Light and heat are the only energy sources in Additive Manufacturing (AM) processes to drive chemical reactions. Among different routes in chemistry, including photo and thermo chemistries, Sonochemistry distinguishes itself by pushing the boundaries of chemistry’s control parameters (energy per molecule, interaction time and pressure) to extremes . Sonochemistry is driven by chemically active cavitation bubbles induced by acoustic waves. Cavitation bubbles act as chemical reactors by creating pressures exceeding 1000 bar and extraordinary temperature over 15000 K with extremely fast heating and cooling rates as 10^12 K/s. Cavitation has been looked at classically as a force of destruction in engineering such as in pump impellers’ erosion damage and it has been adopted evolutionary as a fatal attack mechanism in nature . However, if we could tame chemically active acoustic cavitation and harness it for creation instead of destruction, an alternative and unconventional route for impossible to print materials would emerge.
Direct Sound Printing
The concept of printing with acoustic waves, Direct Sound Printing or in short DSP, is schematically shown in Fig. 1. A focused source of ultrasound creates focused chemically active cavitation regions, Ultra Active Micro Reactor or in short UAMR, where the printing material transforms from liquid to solid instantly. The desired object is created by moving UAMR in the printing liquid, in this case a heat curing resin. Transparent and porous structures can be printed by turning process parameters such as the printing material’s viscosity and acoustic field’s intensity, frequency and duty cycle.
Fig. 1. DSP concept and printed parts. a, DSP process schematic. b, Detailed view of UAMR, bubbles are created in low pressure zones. c, Printed impellers, transparent and porous. e-h, Printed maple leaf, gear, shell and honey comb, respectively.
The origin of the idea
The idea of using acoustic cavitation for printing objects originated from our sonochemiluminescence (SCL) experiments with luminol solution in which chemically active regions emit blue light. We found that using focused ultrasound waves, highly localized and focused chemically active region can be created, sustained and manipulated inside the medium (Fig. 2).
Fig. 2. Highly localized and chemically active region in the luminol solution SCL experiments.
This highly localized region resembles a laser beam spot in Stereolithography. A cavitation bubble creates a region called hotspot in which extremely temporal high pressure and temperature could increase the polymerization rate of the heat curing polymers significantly to the point that it would make on-demand curing possible. On demand curing is captured by high speed imaging as shown in Fig. 3.
Fig. 3. High speed imaging of PDMS polymerization in DSP under extremely high acoustic power.
DSP is first and foremost a printing process. We DSP printed heat curing resins such as Polydimethylsiloxane (PDMS) that can’t be printed directly with any other AM methods. This material is used classically and widely in softlithography for making microfluidic devices through molding processes. Printing such materials opens up wide range of applications such as lab-on-a chip devices, biological machines, flexible conductors and so on. In addition, since in DSP no toxic additive or byproduct is added or produced, the biocompatibility of the printed parts are guaranteed.
Another distinct class of applications arise due to characteristics of acoustic waves that can penetrate deep in opaque materials. As seen in Fig. , acoustic waves can have significant penetration depth in comparison with light. We utilized this aspect to introduce the possibility of a process called Remote Distance Printing (RDP) in which the direct access to the printing location is limited. RDP can be used for remote and on-site maintenance of hidden parts or printing parts inside body without an open surgery. We proved the noninvasive bio printing in-vitro and ex-vivo using tissue phantom and porcine tissue Fig. 4.
Fig. 4. Potential applications of DSP. a, Light-based AM and small cure depth for opaque printing materials. b, DSP and deep penetration of ultrasound in opaque printing materials. c, The concept of an ideal DSP technology for non-invasive inside body printing. d, in-vitro/ex-vivo prove of concept setup. e, Tissue phantom of human skin and muscle. f, A printed maple leaf using the tissue phantom.
DSP is an alternative AM technology taming acoustic cavitation for on-demand curing of heat curing polymers and composites. We expect this alternative multidisciplinary research field attracts researchers with interests in, to name a few, Sonochemistry, Acoustics, Ultrasound, Cavitation, Computational Fluid Dynamics, Additive Manufacturing and Polymerization.
Direct sound printing. Nature Communications 13, 1800 (2022).
 Suslick, K. S., Eddingsaas, N. C., Flannigan, D. J., Hopkins, S. D. & Xu, H. The Chemical History of a Bubble. Acc. Chem. Res. 51, 2169–2178 (2018)
 Koukouvinis, P., Bruecker, C. & Gavaises, M. Unveiling the physical mechanism behind pistol shrimp cavitation. Sci Rep 7, 13994 (2017).
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