Facile fabrication of PDMS microspheres with and without fluorescent dye doping
Abstract
Poly(dimethyl siloxane) (PDMS) microspheres are increasingly gaining importance for a wide range of applications due to their flexibility and inertness. In this paper, we have fabricated PDMS microspheres using facile emulsion formation in water by stirring with and without additional ultrasonic excitation. It was found that the particle size distribution, which can be attributed to the formation rate of cross-linked PDMS networks in the hydrophobic microspheres, depends on the temperature of the aqueous medium. Swelling of the microspheres in acetone was suggested by permeabilization as evidenced by diffusion and encapsulation of fluorescent dyes within the PDMS. Using scanning electron microscopy, the surface morphology of the spheres was confirmed to have no surface roughness or irregularity. Using fluorescence microscopy, we found that the encapsulated dyes randomly and thus uniformly distributed themselves within the cross-linked PDMS networks and retained the fluorescent properties and characteristic emission color, implying their potential for drug carrier.
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Introduction
Among the many important polymeric microparticles, those comprised of poly(dimethylsiloxane) (PDMS) have attracted much attention due to their biocompatibility, thermal stability, optical transparency in the UV-visible region, flexibility, nontoxicity, low polarity, low electrical conductivity, and chemical inertness [1-6]. In addition to industrial and medical applications, PDMS microspheres are also utilized in a number of new scientific disciplines, including sensors, actuators, bioanalysis and additives for polymer resins [7]. For these applications, functional compounds can be incorporated into the PDMS microspheres upon mixing them in less-polar organic solvents. With all of these properties, PDMS microspheres can be used as a matrix to deliver the functional compounds to a desired location through microcirculation. Moreover, PDMS microspheres can be suspended in solution to achieve a fluid with controllable rheological properties [8]. Such a wide range of applications has stimulated great interest in the fabrication of PDMS microspheres and in their microfluidic systems for bioanalyses and beyond. For instance, amphiphilic PDMS microspheres within a few tens to a few hundreds of microns in size have been successfully fabricated by flow-focusing and co-flowing methods [9-10], whereas for smaller PDMS spheres, with dimensions within 2.5 to 25 m, an aqueous emulsion technique has been employed [10-11]. In this technique, the dispersed cross-linked PDMS which is immiscible in water can naturally form micro spheres by virtue of their need to reduce their hydrophobic interactions. By using this technique, the cost of fabrication can be greatly reduced and the process is environmentally friendly.
Driven by the aforementioned potential applications of PDMS microspheres, the objective of this work was focused on low cost and efficient PDMS microsphere fabrication methods. Given the hydrophobic nature of PDMS, in this work we fabricate PDMS microspheres from PDMS elastomer in an aqueous medium by creating a stirred emulsion during crosslinking, with and without sonication. The stirring and the ultrasonic waves are expected to mediate separations of agglomerates of PDMS microdroplets during and after their injection into water, keeping the particles divided and spherical as they cross link and effectively freeze out further coalescence into larger domains. Prior to the cross-linking induced freezing out of droplet fusion and coalescence processes, the small particle size is maintained due to the response of the microspheres to the mechanical or acoustic radiation force exerted on them [12]. With this method, we evaluated the effect of the temperature of the medium on the size distribution of the PDMS microspheres. This is because the rate of cross linking is expected to be strongly temperature dependent. We characterized and examined the PDMS microspheres using bright field microscopy, Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM). Subsequently, we also evaluated the incorporation of ionic fluorescent dyes within the PDMS microspheres.
Conclusion
In summary, we have demonstrated that PDMS microspheres can be fabricated simply by stirring induced shear and sonication methods using water as a medium. These simple methods are able to fabricate and to control the inter-sphere cross-linking of PDMS microspheres by adjusting the temperature of the water medium. We found that the particle size tends to be smaller and the size distribution is narrower with temperature, indicating that the crosslinking rate for PDMS networks in the hydrophobic microspheres is slightly increased. We also showed that the PDMS microspheres undergo swelling process, incorporating fluorescent dyes, upon replacing the medium from water to an organic solvent with dissolved dyes. Upon incorporation of the dyes, the cross-linked PDMS network in the microspheres remain intact. Thus, the PDMS microspheres can act as a matrix to encapsulate fluorescence dyes, making them fluorescent microspheres. Using scanning electron and fluorescence microscopy, we found that the fluorescent dyes are randomly distributed in the cross-linked PDMS networks and the dyes do not change either their own chemical structures or the crosslinking PDMS networks in the microspheres. This work provides and paves the way for utilization of PDMS microspheres as matrices for medicinal, surfactants, electron donor, or electron acceptor compounds for various applications.