A Disposable Pneumatic Microgripper for Cell Manipulation with Image-Based Force Sensing

Abstract

A new design for a single-use disposable pneumatic microgripper is presented in this paper. It enables very cost-effective batch microfabrication in SU-8 with a single lithography mask by shifting manufacturing complexity into reusable components. An optically readable force sensor with potential to be used in a feedback loop has been integrated in order to enable gripping with a controlled force. The sensors are first examined separately from the gripper and exhibit good linearity. The gripper function utilizes the disposable gripper element together with a reusable gripper fixture. During experiments, the pneumatically actuated microgripper can vary the gripping force within a range of a few mN (up to 5.7 mN was observed). This microgripper is planned to be used in a liquid environment for gripping larger aggregates of cells in combination with the patch clamp technique. This approach will allow Langerhans islets suspended in an electrolyte solution to be grasped and held during electrophysiological measurements without cell damage.

Keywords: SU-8; image-based force sensor; microgripper; patch clamp technique; pneumatic actuation.

Figure 1

A pneumatic gripper design without force sensing capability as shown by Hoxhold [6], predecessor of the current iteration.

Figure 2

Concept sketch of the envisaged handling system allowing the patch clamp analysis of small cell clusters. A cell sample (1) is submerged in an electrolyte solution in a sample dish; (2) the sample is gripped with the aid of a disposable microgripper; (3) that is mounted on a reusable pneumatic actuator fixture; (4) the sample is contacted via a pipette; (5) filled with the same electrolyte. An electric current can thus be measured through the pipette, the sample, and the surrounding electrolyte.

Figure 3

Schematic visualization of the manufacturing process for the disposable gripper component: (a). The silicon carrier wafer; (b). The sputtering of Cr and Cu; (c). Depositing the SU-8; (d). Structuring the SU-8; (e). The release by etching of the Cu sacrificial layer.

Figure 4

The disposable gripper component made of SU-8 with a larger partial view of the force measurement structure in one of its arms. (1): The flexible part of the pneumatic actuator. (2): The rigid pressure barrier of the pneumatic actuator. (3): The compliant hinge transmission. (4): Force sensors. (5): Gripping jaws. (6): The optical readout structure. (7): The force sensor beams.

Figure 5

Simplified schematic view of the compliant hinge transmission. (a): No actuator force. (b): The actuator pushes (positive pressure). (c): The actuator pulls (negative pressure) and the gripping force sensors get deflected according to the force applied to the sample.

Figure 6

Model used for simulation.

Figure 7

The simulation results of the sensor deflection (blue points) as a function of the geometric design parameters of spring beam length and width. The thickness of the SU-8 layer was set to 400 µm and the applied force to 30 mN. The colored area is numerically approximated to the simulation data by means of third-degree polynomials.

Figure 8

The central gripper component inserted into its mounting.

Figure 9

Force–deflection diagrams determined by image evaluation of the fully functional grippers for calibration of their force measurement structures: (a) Gripper A, side 1: offset: −0.104 mN, R²: 0.9740; side 2: offset: −0.297 mN, R²: 0.9697 (b) Gripper B, side 1: offset: −2.220 mN, R²: 0.9856; side 2: offset: −1.279 mN, R²: 0.9846.

Figure 10

Gripping force sensor in an experiment with a gripped rubber sample. The deformation of the force-measuring structure and the displacement in the vernier readout structure are clearly visible.

Figure 11

Forces determined by the elastic force measurement structures on gripper A, side 1, against the applied actuator pressure. (a) Values are read on the vernier scale; (b) Values are determined by measuring the image.

Figure 12

A demonstration of the gripping process on a rubber test body with the newly designed microgripper.