(K-410) Fabrication of Polydimethylsiloxane Microspheres and their Use in Cartilage Tissue Engineering

An 80 cm long aluminum tubing with an inner diameter of approximately 6 cm was heated using two flexible heaters wrapped around the tube and insulated by ceramic fiber insulation. A temperature controller was used to maintain the temperature of 300°C inside the tube.

PDMS base and PDMS curing agent were mixed at a ratio of 10:1 for one minute. The mixture was transferred to a 10 mL syringe equipped with a 30-gauge, flat needle tip. The PDMS-filled syringe was positioned in a syringe pump with precise alignment so that the needle tip pointed directly toward the center of the aluminum tubing. The distance between the top entrance of the tubing and the needle tip was maintained at 9 cm.

To collect the PDMS microspheres, a piece of aluminum foil was at the bottom exit of the aluminum tubing. During this experiment, the PDMS mixture was subjected to a range of voltages varying from 3.25 kV to 11 kV, achieved using a high-voltage power supply connected to the needle tip. The syringe pump operated at a constant flow rate of 300 mL/hr. During this process, PDMS spheres were collected on aluminum foil. PDMS spheres were generated and collected on the aluminum foil placed below the tubing. However, at the 11kV, the PDMS aerosol exhibited a tendency to disperse in a sideway direction, so the microspheres were collected using an aluminum foil attached to the side of the tubing.

The study revealed that there is an inverse relationship between the applied voltage and the diameter of microspheres. Voltages below 6.43kV and above 10kV resulted in incomplete polymerization of PDMS droplets before reaching the aluminum foil at the bottom and the side of the tubing, respectively. This led to a reduced collection of microspheres for analysis. In contrast, voltages in the above-mentioned range promoted the formation of nearly all electrospray droplets into polymerized solid spherical shapes before reaching the aluminum foil. The distribution curves of these microspheres exhibited a bell curve pattern, indicating a feasible collection of microspheres at a desired size. However, because of the issues encountered with voltages 6.43, the distribution curve of microspheres obtained at this setting did not exhibit the expected bell curve shape. This presents a limitation in producing consistently sized microspheres at certain voltages. Additionally, temperature profiling within the tubing demonstrated that a minimum of 266°C was necessary for the successful fabrication of polymerized microspheres. 

Various factors can influence the synthesis of microspheres. The flow rate of PDMS may affect the size distribution of the microspheres. The generation of electric fields may be easily interfered with by the humidity of the environment due to the dipole of water molecules. The viscosity of PDMS changes with temperature, adding complexity to the fabrication process. The precise control of experimental parameters may be challenging, leading to some variations in the size of microspheres when the procedure is replicated. Despite these variations, the overall trend of an inverse relationship between the diameter of microspheres and applied voltage is expected to remain consistent. 

In conclusion, this study successfully demonstrated the fabrication of PDMS microspheres using electrospray drying, offering control over their size by manipulating the applied voltage. The results established an inverse relationship between the voltage and microsphere diameter. This method opens up a wide range of applications, such as fluorescent detection of oxygen, which we are currently investigating for cell culture.