Computer simulation of polymer chain scission in biodegradable polymers

Biodegradable polymers degrade due to the hydrolysis (chain scission) of the polymer chains. Two theories of hydrolysis are that 1) scissions occur randomly at any bond in chains, and 2) scissions occur in the final bond at chain ends. In this study, a simulation tool was developed to simulate both random chain scission and chain end scission. The effect of each type of scission was analysed. Random scissions were found to have over 1000 time’s greater impact on molecular weight reduction than end scissions. For the degradation of poly lactic acid by random scission, it was found that Mn must reduce to <5000 g/mol in order for a polymer to exhibit significant mass loss due to the diffusion of water-soluble short chains. In contrast, end scission was able to produce a significant fraction of water-soluble chains with little or no effect on Mn. The production rate of water-soluble chains was linearly related to end scission but increase in an accelerated manner due to random scission. Molecular weight distributions were fitted to experimental data for the degradation of poly D-lactic acid

An atomic finite element model for biodegradable polymers. Part 2. A model for change in Young’s modulus due to polymer chain scission

Atomic simulations were undertaken to analyse the effect of polymer chain scission on amorphous poly(lactide) during degradation. Many experimental studies have analysed mechanical properties degradation but relatively few computation studies have been conducted. Such studies are valuable for supporting the design of bioresorbable medical devices. Hence in this paper, an Effective Cavity Theory for the degradation of Young's modulus was developed. Atomic simulations indicated that a volume of reduced-stiffness polymer may exist around chain scissions. In the Effective Cavity Theory, each chain scission is considered to instantiate an effective cavity. Finite Element Analysis simulations were conducted to model the effect of the cavities on Young's modulus. Since polymer crystallinity affects mechanical properties, the effect of increases in crystallinity during degradation on Young's modulus is also considered. To demonstrate the ability of the Effective Cavity Theory, it was fitted to several sets of experimental data for Young's modulus in the literature

The effect of geometry on mechanical properties of biodegradable polylactic-acid tensile-test specimens by material extrusion

Additive manufactured biomedical devices have been widely used in the biomedical fields due to the development of biomaterials and manufacturing techniques. Biodegradable Polylactic Acid-based polymers are the most common material that can be manufactured using material extrusion, one of the most widely known additive manufacturing methods. However, medical grade polymers are too expensive for degradation studies with common tensile specimens. Therefore, this paper aims to reduce the volume of the material used for manufacturing tensile specimen by introducing a new tensile specimen, Micro-X tensile specimen, developed for steel. Young’s Modulus and Ultimate Tensile Strength of micro-X tensile specimens were compared with the ASTM D1708 standard specimens. The experimental results showed that there is no significant difference in terms of mechanical properties. Furthermore, the micro-X tensile specimen was reduced the volume and as well as the cost by approximately 91% in comparison to ASTM D1708 standard tensile specimen

Degradation mechanisms of bioresorbable polyesters. Part 2, Effects of initial molecular weight and residual monomer

This paper presents an understanding of how initial molecular weight and initial monomer fraction affect the degradation of bioresorbable polymers in terms of the underlying hydrolysis mechanisms. A mathematical model was used to analyse the effects of initial molecular weight for various hydrolysis mechanisms including noncatalytic random scission, autocatalytic random scission, noncatalytic end scission or autocatalytic end scission. Different behaviours were identified to relate initial molecular weight to the molecular weight half-life and to the time until the onset of mass loss. The behaviours were validated by fitting the model to experimental data for molecular weight reduction and mass loss of samples with different initial molecular weights. Several publications that consider initial molecular weight were reviewed. The effect of residual monomer on degradation was also analysed, and shown to accelerate the reduction of molecular weight and mass loss. An inverse square root law relationship was found between molecular weight half-life and initial monomer fraction for autocatalytic hydrolysis. The relationship was tested by fitting the model to experimental data with various residual monomer contents

Review of additive manufactured tissue engineering scaffolds: relationship between geometry and performance

Material extrusion additive manufacturing has rapidly grown in use for tissue engineering research since its adoption in the year 2000. It has enabled researchers to produce scaffolds with intricate porous geometries that were not feasible with traditional manufacturing processes. Researchers can control the structural geometry through a wide range of customisable printing parameters and design choices including material, print path, temperature, and many other process parameters. Currently, the impact of these choices is not fully understood. This review focuses on how the position and orientation of extruded filaments, which sometimes referred to as the print path, lay-down pattern, or simply "scaffold design", affect scaffold properties and biological performance. By analysing trends across multiple studies, new understanding was developed on how filament position affects mechanical properties. Biological performance was also found to be affected by filament position, but a lack of consensus between studies indicates a need for further research and understanding. In most research studies, scaffold design was dictated by capabilities of additive manufacturing software rather than free-form design of structural geometry optimised for biological requirements. There is scope for much greater application of engineering innovation to additive manufacture novel geometries. To achieve this, better understanding of biological requirements is needed to enable the effective specification of ideal scaffold geometries

Extra-wide deposition in extrusion additive manufacturing: A new convention for improved interlayer mechanical performance

