High-intensity focused ultrasound-induced mechanochemical transduction in synthetic elastomers

Abstract

While study in the field of polymer mechanochemistry has yielded mechanophores that perform various chemical reactions in response to mechanical stimuli, there is not yet a triggering method compatible with biological systems. Applications such as using mechanoluminescence to generate localized photon flux in vivo for optogenetics would greatly benefit from such an approach. Here we introduce a method of triggering mechanophores by using high-intensity focused ultrasound (HIFU) as a remote energy source to drive the spatially and temporally resolved mechanicalto-chemical transduction of mechanoresponsive polymers. A HIFU setup capable of controlling the excitation pressure, spatial location, and duration of exposure is employed to activate mechanochemical reactions in a cross-linked elastomeric polymer in a noninvasive fashion. One reaction is the chromogenic isomerization of a naphthopyran mechanophore embedded in a polydimethylsiloxane (PDMS) network. Under HIFU irradiation evidence of the mechanochemical transduction is the observation of a reversible color change as expected for the isomerization. The elastomer exhibits this distinguishable color change at the focal spot, depending on ultrasonic exposure conditions. A second reaction is the demonstration that HIFU irradiation successfully triggers a luminescent dioxetane, resulting in localized generation of visible blue light at the focal spot. In contrast to conventional stimuli such as UV light, heat, and uniaxial compression/tension testing, HIFU irradiation provides spatiotemporal control of the mechanochemical activation through targeted but noninvasive ultrasonic energy deposition. Targeted, remote light generation is potentially useful in biomedical applications such as optogenetics where a light source is used to trigger a cellular response.high-intensity focused ultrasound | mechanochromism | mechanophores | mechanoluminescence | spatiotemporal control M echanical force exerted on a covalent bond acts as an energetic stimulus to drive specific chemical reactivity (mechanochemistry) (1). Particularly, mechanophores, i.e., mechanically sensitive molecules, have attracted attention due to their role in eliciting various chemical transformations in polymers (2). Extensive work in molecular design has revealed a rich library of mechanophores, which when subjected to mechanical stress undergo selective bond cleavage to unveil color change (3, 4), fluorescence (5), new reactive functionality (6), catalytic activity (7), light emission (8), and small-molecule release (9), among other responses. The ability of mechanochromic mechanophores to "visualize" the mechanical response to applied force provides insight for identifying the damage location and the material state in polymeric materials. Of particular importance to extending functionality of these mechanophores is the development of novel force-control tools capable of noninvasively triggering their activation. Mechanochemical responses have been observed when mechanical energy is applied to bulk-phase m...

On the acoustic fountain types and flow induced with focused ultrasound

“…1993); additional details on the transducer can be found in Kim et al. (2019).

Figure 8.( a ) Shape characteristics of intermediate fountains for the 550 kHz (0.7 MPa) and 1.1 MHz (1.3 MPa) transducers.

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“…The height of the water surface was fixed to 60 mm to guarantee that the transmitted acoustic energy was focused on the free surface. In the water, the output voltage signal was detected at the focal spot using a fibreoptic hydrophone (Precision Acoustics) and converted to peak acoustic pressure by applying hydrophone sensitivity (96.00 mV MPa −1 for 550 kHz and 112.04 mV MPa −1 for 1.1 MHz) (Kim et al 2019). The peak acoustic pressure was varied in the range from 0.5 to 1.4 MPa and focused on the free surface.…”

“…necrosis of cancer cells as a new therapeutic modality (Kennedy 2005). The need to deliver spatiotemporally controlled moderated acoustic energy to a target area is a critical ability of ultrasound in medicine (Kim et al 2019). The moderate use of the acoustic energy offers the solutions for clinical challenges, e.g.…”

Cavitation in soft matter

“…Another important tool in understanding the stress profile in a cavitation event could be the emergence of so-called "mechanochemistry." Mechanochromism, a subsection of mechanochemistry, produces color change with mechanical force and is inspired by natural phenomena, such as sonoluminescence (114). In this field, spiropyran-based materials have received significant attention with their ability to turn into a colored merocyanine form in response to applied stress (115,116).…”

Sono-optogenetics facilitated by a circulation-delivered rechargeable light source for minimally invasive optogenetics

“…1 A ), owing to the sufficient penetration depth of 200 µm for 473-nm photons (52) and the pervasive cerebral vasculature penetrating into every region of the brain (53). In comparison to organic mechanophores that emit light during the irreversible break of chemical bonds (28), the rechargeability of ZnS:Ag,Co@ZnS nanoparticles with 400-nm excitation allows for repetitive optogenetic stimulation in the brain after a single i.v. injection, making our method suitable for animal studies that last hours to days.…”