Lasers and bubbles: Thermocavitation for needle-free jet injections

This thesis describes thermocavitation in microfluidic confinement, which is the generation of a vapor bubble by laser-heating of the liquid. During the growth of this bubble, it pushes the remaining liquid in the microfluidic channel out, creating a fast jet which can penetrate the skin. Therefore, this technique can be used to inject medication or other liquids, such as tattoo ink, without the use of a needle. Good control of the jet velocity requires understanding of the laser-liquid interaction, which is the focus of this thesis.
The two most commonly used lasers, pulsed and continuous-wave (CW) lasers were compared. Both lasers were found to generate similar bubble dynamics, but the CW laser is slightly less efficient and reproducible. However, CW lasers are more affordable and smaller in size, which could increase their potential to be used in a commercial handheld device.
To further investigate thermocavitation, numerical simulations of the laser heating were compared with experimental results of the CW-generated bubbles. These simulations suggest that nucleation always occurred when the maximum temperature was around 237 °C. Therefore, varying the laser beam size and power allows for control over the bubble dynamics.
Absorption of the optical energy by a thin gold layer on the channel wall was investigated as an alternative to volumetric absorption by the liquid itself. Although this heating through the gold layer could generate bubbles and jets, it was less efficient and degradation of the layer occurred. Improvement through simulations and different choice of materials may overcome these problems.
Finally, surface modifications were applied to the microfluidic channel in order to improve the jet dynamics. Alternating hydrophobic and hydrophilic stripes along the jet axis enable further control over the jet formation, through two influences: shaping the initial meniscus, as well as keeping the jet straight along the hydrophilic stripe.
Overall, the results in this thesis contribute to the understanding and control of laser-generated bubbles in microfluidic confinement and the resulting jet. These results are vital for further development of laser-actuated jet injection.