Recent studies have contested long-standing assumptions that mechanical anisotropy is caused by weak interlayer bonding and demonstrated that microscale geometry (the groove between extruded filaments) is the major cause of anisotropy in extrusion additive manufacturing (AM). Inspired by those finding, this study investigates the potential for a new convention for print-path design to improve mechanical properties by setting extrusion width to be at least 250 % of nozzle diameter. The new convention enabled an almost 50 % improvement in mechanical performance, which was supported by finite element analysis data, whilst simultaneously reducing the printing time by 67 %. Whereas a typical extrusion AM part uses several side-by-side extrusions, here, three 0.4-mm-wide extrusions are replaced with a single extra-wide 1.2-mm extrusion; two 0.6-mm-wide extrusions are also studied. The contact area between layers of the extra-wide extrusion was 90 % as opposed to 63 % for the conventional approach. The improved contact area led to a 40–48 % enhancement of strength, strain-at-fracture and toughness. This study presents a compelling case for a methodological shift to extra-wide extruded-filament deposition and explains the underlying cause of anisotropic strength observed in previous studies. Two case studies demonstrate practical applicability for a print run of 1000 nylon visors and lower-limb polylactide prosthetic sockets, for which extra-wide filaments more than doubled load-bearing capabilities. Polylactide material was used for most of the study; potential for translation to other materials is discussed

Interfacial fracture of 3D-printed bioresorbable polymers

A micro specimen for tensile testing was designed with two primary aims: (i) to characterise interface fracture behaviour between fused 3D-printed polymer filaments; and (ii) to minimise material use of high-cost medical-grade polymer since a high number of specimens are required for time-series studies (e.g. polymer degradation). Polylactide specimens were fabricated on an extrusion 3D-printer as a single-filament-wide wall. The widths of filaments were set individually, with a custom machine-control code, to achieve a higher width in the grip sections of specimens and a narrower width in their gauge section. On average, the interface between filaments was 114 µm narrower than the widest point of the filaments. Each specimen was tested in the build direction to determine the interfacial strength between 3D-printed layers. Optical microscopy was employed to characterise geometry of specimens and fracture surfaces. Samples fractured in the gauge section and the fracture surface demonstrated brittle characteristics. The specimens utilised an order of magnitude less material than ASTM D638 samples, whilst maintaining repeatability for tensile strength similar to that in other studies. The average strength was 49.4 MPa, which is comparable to data in the literature. Further optimisation of the specimen design and 3D printing strategy could realise greater reductions in material use

Data for article: FullControl GCode Designer - open-source software for unconstrained design in additive manufacturing

Supporting data for journal article - download the videos for best quality (rather than streaming on the website), or view on YouTube:Video S1: https://youtu.be/KlxuZ5JnA0k (FullControl GCode Designer - Highlight Video)Video S2: https://youtu.be/yGtyw3oLOVQ (Technical Introduction to FullControl GCode Designer)Video S3: https://youtu.be/2Lkt4ZxSZhk (CONVEX Demonstration Video)Video S4: https://youtu.be/e68Vc69yAo8 (Lattice Concept Shoe Sole Demonstration Video)Video S5: https://youtu.be/VOEz_vddlMw (Sine Wave Bridging Demonstration Video)More information is available in the Supplementary Data file.</div

FullControl GCode Designer: Open-source software for unconstrained design in additive manufacturing

A new concept is presented for the design of additive manufacturing procedures, which is implemented in open-source software called FullControl GCode Designer. In this new design approach, the user defines every segment of the print-path along with all printing parameters, which may be related to geometric and non-geometric factors, at all points along the print-path. Machine control code (GCode) is directly generated by the software, without the need for any programming skills and without using computer-aided design (CAD), STL-files or slicing software. Excel is used as the front end for the software, which is written in Visual Basic. Case studies are used to demonstrate the broad range of structures that can be designed using the software, including: precisely controlled specimens for printer calibration, parametric specimens for hardware characterisation utilising hundreds of unique parameter combinations, novel mathematically defined lattice structures, and previously inconceivable 3D geometries that are impossible for traditional slicing software to achieve. The FullControl design approach enables unconstrained freedom to create nonplanar 3D print-paths and break free from traditional restrictions of layerwise print-path planning. It also allows nozzle movements to be carefully designed - both during extrusion and while travelling between disconnected extrusion volumes - to overcome inherent limitations of the printing process or to improve capabilities for challenging materials. An industrial case study shows how explicit print-path design improved printer reliability, production time, and print quality for a production run of over 1000 parts. FullControl GCode Designer offers a general framework for unconstrained design and is not limited to a particular type of structure or hardware; transferability to lasers and other manufacturing processes is discussed. Parametric design files use a few bytes or kilobytes of data to describe all details that are sent to the printer, which greatly improves shareability by eliminating any risk of errors being introduced during STL file conversion or due to different users having inconsistent slicer settings. Adjustable parameters allow GCode for revised designs to be produced instantly, instead of the laborious traditional routine using multiple software packages and file conversions. The FullControl design concept offers new opportunities for creative and high-precision use of additive manufacturing systems. It facilitates design for additive manufacturing (DfAM) at the smallest possible scale based on the fundamental nature of the process (i.e. assembly of individual extrusions). The software and source code are provided as supplementary data and ongoing updates to improve functionality and the user interface will be available at www.fullcontrolgcode.com

Advanced metal transfer additive manufacturing of high temperature turbine blades

During the pursuit of this research, a novel method for the additive manufacture of metal parts has been developed. To this end, a way to create the complex geometrical structure of Inconel 625 jet turbine blades has been engineered. This advanced hybrid PETG and 90% Inconel 625 powder has first been Additive Manufactured into the shape of a turbine blade. The resulting green part produced was initially debinded at a temperature of 350 °C followed by heating to a sintering temperature of 1350 °C. This resulted in the transformation of a part into a solid Inconel 625 structure, which was later tested to understand the microstructural and mechanical properties of the material. It was found that although there was a slight degree of porosity, the structures were still mechanically sound, up to a temperature of 600 °C. The turbine blades were later machined to high tolerance 0.2 µm finish surface as is required for such components. This novel means for the fabrication of such complex and ultimately expensive to create structures allows a revolution in manufacture capabilities through the use of 3D Metal transfer printing technology