The waves that make the pattern: a review on acoustic manipulation in biomedical research

Abstract

Novel approaches, combining technology, biomaterial design, and cutting-edge cell culture, have been increasingly considered to advance the field of tissue engineering and regenerative medicine. Within this context, acoustic manipulation to remotely control spatial cellular organization within a carrier matrix has arisen as a particularly promising method during the last decade. Acoustic or sound-induced manipulation takes advantage of hydrodynamic forces exerted on systems of particles within a liquid medium by standing waves. Inorganic or organic particles, cells, or organoids assemble within the nodes of the standing wave, creating distinct patterns in response to the applied frequency and amplitude. Acoustic manipulation has advanced from micro- or nanoparticle arrangement in 2D to the assembly of multiple cell types or organoids into highly complex in vitro tissues. In this review, we discuss the past research achievements in the field of acoustic manipulation with particular emphasis on biomedical application. We survey microfluidic, open chamber, and high throughput devices for their applicability to arrange non-living and living units in buffer or hydrogels. We also investigate the challenges arising from different methods, and their prospects to gain a deeper understanding of in vitro tissue formation and application in the field of biomedical engineering.

Keywords: Biofabrication; Faraday waves; Pattern; Sound; Standing waves.

Graphical abstract

Fig. 1

Use of acoustic manipulation on different length ​and time scales. Acoustic manipulation or sound patterning can be applied over seconds, minutes, or even hours or days to perform long-term stimulation. On the length scale, nano- or microparticles can be manipulated by acoustic tweezers, individual cells can be patterned within 2 or 2.5 dimensions, whereas 3D constructs were prepared by cell spheroid assembly.

Fig. 2

Schematic of the three main setups used for acoustic manipulation. (A) By use of ultrasound (100 ​kHz and 10 ​MHz), a piezoelectric element, most often consisting of LiNbO₃, is addressed via interdigital transducers (IDT), inducing SAWs in a microchannel (polydimethylsiloxane chamber [PDMS]). (B) BAWs are induced by ultrasound, transduced to a ceramic piezo element (e.g. ZnO/Si). A reflector, covering the petri dish or container induces an opposing wave that overlaps with the incoming wave resulting in a vertical standing wave. (C) Low frequencies (40–200 ​Hz) are applied by a frequency generator to an open-top container or Petri dish that is placed on the generator. Faraday waves are formed at the liquid–air interface. In each scenario, cells (illustrated in pink) are moved toward ​the wave nodes by acoustic radiation or hydrodynamic forces.

Fig. 3

Schematic of acoustic manipulation on different length scales. (A) On the nm to μm scale, acoustic tweezers are used to trap particles or cells and position them in space. (B) On the μm to mm scale, particles or cells (or a combination thereof) are assembled in the x–y axis to form defined patterns. C) On the mm to cm scale, cell aggregates (spheroids or organoids) are assembled to form three-dimension in vitro engineered tissue constructs.

Fig. 4

Schematic of acoustic manipulation on different time scales. (A) Within seconds, sound can assist in cell seeding within fibrous scaffolds. (B) Within seconds or minutes, cell assembly can be achieved by acoustic waves. (C) Over minutes or hours, acoustic manipulation can be leveraged to induce matrix remodeling of natural or synthetic scaffolds. (D) Cell differentiation can be promoted by acoustic manipulation over days or weeks.