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Acoustic Nozzles Improve the Performance of 3D Printed Parts

Usually when we’re talking about 3D printing in terms of acoustics, it has to do with making a good set of speakers. But, recent research has determined that acoustic signal processing could be used to monitor the build quality of a 3D printed part while in progress. There are 3D printable sound-shaping super-materials, and 3D printed objects have even been implanted with sound data for tagging purposes. New research out of Nanyang Technological University (NTU) in Singapore looks at using acoustics to manipulate microparticles inside the actual 3D printing ink itself to improve the final object’s performance and functionality.

Properly orienting and aligning the fibers in a polymer matrix could help transfer loads from critical areas for better performance, and creating 3D scaffolds with a controlled hierarchical structure at the nano- and micro-scale levels could increase their mechanical strength, which is good for cell and tissue regeneration and load-bearing bone defect repair. In addition, using surface acoustic waves to focus microparticles inside the microchannel could delay accumulation on the wall, which can improve extrusion-based 3D printing.

Schematic diagram of experimental setup.

Researchers from NTU recently published a paper on their 3D printing work with acoustics, titled “Cells alignment and accumulation using acoustic nozzle for 3D printing.”

The abstract reads, “Arrangement or patterning of microparticles/cells would enhance the efficiency, performance, and function of the printed construct. This could be utilized in various applications such as fibers reinforced polymer matrix, hydrogel scaffold, and 3D printed biological samples. Magnetic manipulation and dielectrophoresis have some drawbacks, such as time-consuming and only valid for samples with specific physical properties. Here, acoustic manipulation of microparticles in the cylindrical glass nozzle is proposed to produce a structural vibration at the specific resonant frequency. With the acoustic excitation, microparticles were accumulated at the center of the nozzle and consequently printed construct at the fundamental frequency of 871 kHz. The distribution of microparticles fits well with a Gaussian distribution. In addition, C2C12 cells were also patterned by the acoustic waves inside the cylindrical glass tube and in the printed hydrogel construct. Overall, the proposed acoustic approach is able to accumulate the microparticles and biological cells in the printed construct at a low cost, easy configuration, low power, and high biocompatibility.”

Morphology and distribution of the cells in 5% GelMA without the acoustic excitation on (a) day 1, (b) day 4, (c) day 7, and with the acoustic excitation on (d) day 1,(e) day 4, (f) day 7.

The team numerically and experimentally studied the structural vibration of a cylindrical tube, as well as the patterning of the microparticles and cells inside of it.

“Firstly, the resonant frequency was numerically predicted and validated with experiment,” the researchers wrote. “Subsequently, the distribution of microparticles and biological cells inside the cylindrical tube and printed construct was investigated. Lastly, the growth of biological cells undergone the acoustic excitation was monitored for up to 7 days.”

During an acoustic excitation, a mixture of C2C12 cells embedded in 2 ml of 5% GelMA was printed on a 4″ petri dish, with the nozzle perpendicular to the print bed. The researchers discovered that during the excitation, most of the microparticles that were initially suspended in fluid ended up accumulating at the center of the glass tube. There seemed to be a good overall agreement between the experimental results and numerical simulation of the excitation frequency, along with the location of pressure nodes in the glass tube.

The researchers further evaluated their acoustic nozzle’s performance using C2C12 muscle cells, and determined that without the excitation during printing, the distribution of the cells in the tube was very random.

Microparticle distribution in the cylindrical tube (a) without and (b) with the acoustic excitation at 877 kHz.

“Results of simulation and experiment are agreeable with a slight difference in the resonant frequency (< 2%). In the experiment, microparticles were accumulated at the center of the nozzle and consequently printed construct. The distribution of microparticles fits well in a Gaussian curve with a standard deviation of (V = 0.16 mm). Furthermore, the acoustic excitation could also be used for patterning biological cells in the 3D printed construct of GelMA,” the researchers concluded. “Subsequently, the distribution of cells was quite dense at the center of the printed structure, and accumulated C2C12 cells had greater growth and differentiation in comparison to the suspended ones in the control group.”

