Determination of the Interfacial Rheological Properties of a Poly(DL-lactic acid)-Encapsulated Contrast Agent Using In Vitro Attenuation and Scattering

Abstract

The stabilizing encapsulation of a microbubble based ultrasound contrast agent (UCA) critically affects its acoustic properties. Polymers, which behave differently from commonly used materials—e.g. lipids or proteins—for the monolayer encapsulation, hold potential for better stability and control over encapsulation properties. Air-filled microbubbles coated with Poly (D, L-lactide) (PLA) are characterized here using in vitro acoustic experiments and several models of encapsulation. The interfacial rheological properties of the encapsulation are determined according to each of these models using attenuation of ultrasound through a suspension of these microbubbles. Then the model predictions are compared with scattered nonlinear—sub- and second harmonic—responses. For this microbubble population (average diameter 1.9 μm), the peak in attenuation measurement indicates a weighted average resonance frequency of 2.5–3 MHz, which, in contrast to other encapsulated microbubbles, is lower than the resonance frequency of a free bubble of similar size (diameter 1.9 μm). This apparently contradictory result stems from the extremely low surface dilatational elasticity (around 0.01–0.07 N/m) and the reduced surface tension of the PLA encapsulation as well as the polydispersity of the bubble population. All models considered here are shown to behave similarly even in the nonlinear regime because of the low value of the surface dilatational elasticity. Pressure dependent scattering measurements at two different excitation frequencies (2.25 and 3 MHz) show strongly non-linear behavior with 25–30 dB and 5–20 dB enhancements in fundamental and second-harmonic responses respectively for a concentration of 1.33 μg/mL of suspension. Subharmonic responses are registered above a relatively low generation threshold of 100–150 kPa with up to 20 dB enhancement beyond that pressure. Numerical predictions from all models show good agreement with the experimentally measured fundamental response, but not with the second harmonic response. The characteristic features of subharmonic response and the steady response beyond the threshold are matched well by model predictions. However, prediction of the threshold value depends on property values and the size distribution. The variation in size distribution from sample to sample leads to variation in estimated encapsulation property values—the lowest estimated value of surface dilatational viscosity better predicts the subharmonic threshold.

Encapsulated microbubbles and echogenic liposomes for contrast ultrasound imaging and targeted drug delivery

“…Note that the surface tension used to characterize an air-liquid interface, either pure or contaminated with surfactants, is also an interfacial rheological property [109]. Over the years, we have developed a two-pronged interfacial rheological characterization effort which includes one experiment to determine the characteristic properties of the encapsulation and a second independent experiment that validates the characterization [109–112]. The independent validation distinguishes this effort from other similar modeling studies.…”

“…We use the experimentally obtained attenuation data to determine the unknown model parameters and then validate our model predictions against nonlinear scattering [110–112]. Usually, attenuation experiments are performed at low amplitude excitations.…”

“…Due to the ill posed nature of the problem, the fitting process is difficult and also very sensitive to several factors like polydispersity of the suspension, initial guess of parameters, etc. [112]. Also, attenuation data might not always reflect the linearized dynamics.…”

“…The PLA microbubbles were developed by Prof. Margaret A. Wheatley at the Biomedical Engineering Department, Drexel University [11, 12, 143–148]. The detailed description of the experimental setups used to study acoustic scattering from and attenuation through a suspension of above mentioned contrast agents can be found in our earlier publications [110, 112, 149], and hence, not discussed here.…”

“…Subharmonic imaging has also been investigated for high frequency imaging applications [159–162] and noninvasive blood pressure estimation [163–174]. We have been investigating scattered subharmonic response from contrast microbubbles both experimentally and through numerical simulations [110–112, 120, 125, 175]. Figure 4 below shows both the experimentally measured and simulated subharmonic responses from Sonazoid and PLA coated microbubbles.…”