Ultrafast vapourization dynamics of laser-activated polymeric microcapsules.

Precision control of vapourization, both in space and time, has many potential applications; however, the physical mechanisms underlying controlled boiling are not well understood. The reason is the combined microscopic length scales and ultrashort timescales associated with the initiation and subsequent dynamical behaviour of the vapour bubbles formed. Here we study the nanoseconds vapour bubble dynamics of laser-heated single oil-filled microcapsules using coupled optical and acoustic detection. Pulsed laser excitation leads to vapour formation and collapse, and a simple physical model captures the observed radial dynamics and resulting acoustic pressures. Continuous wave laser excitation leads to a sequence of vapourization/condensation cycles, the result of absorbing microcapsule fragments moving in and out of the laser beam. A model incorporating thermal diffusion from the capsule shell into the oil core and surrounding water reveals the mechanisms behind the onset of vapourization. Excellent agreement is observed between the modelled dynamics and experiment.

Influence of shell properties on high-frequency ultrasound imaging and drug delivery using polymer-shelled microbubbles.

This two-part study investigated shell rupture of ultrasound contrast agents (UCAs) under static overpressure conditions and the subharmonic component from UCAs subjected to 20-MHz tonebursts. Five different polylactide-shelled UCAs with shell-thickness-to-radius ratios (STRRs) of 7.5, 30, 40, 65, and 100 nm/¿m were subjected to static overpressure in a glycerol-filled test chamber. A video microscope imaged the UCAs as pressure varied from 2 to 330 kPa over 90 min. Images were postprocessed to obtain the pressure threshold for rupture and the diameter of individual microbubbles. Backscatter from individual UCAs was investigated by flowing a dilute UCA solution through a wall-less flow phantom placed at the geometric focus of a 20-MHz transducer. UCAs were subjected to 10- and 20-cycle tonebursts of acoustic pressures ranging from 0.3 to 2.3 MPa. A method based on singular-value decomposition (SVD) was employed to obtain a cumulative subharmonic score (SHS). Different UCA types exhibited distinctly different rupture thresholds that were linearly related to their STRR, but uncorrelated with UCA size. The rupture threshold for the UCAs with an STRR = 100 nm/µm was more than 4 times greater than the UCAs with an STRR = 7.5 nm/µm. The polymer-shelled UCAs produced substantial subharmonic response but the subharmonic response to 20- MHz excitation did not correlate with STRRs or UCA-rupture pressures. The 20-cycle excitation resulted in an SHS that was 2 to 3 times that of UCAs excited with 10-cycle tonebursts.

Monodisperse polymeric particles prepared by ink-jet printing: double emulsions, hydrogels and polymer mixtures.

Submerged ink-jetting produces a monodisperse emulsion that can be converted into monodisperse particles. As the initial droplet size is known and the final particle size can be easily measured, such a method can be used to quantify the shrinkage and the swelling of polymer particles made from double emulsions, polymer mixtures and hydrogel forming polymers. It is found that at the same starting concentration and initial emulsion droplet size poly-lactide-co-glycolide particles made from an ink-jetted emulsion have the same size as particles ink-jetted from a solution, however with a more porous structure. The total pore volume, however, is negligible compared to the polymer volume of the particle. If polymers containing a poly-ethylene glycol block are included, particles with internal porosity are formed, even if no double emulsion process is applied. Still the final particle size is the same. Only if typical hydrogel forming polymers are used, in which water is distributed more homogeneously, significantly higher particle diameters are found; for a four-arm PEG-poly-caprolacton a degree of swelling of 3.3 is found.

Effect of molecular weight, crystallinity, and hydrophobicity on the acoustic activation of polymer-shelled ultrasound contrast agents.

Polymer-shelled microbubbles are applied as ultrasound contrast agents. To investigate the effect of the polymer on microbubble preparation and acoustic properties, polylactides with systematic variations in molecular weight, crystallinity, and end-group hydrophobicity were used. Polymer-shelled cyclodecane filled capsules were prepared by emulsification, and the cyclodecane was removed by lyophilization to obtain hollow capsules. Complete removal of cyclodecane from the microcapsules was only achieved for short chain (about M(w) 6000) crystalline polymers. The pressure threshold for acoustic destruction of the microbubbles was found to increase with molecular weight. Noncrystalline polymers showed a higher threshold for destruction than crystalline polymers. Hydrophobically modified short chain crystalline polymers showed the steepest increase in acoustic destruction after the threshold as a function of the applied pressure, which is a favorable characteristic for ultrasound mediated drug delivery. Microcapsules made with such polymers had an inhomogeneous surface including pores through which cyclodecane was lyophilized efficiently.

Oil-filled polymer microcapsules for ultrasound-mediated delivery of lipophilic drugs.

The use of ultrasound contrast agents as local drug delivery systems continues to grow. Current limitations are the amount of drug that can be incorporated as well as the efficiency of drug release upon insonification. This study focuses on the synthesis and characterisation of novel polymeric microcapsules for ultrasound-triggered delivery of lipophilic drugs. Microcapsules with a shell of fluorinated end-capped poly(L-lactic acid) were made through pre-mix membrane emulsification and contained, apart from a gaseous phase, different amounts of hexadecane oil as a drug-carrier reservoir. Mean number weighted diameters were between 1.22 microm and 1.31 microm. High-speed imaging at approximately 10 million fames per second showed that for low acoustic pressures (1 MHz, 0.24 MPa) microcapsules compressed but remained intact. At higher diagnostic pressures of 0.51 MPa, microcapsules cracked, thereby releasing the encapsulated gas and model lipophilic drug. Using conventional ultrasound B-mode imaging at a frequency of 2.5 MHz, a marked enhancement of scatter intensity over a tissue-mimicking phantom was observed for all differently loaded microcapsules. The partially oil-filled microcapsules with high drug loads and well-defined acoustic activation thresholds have great potential for ultrasound-triggered local delivery of lipophilic drugs under ultrasound image-guidance.

Large area liquid crystal monodomain field-effect transistors.

Butyl, hexyl, and decyl derivatives of the liquid-crystalline organic semiconductor 5,5' '-bis(5-alkyl-2-thienylethynyl)-2,2':5',2' '-terthiophene were synthesized and studied with respect to their structural, optical, and electrical properties. By means of an optimized thermal annealing scheme the hexyl and decyl compounds could be processed into self-assembled monodomain films of up to 150 mm in diameter. These were investigated with X-ray diffractometry, which revealed a clearly single-crystalline monoclinic morphology with lamellae parallel to the substrate. Within the lamellae the molecules were found to arrange with a tilt of about 50 degrees with the rubbing direction of the polyimide alignment layer. The resulting, close side-to-side packing was confirmed by measurements of the UV/vis absorption, which showed a dichroic ratio of 19 and indicated H-aggregation. AFM analyses revealed self-affinity in the surface roughness of the monodomain. The compounds showed bipolar charge transport in TOF measurements, with hole mobilities reaching up to 0.02 cm(2)/Vs and maximum electron mobilities around 0.002 cm(2)/Vs. The hexyl derivative was processed into large-area monodomain top-gate field-effect transistors, which were stable for months and showed anisotropic hole mobilities of up to 0.02 cm(2)/Vs. Compared to multidomain bottom-gate transistors the monodomain formation allowed for a mobility increase by 1 order of magnitude.