion bioreactors equipped with regular shape scaffolds is proposed. We identify cases in which dynamic culturing is necessary using a zero‐dimensional mass transport and reaction model. Then, based on the three‐dimensional (3‐D) rendering of the flow field inside the bioreactor, we identify regions where cellular growth may become critical; finally, using a 1‐D mass transport and reaction model, we calculate the minimal perfusion flow necessary to maintain the cellular growth rate above a target threshold. The developed approach is used to analyze culturing conditions inside an indirect perfusion bioreactor equipped with a lattice scaffold. Regions where the perfusion flow is inadequate to foster cellular growth at the desired rate are identified. The perfusion flow required to maintain the target growth rate inside the bioreactor is calculated. © 2013 American Institute of Chemical Engineers AIChE J, 59: 3131–3144, 2013

Green ceramic machining: A top-down approach for the rapid fabrication of complex-shaped ceramics

Green ceramic machining (GCM) has been investigated as an alternative method for the mould-free fabrication of complex-shaped ceramics, where ceramics in ‘unsintered’ green state are machined to complex-shaped via CAD/CAM technology. Recent advancement of CNC machining technology makes GCM a potential alternative for the rapid fabrication of ceramics, especially for some one-of-a-kind products. In this work, we investigated two processing routes to make ceramic green bodies which were suitable for green machining. The effects of processing methods on the green ceramic strength and toughness, machinability and surface finishing of the final ceramics were discussed. Examples of applications of the GCM in the fabrication of ceramic lattice scaffolds for bone tissue engineering and dental restorations have been demonstrated. The results show that gelformed ceramics have better mechanical properties and machinability than protein coagulation cast ceramics. But the debinding for gelformed thick wall components was found more difficult. It is therefore more suitable for the fabrication of ceramic microcomponent or thin sheet structures such as ceramic lattice scaffolds. Protein coagulation cast ceramics have good machinability and surface finishing for thick wall ceramic components. However, inhomogeneous and anisotropic shrinkage of complex-shaped ceramics were observed during sintering stage. Possible causes and solutions have been discussed.

Customization of Load-Bearing Hydroxyapatite Lattice Scaffolds

Robocasting, a solid freeform fabrication technique, was used to develop lattices of hydroxyapatite (HA) that show promise as load-bearing scaffolds for bone repair. Additionally, the lattice scaffolds can be shaped into customized implants. A computed tomography (CT) scan of a damaged mandible was utilized to noninvasively design a lattice implant that could fill the damaged region. In a case study, the fit of the custom lattice scaffold was tested in a damaged mandible and “fit like a glove.” Such implants may be able to replace numerous autograft procedures, thereby reducing surgical cost and complications associated with bone harvesting surgery.