Abstract: The assembly line balancing problem is a classical optimisation problem whose objective is to assign each production task to one of the stations on the assembly line so that the total efficiency of the line is maximized. This study proposes a novel hybrid method to solve the simple version of the problem in which the number of stations is fixed, a problem known as SALBP-2. The hybrid differs from previous approaches by encoding individuals of a genetic algorithm as instances of a modified problem that contains only a subset of the solutions to the original formulation. These individuals are decoded to feasible solutions of the original problem during fitness evaluation in which the resolution of the modified problem is conducted using a dynamic programming based approach that uses new bounds to reduce its state space. Computational experiments show the efficiency of the method as it is able to obtain several new best-known solutions for some of the benchmark instances used in the literature for comparison purposes.

Abstract: The simple assembly line balancing problem (SALBP) involves the determination of the assignment of elementary assembly operations to the workstations of the assembly line for the manufacture of a final product, with the objective of maximising assembly efficiency. In addition to its practicality, the SALBP can be considered as an extension of the bin packing problem (BPP) to account for the precedence relations between items. These constraints introduce an ordering component to the problem, which increases the complexity of SALBP resolution. However, previous studies indicated that precedence constraints do not play an important role in the capacity of state-of-the-art procedures to solve benchmark instances to optimality. In this study, we analysed the influences of different features of an SALBP instance on the performance of state-of-the-art solution methods for the abovementioned problem. First, we provide an alternative proof of complexity for the SALBP that uses precedence constraints to demonstrate its non-deterministic polynomial time (NP)-complete status, followed by a new set of benchmark instances directed towards an empirical analysis of the different features of SALBP instances. The experimental results revealed that the packing features of the SALBP are a major source of the perceived difficulty for any instance; however, precedence constraints play a role in the performance of these solution procedures. Specifically, the number of precedence constraints plays an important role in the results obtained from state-of-the-art methods. In addition to the analysis, certain issues that were identified in the publicly available implementations of the state-of-the-art method for resolving this problem were addressed in this study.

Abstract: Direct ink writing (DIW) is a promising extrusion-based 3D printing technology, which employs an ink-deposition nozzle to fabricate 3D scaffold structures with customizable ink formulations for tissue engineering applications. However, determining the optimal DIW process parameters such as temperature, pressure, and speed for the specific ink is essential to achieve high reproducibility of the designed geometry and subsequent mechano-biological performance for different applications, particularly for porous scaffolds of finite sizes (total volume > 1000 mm3) and controlled pore size and porosity. The goal of this study was to evaluate the feasibility of fabricating Polycaprolactone (PCL) and bio-active glass (BG) composite-based 3D scaffolds of finite size using DIW. 3D-scaffolds were fabricated either as cylinders (10 mm diameter; 15 mm height) or cubes (5 × 5 × 5 mm3) with height/width aspect ratios of 1.5 and 1, respectively. A rheological characterization of the PCL-BG inks was performed before printing to determine the optimal printing parameters such as pressure and speed for printing at 110 °C. Microstructural properties of the scaffolds were analyzed in terms of overall scaffold porosity, and in situ pore size assessments in each layer (36 pores/layer; 1764 pores per specimen) during their fabrication. Measured porosity of the fabricated specimens&#65533;PCL: =46.94%, SD = 1.61; PCL-10 wt%BG: = 48.29%, SD = 5.95; and PCL-20 wt% BG: =50.87%, SD = 2.45&#65533;matched well with the designed porosity of 50%. Mean pore sizes&#65533;PCL [ = 0.37 mm (SD = 0.03)], PCL-10%BG [ = 0.38 mm (SD = 0.07)] and PCL-20% BG [ = 0.37 mm (SD = 0.04)]&#65533;were slightly fairly close to the designed pore size of 0.4 mm. Nevertheless there was a small but consistent, statistically significant (p < 0.0001) decrease in pore size from the first printed layer (PCL: 0.39 mm; PCL-10%BG: 0.4 mm; PCL-20%BG: 0.41 mm) to the last. SEM and micro-CT imaging revealed consistent BG particle distribution across the layers and throughout the specimens. Cell adhesion experiments revealed similar cell adhesion of PCL-20 wt% BG to pure PCL, but significantly better cell proliferation &#65533; as inferred from metabolic activity &#65533; after 7 days, although a decrease after 14 days was noted. Quasi-static compression tests showed a decrease in compressive yield strength and apparent elastic modulus with increasing BG fraction, which could be attributed to a lack of adequate mechanical bonding between the BG particles and the PCL matrix. The results show that the inks were successfully generated, and the scaffolds were fabricated with high resolution and fidelity despite their relatively large size (>1000 mm3). However, further work is required to understand the mechano-biological interaction between the BG particle additives and the PCL matrix to improve the mechanical and biological properties of the printed structures.