TABLE IV.
Common methods used to actively modulate cell encapsulation within microfluidic devices.
| Methods | Order of magnitude of rate (Hz, or droplet per second) | Advantages | Disadvantages | Principles | Reference |
|---|---|---|---|---|---|
| Extrinsic | |||||
| Electronic | 101–102 | Well-studied fundamental theories/ the application of electrical field may not be compatible with the integrity or viability of biological samples. | Low production rate and potential harm of magnetic nanoparticles toward biological sample. | The application of electrical fields can manipulate the displacement of water–oil interface, and thereby accelerate the droplet pinch-off. | 89 and 91–99 |
| Magnetic | 102 | Easy to implement and manipulate | Converting the fluid into ferrofluid, by the addition of magnetic nanoparticles for example, can be used to control the fluid motion. | 63, 64, 101 | |
| Optical | |||||
| Laser | 103–104 | High production rate, precise control | potential harm of laser towards biological samples and requirement of integrating complex laser equipment into the system | Laser-induced cavitation is used to disturb immiscible interphase. | 103 |
| Off-chip affiliation | 102 | Non-invasive manipulation of fluid hydraulic pressure | The application of off-chip pressure source to induce hydraulic pressure cycles, and thereby manipulate droplet generation. | ||
| Microvibrator | very low production rate of droplets | 90 | |||
| Mechanical valve | 108 and 109 | ||||
| On-chip affiliation | 103 | Control over the production rate and droplet morphologies | very low production rate | On-chip actuators or valves to induce mechanical pulses can be used to control the droplet generation. | |
| Piezoelectric actuator | 113 | ||||
| SAW actuator | 111 | ||||
| Pneumatic valves | 116 and 119 | ||||
| Intrinsic | |||||
| Electronic | Not given | Able to achieve on-demand droplet generation as the response time is within the order of millisecond. | Potential harm of electrical field to biological samples | The ordering of suspended non-conductive but electrically active particles under the application of electrical field can change the fluid viscosity. | 94 |
| Thermal | Not given | The application of heat may not be compatible with the integrity or activity of biological samples. | Potential harm of heat to biological samples | The change in fluid viscosity and volume along with the change in temperature can be used to modulate the viscous and interfacial tension force profile within microchannel, and thereby manipulate droplet generation. | 121 and 122 |
| Optical/dielectrophoresis tweezers | <1 | Extremely precise control over cell displacement, near 100% encapsulation efficiency | extremely low encapsulation rate | Application of optical or electrical field to control the cell displacement. | 17–19 |