Polymeric microcapsules with internal cavities for ultrasonic imaging: efficient fabrication and physical characterization

Polymeric microcapsules are of great potential in ultrasonic imaging due to their characteristic hollow structure. Water-in-oil-in-water (W₁/O/W₂) double emulsion-solvent evaporation technique is a versatile strategy applicable to most hydrophobic polymers for fabricating microcapsules; however, the adjustment of the size and inner structure of resultant microcapsules have not been systematically studied until now. Here, we evaluate in detail the parameters in double emulsification and find that the W₁/O volume ratio is a pivotal parameter which controls the hollow structure of microcapsules. In addition, an appropriate concentration of emulsifier in W₂ is essential to guarantee the hollow structure as well. For quantitatively characterizing the hollow structure of microcapsules, we propose the concept of Hollow Ratio (HR) and Hollow Degree (HD) to evaluate the percent of hollow microcapsules in products and the hollow characteristic of the microcapsules. Our study demonstrates that the HR of microcapsules can vary between 25% and 98% by only adjusting the W₁/O volume ratio. The size of microcapsule has a close relationship to its HD. Moreover, the microcapsules with both single cavity and multicavities have been fabricated by altering the energy of the second emulsification. Further, acoustic studies reveal that the microcapsules with different HD display obviously different sound attenuation spectrum and resonance frequency, which demonstrates that the adjustment of hollow structure should be an effective approach to control the acoustical properties of microcapsules for ultrasonic imaging.