Design of ultrasonically-activatable nanoparticles using low boiling point perfluorocarbons

Abstract

Recently, an interest has developed in designing biomaterials for medical ultrasonics that can provide the acoustic activity of microbubbles, but with improved stability in vivo and a smaller size distribution for extravascular interrogation. One proposed alternative is the phase-change contrast agent. Phase-change contrast agents (PCCAs) consist of perfluorocarbons (PFCs) that are initially in liquid form, but can then be vaporized with acoustic energy. Crucial parameters for PCCAs include their sensitivity to acoustic energy, their size distribution, and their stability, and this manuscript provides insight into the custom design of PCCAs for balancing these parameters. Specifically, the relationship between size, thermal stability and sensitivity to ultrasound as a function of PFC boiling point and ambient temperature is illustrated. Emulsion stability and sensitivity can be 'tuned' by mixing PFCs in the gaseous state prior to condensation. Novel observations illustrate that stable droplets can be generated from PFCs with extremely low boiling points, such as octafluoropropane (b.p. -36.7 °C), which can be vaporized with acoustic parameters lower than previously observed. Results demonstrate the potential for low boiling point PFCs as a useful new class of compounds for activatable agents, which can be tailored to the desired application.

FIGURE 1

Exposing pre-formed PFC microbubbles to decreased ambient temperature and increased ambient pressure results in condensation of the gaseous core. The decreased size results in an increased Laplace pressure, which serves to preserve the particle in the liquid state. Once exposed to increased temperature and energy delivered via ultrasound, vaporization of the droplet core results in a larger, highly echogenic gas microbubble.

FIGURE 2

Dynamic light scattering results for various nanodroplet formulations (N=3 for each group). Both DFB and 1:1 DFB + OFP droplets resulting from polydisperse bubbles showed peaks at 295 nm in diameter.

FIGURE 3

Perfluorocarbon droplet distributions in vitro over a 1-hour period: Pure decafluorobutane at a) 22°C and b) 37°C; DFB + OFP mixture at c) 22°C and d) 37°C; and pure octafluoropropane at e) 22°C and d) 37°C. The distribution at each timepoint was scaled to the relative mean concentration (concentration-weighted) to simultaneously reflect changes in concentration over the time period.

FIGURE 4

Change in concentration over time for droplet samples of each perfluorocarbon at a) 22°C and b) 37°C.

FIGURE 5

A microscale droplet of OFP exposed to a 0.25 µs pulse at approximately 0.55 MPa vaporizes to form a gas bubble approximately 5-fold larger

FIGURE 6

Vaporization pressure for microscale DFB and OFP droplets at 22°C and 37°C. The vaporization threshold increased with increasing boiling point and decreasing diameter for the small range observed.

FIGURE 7

Droplets composed of a mixture of PFCs vaporized at 22 °C with rarefactional pressures between each of the composing PFCs – indicating condensation of the mixed gases resulted in a miscible dual-PFC core. By adjusting the ratio of PFCs, the vaporization threshold may be further ‘tuned’.

FIGURE 8

Brightfield (a, c) and fluorescence (b, d) microscopy illustrating that the shell is preserved when DiI-labeled DFB microbubbles (a, b) are condensed to the liquid state (c, d), the shell is preserved through the change in volume. (note that the droplet in c–d did not result directly from condensation of the precursor bubbles in a–b). Scale bar is 5 micrometers.

Figure 9

OFP droplet sample exposed to a 0.25 0µs pulses pulse at 1.1 MPa initiated at t = 0 forms bubbles in the 1 – 5 µm range, confirming the presence of viable nanoscale OFP droplets.