In micro-dispensing applications, printhead activation mechanism, its design and operating parameters are integrated together to affect the droplet generation process. These factors give each printhead advantages and limitations over the others in specific fabrication. Hence, multiple printheads on micro level fabrication are preferred to perform multi-material dispensing task. In this paper, the mechanisms of two commonly used micro printheads, solenoid actuated micro-valve and piezoelectric printhead are discussed. Comprehensive experiments are conducted to characterize their performance and the results are analyzed so as to explore optimal droplet formation condition. With regards to the operational parameters' influence on droplet formation, micro-valve is investigated in terms of pressure, and operational on time, and piezoelectric printhead is investigated based on pulse amplitude, and width of driving pulse. Nozzle size, a key design parameter in the two printheads, is also studied according to its influence on dispensing capability. To facilitate dispenser selection, the two printheads are compared based on droplet size, droplet stability, droplet velocity, and dispensing viscosity of successful ejection. Other factors such as chemical compatibility, time consumption in determining optimal condition and reliability of dispensing process are also reported to play an essential role in this selection. Our investigation on the relationship between related parameters and dispensing performance will not only benefit dispenser selection in multi-material dispensing application, but also build a solid background to develop multiple printhead system for fabrication of bioengineering components.

Nerve guidance conduits (NGCs) are tubular tissue engineering scaffolds used for nerve regeneration. The poor mechanical properties and porosity have always compromised their performances for guiding and supporting axonal growth. Therefore, in order to improve the properties of NGCs, the computational design approach was adopted to investigate the effects of different NGC structural features on their various properties, and finally, design an ideal NGC with mechanical properties matching human nerves and high porosity and permeability. Three common NGC designs, namely hollow luminal, multichannel, and microgrooved, were chosen in this study. Simulations were conducted to study the mechanical properties and permeability. The results show that pore size is the most influential structural feature for NGC tensile modulus. Multichannel NGCs have higher mechanical strength but lower permeability compared to other designs. Square pores lead to higher permeability but lower mechanical strength than circular pores. The study finally selected an optimized hollow luminal NGC with a porosity of 71% and a tensile modulus of 8 MPa to achieve multiple design requirements. The use of computational design and optimization was shown to be promising in future NGC design and nerve tissue engineering research.

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The formation mechanism of the pore defects and the surface ripples takes an crucial position in the quality control of laser powder bed fusion process. This paper developed the melt pool evolution in LPBF process, and revealed the effects of flow state and temperature field distribution of the melt pool with various process parameters, from which the formation mechanism of pore defects and surface ripples had been clarified by computational fluid dynamics methodology. The discrete element method was applied to reproduce the powder bed with a set of processing parameters. Then free surface of the melt pool was calculated employing the volume of fluid method. It was found that surface ripples of the melt pool and irregular pore defects in the lap joints appeared due to the lack of fusion at a laser energy density of 32 J/mm3 under the pre-set conditions. As the laser scanning speed dropped to 600 mm/s, keyholes generated inside the melt pool due to the recoil pressure. Similar phenomenon of pore defects and surface ripples were observed in the experiments. The measured and simulated average widths of the molten track showed good agreement with the relative errors of less than 6%, which verified the accuracy of simulated formation. Our work shed new light on quality control of LPBF fabricated parts for future.