Development of acoustic nozzle for 3D printing
Date of Issue2019-04-08
School of Mechanical and Aerospace Engineering
Singapore Centre for 3D Printing
A random distribution of microparticles/cells increases the chance of deposition on the inner surface of the nozzle, leading to obstruction/clogging of nozzle constriction. Nozzle clogging in 3D printers and other high-resolution machines is a common problem resulting in the loss of time, budget, productivity, part uniformity, and integrity of the printed part. Nozzle clogging is mainly due to the deposition of microparticles, which affects the accuracy and reliability of printing as well as the choice of printable materials. In jetting and extrusion, the surface tension and viscosity of fluid medium and the concentration of solid microparticles/cells are limited to a certain range. In addition, the obstruction of flow path could increase the mechanical stress in that region. High mechanical stress on biomaterials and cells during bioprinting decreases the viability of cells on the printing scaffold. Current methods (e.g. surfactant, limited volume fraction and size of aggregates) cannot reduce clogging effectively and have limitations. Addition of surfactant can damage cells resulting in low cell proliferation. In the circuit printing (inkjet-based), the volume fraction of aggregates should be controlled to avoid clogging. But low volume fraction causes impairment in the electrical performance of the printed structure. In this study, a method of utilizing ultrasound was proposed, developed, and evaluated in order to reduce the nozzle clogging, improve the printing stability and accuracy of the nozzle-based 3D printer. Such method could also be applied to other high-resolution machines with little modification. The proposed method is to use acoustic waves to align microparticles/cells through the constriction/nozzle and, subsequently, in the printed construct. Also, it is to evaluate the effect of acoustic waves to suppress nozzle clogging. In the first part of this study, the effect of standing surface acoustic wave (SSAW) on the reduction of microparticle accumulation was studied in a microchannel. The fluid medium consists of deionized water and sodium alginate with different concentrations ranging from 0% to 5%. The experimental results had a good agreement with the numerical simulation. The experimental results showed that SSAW is capable of reducing the microparticle accumulation area effectively in the low alginate concentration solution. Meanwhile, such capability decreased in the high concentration of alginate. Additionally, to enhance the tunability of SSAW, a dual-frequency excitation was utilized in the microchannel. The dual-frequency excitation method utilizes a superposition of SSAW at the fundamental (f1) and third harmonic (f3) frequencies allowing the number and location of the pressure node to be controlled more flexibly. In the later part of this study, an acoustic excitation of the structural vibration was used for focusing the microparticles/cells towards the center of the cylindrical tube. It was found that the focusing time and width of microparticles in the cylindrical tube increase with the concentration of sodium alginate and microparticles in the ink. Subsequently, the ink was printed from the nozzle consistently. Most of the microparticles are distributed in the central part of the printing structure. In comparison to the conventional printing strategy, acoustic excitation could significantly reduce the width of accumulated microparticles in the printing structure (p-value < 0.05). In addition, the microparticle motion at the higher harmonics (385 kHz and 657 kHz) was also studied. Lastly, the C2C12 cells (myoblast muscle cells) were printed out from the nozzle through the cylindrical tube using the acoustic excitation. The acoustically-patterned C2C12 cells in the three-dimensional printed gelatin methacrylate (GelMA) construct were monitored for 7 days for their growth and morphology. Overall, the acoustic approach is able to accumulate microparticles/cells in the printed construct at a low cost, simple configuration, and low power, but high biocompatibility. In the future, acoustic patterning of various biological cell types in printed construct could be investigated. As acoustic method has a capability to manipulate the microparticle/biological cells depending on their physical properties (compressibility, density and size).