Algorithm to determine orientation distribution function from microscopic images of fibrous networks: Validation with X-ray microtomography.

Quantitative analysis of fibre orientation in a random fibrous network (RFN) is important to understand their microstructure, properties and performance. 2D fibre orientation distribution presents an in-plane fibre orientation without any information on fibre orientation in thickness direction. This research introduces a fully parametric algorithm for computing 3D fibre orientation as thickness is important for high-density or thick fibrous networks. The algorithm is tested for 3 major classes of nonwoven fabrics called low- (L), medium- (M) and high-density (H) ones. H fabric density is 6-8 times larger than the L fabric density. M fabric density (traditional intermediate fabric density) is 3-4 times larger than the L fabric density. Voxel models of experimental nonwoven webs were generated by an X-ray micro-CT (µCT) system and evaluated with the algorithm. Statistical results showed that a fraction of fibres orientated along the thickness direction increases as fibre density grows. To validate the accuracy of findings, deterministic voxelated virtual fibrous structures, created using mathematical functions were used. This novel algorithm is able to produce a 3D orientation distribution function (ODF) for any RFN including, models of nonwovens produced with various manufacturing parameters, experimentally verified and validated with X-ray µCT. Also, it can compute 2D ODFs of various types of RFNs to evaluate 2D behaviour of fibrous structures. The obtained results are useful for applications in many fields including finite element analysis, computational fluid dynamics, additive manufacturing, etc.

Influence of interface in electrical properties of 3D printed structures.

The aligned bond interfaces resulting from the layer-by-layer nature of material extrusion-based additive manufacturing (MEAM) leads to anisotropic properties in printed parts. This study examines the anisotropy in electrical impedance and its variation with print parameters. Samples consisting of a stack of filaments are used to study the interfaces, which are the fundamental building block of MEAM, in a controlled manner. Anisotropy was quantified using the ratio of the impedance measured across (Z-specimen) and along (F-specimen) the fiber orientation. Although the conductivity of the material was found to change with extrusion temperature, the Z/F ratio was found to be constant (2.15 ± 0.23), regardless of the variation in thermal conditions imposed by varying extrusion temperature and print speed. By varying the distance over which impedance was measured, impedance scaling was understood. The scaling was found to be dependent on the extrusion temperature regardless of the variation of print speed by 266%; ~12.5 Ω per interface for 190 °C while ~6.5 Ω per interface for 230 °C, one-third of which was found to be contributed by fiber. While studying the cause for significant impedance at the interface, scanning electron microscopy study shows absence of airgaps at the interface, and energy dispersion spectroscopy shows absence of oxidation at the interface. The implications of specimen design and characterization proposed here allows for examination of a wide range of print parameters with reduction in material, time, and cost. Thus, by investigating the role of print parameters and scaling of impedance with interfaces, we seek to provide a framework to model and predict electrical behavior of electric sensors and actuators made with MEAM.

Effects of porosity on drug release kinetics of swellable and erodible porous pharmaceutical solid dosage forms fabricated by hot melt droplet deposition 3D printing.

3D printing has the unique ability to produce porous pharmaceutical solid dosage forms on-demand. Although using porosity to alter drug release kinetics has been proposed in the literature, the effects of porosity on the swellable and erodible porous solid dosage forms have not been explored. This study used a model formulation containing hypromellose acetate succinate (HPMCAS), polyethylene oxide (PEO) and paracetamol and a newly developed hot melt droplet deposition 3D printing method, Arburg plastic free-forming (APF), to examine the porosity effects on in vitro drug release. This is the first study reporting the use of APF on 3D printing porous pharmaceutical tablets. With the unique pellet feeding mechanism of APF, it is important to explore its potential applications in pharmaceutical additive manufacturing. The pores were created by altering the infill percentages (%) of the APF printing between 20 and 100% to generate porous tablets. The printing quality of these porous tablets was examined. The APF printed formulation swelled in pH 1.2 HCl and eroded in pH 6.8 PBS. During the dissolution at pH 1.2, the swelling of the printing pathway led to the gradual decreases in the open pore area and complete closure of pores for the tablets with high infills. In pH 6.8 buffer media, the direct correlation between drug release rate and infills was observed for the tablets printed with infill at and less than 60%. The results revealed that drug release kinetics were controlled by the complex interplay of the porosity and dynamic changes of the tablets caused by swelling and erosion. It also implied the potential impact of fluid hydrodynamics on the in vitro data collection and interpretation of porous solids.

Mechanical and hydrolytic properties of thin polylactic acid films by fused filament fabrication.

Thin polymeric films are widely used as medical applications such as cell culture, stent, drug delivery and mechanical fixation. One of the most commonly used materials is polylactic acid (PLA) - a material, which is non-toxic, biodegradable and biocompatible. Fused filament fabrication (FFF) is a preferable additive manufacturing technique to manufacture polymers, where PLA is one of the most common materials. FFF is a promising technique for customised biomedical applications due to its relatively low cost and geometrical flexibility where biomedical applications are patient tailored. This study is the first to consider FFF monolayered thin films of PLA in terms of mechanical and hydrolytic properties at 37 °C in vitro degradation. Throughout degradation, the reduction in mechanical properties was examined by analysing molecular weight and thermal properties. FFF monolayered PLA underwent autocatalytic bulk degradation with no proof of significant mass loss. Young's modulus, ultimate tensile strength and molecular weight reduced by approximately 60%, 86%, and 80% after 280 days, respectively, while the degree of crystallinity increased by 143% in comparison to benchmark thin films at day 0. It was found that the decrease in mechanical properties was more sensitive to the increase in crystallinity in the early stage of the degradation, while the molecular weight was more dominant in the late stage of the degradation. This study provides practical information in terms of mechanical properties to support medical device designers in a range of potential end-use biomedical applications to achieve safe functional products over the required degradation lifetime.

Layer-dependent properties of material extruded biodegradable Polylactic Acid.

Polylactic acid (PLA) is a biodegradable, biocompatible and non-toxic biopolymer with good mechanical properties, and is commonly used for the additive manufacture of PLA-based biomedical devices. Such devices are available in a range of sizes and thicknesses, with smaller devices capable of being realised via additive manufacturing in just a few layers. Due to their thermal history and thermal degradation, the thermal, molecular weight and mechanical properties of each layer was different when the raw material was melted, and the in-course layer was deposited to the previous layer. This study investigated the effect of the number of layers on mechanical, thermal and molecular weight properties, and the relationship between them. Material extruded ISO 527-2 type 5A specimens with 1-, 2-, 3-, 4-, 5-, 7- and 10-layers were prepared with the cutting die. Results indicated that the degree of crystallinity was found to decrease from 8% to 0.5% with an increasing number of layers. This was likely due to different cooling rates, where the molecular weight was lowest for 1-layer and increased with the increasing number of layers until it almost reached that of the bulk material. Additionally, ultimate tensile strength and strain increased with an increasing number of layers, while Young's Modulus decreased due to heterogeneous material structure. Of all obtained results, there was no significant difference between 5- and 10-layer in terms of mechanical and thermal properties.