Co-authors of the paper are Yannapol Sriphutkiat, Surasak Kasetsirikul, Dettachai Ketpun, and Yufeng Zhou.

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Could Your Next Headphones Use 3D Printed Metamaterials?

We’ve often seen 3D printing used to fabricate headphones and earbuds. But a team of researchers from the Department of Electrical and Electronic Engineering in the University of Strathclyde in Glasgow are taking this idea to the next level with acoustic metamaterials. Metamaterials, which can morph according to their environment, make up a new class of finely-engineered surfaces that can perform nature-defying tasks like 3D printing holograms and shaping sound waves. Acoustic metamaterials have the unique ability to attenuate sound by breaking the mass-density law, due to properties such as negative effective density and bulk modulus – no easy feat when it comes to small devices since thin walls are easily penetrated by acoustic waves.

The researchers, who are all affiliated in various capacities with the university’s Centre for Ultrasonic Engineering (CUE), recently published a paper, titled “Enhancing the Sound Absorption of Small-Scale 3D Printed Acoustic Metamaterials Based on Helmholtz Resonators,” detailing how their SLA 3D printed acoustic metametarials, based on Helmholtz resonators, can be used for small-scale sound absorption applications.

The abstract reads, “The directional response due to the position of the acoustic source on the sound attenuation provided by the metamaterial is investigated by controlling the location of a loudspeaker with a robot arm. To enhance and broaden the absorption bands, structural modifications are added such that overtones are tuned to selected frequencies, and membranes are included at the base of the resonators. This design is made possible by innovative 3D printing techniques based on stereolithography and on the use of specific UV-curable resins. These results show that these designs could be used for sound control in small-scale electroacoustic devices and sensors.”

To cut a material’s sound transmission by half, you have to double its acoustic frequency, density, or thickness. When an acoustic metamaterial has negative parameters, stop bands will form where the sound is deeply attenuated at certain frequencies.

“Acoustic metamaterials can break the mass-density law by exploiting the stop bands formed in the proximity of the resonance frequencies of their unit cells,” the researchers explain. “These material structures are often based on Helmholtz resonators and membranes. The frequency band that is attenuated by using these kind of unit cells is nevertheless narrow, hence solutions such as coupling of multiple resonances and leveraging the losses of the materials are generally used to make the attenuation broadband.”

Kuka robotic arm configuration.

The team’s paper presents a basic design of these small-scale, sub-wavelength 3D printed acoustic metamaterials, which use Helmholtz resonators arrays to generate stop bands where sound attenuation increases with the number of unit cells. A loudspeaker, guided by a KUKA robot arm through a quarter-hemisphere trajectory, illustrated that an absorption band forms “for every angle of incidence of the impinging sound wave.” A reference microphone was used to measure sound transmission in the air, while the transmission above the sample was measured by a second microphone.

The paper also explains two methods of enhancing the stop band, the first of which requires the resonators’ overtones to be tuned more closely to the fundamental frequency; this causes the band to grow wider. The second method involves printing membranes at the resonators’ base.

“In this work a novel fabrication technique is used, where thin membranes are fabricated inside Helmholtz resonators, the two units consisting of different materials,” the researchers wrote. “The presented manufacturing technique could result in rapid prototyping of metamaterials and contribute to the advancement of this field into the industrial environment. Simple physical models of the presented metamaterials are included in each section, nevertheless this work is mostly based on experimental results and aims at developing metamaterials that could be included in real devices. Potential applications of this work include noise cancellation for devices such as headphones, hearing aids and other sensors.”

Work is continuing to advance acoustic metamaterials in applications like acoustic cloaking, sound focusing and waveguiding, and imaging and computation, but the results are not often integrated into functional devices. But if studies like this one by the University of Strathclyde researchers can be validated, we could be looking at vastly improved headphones in the near future.


Co-authors of the paper are IEEE Student Member Cecilia Casarini, Benjamin Tiller, Carmelo Mineo, Charles N. MacLeod, IEEE Senior Member Professor James F. C. Windmill, and Joseph C. Jackson.

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