Surface functionalisation for microhandling

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Surface functionalisation for microhandling

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Surface functionalisation for microhandling

Objectives and Context

Manufactured products are getting smaller and smaller and are integrating more and more functionalities in small volumes. Several application fields are concerned such as telephony, bio-engineering, telecommunications or more generally speaking the Micro-Electro-Mechanical-Systems (MEMS). The assembly of these microproducts is a great challenge because of the microscopic size of the components. In fact, the major difficulty of micro-assembly come from the particularity of the micro-objects behaviour which depend on surface forces. In microscale, the surface strengths are predominant compare to the weight and the objects tends to stick the microgrippers. The manipulation of a micro-object requires handling, positioning, and releasing it without disturbances of the surface forces such as electrostatic forces, van der Waals forces or capillary forces. The release is the most critical phase which is usually hindered by adhesion. Several methods have been proposed in the last ten years to improve micromanipulation. We propose a new contact handling system that chemically controls the surface forces between the object and the gripper. We have already shown that the medium (air or liquid) can modify or cancel the adhesion force. The major objective is to control the adhesion force or to create a repulsive force to guarantee a reliable release.
Now, the surface properties of a material can be controlled by surface functionalisation in a liquid. The charge density of functionalised surfaces must depend on the pH in order to control the adhesion force in liquid medium using the pH.

Force measurement by AFM

In Figure 1, the distance force measurement done by AFM have shown the interaction forces inversion during the variation of the pH of the solution.

a) pH nat

b) ph 9

Figure 1: Force-distance curve for the APTES functionalised substrate in liquid medium at different pH (spring constant0.3 N/m): a) pH natural (near 5.5), b) pH 9. The dash line (blue) are the approach of the surface near the cantilever, and the full line

In this figure, we note that at natural pH, an attractive pull-in and an important pull-of is measured whereas the pull-in force is inverted, became repulsive and the adhesion forces disappears when the pH increase.
This result is a great interest for the micromanipulator community. So, the pH control is in fact able to switch from an attractive (grasping) to a repulsive behaviour (release).

Micromanipulation tasks

This behaviour described above can be used to control the grasping and the release of a micro-object manipulated with a microgripper. Two scenarii can be presented (see in Figure 2). In the first case, grasping and release occur in two different medium in order to guarantee adhesion during the grasping and repulsion during the release. In the second case, the pH of the medium has to be changed during manipulation. A microfluidic system could be build to
induce laminar flow of acidic or basic solution sequentially in the manipulation range. The laminar flow should be able to switch rapidly the pH without disturbing the position of the object on the substrate.

first scenario

second scenario

Figure 2: Scenarii of robotic microhandling coupling with chemical functionnalisation

The Figure 3 shows first experiments made with the AFM with a tipless cantilever functionalised and a free functionalisation glass sphere. At natural pH a glass sphere, whose diameter is around 50 micrometers, is 'grasped' with the tipless cantilever (Figure 3, left). For this, the cantilever was lowered down to the sphere and a force of 20 µN was applied. When the cantilever went up the borosilicate sphere stayed attached to the cantilever thanks to the attractive force (natural pH) and the adhesion force which are near 60 nN and 387 nN respectively. Afterwards, the pH was increased by addition of NaOH solution. The behaviour was inverted and when pH was near 9, the sphere
has been released (Figure 3, right). Indeed, the attractive and adhesion force disappeared and an repulsive force of 282 nN appeared between the cantilever and the sphere.

grasping