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Introduction to Haptic Optical Tweezers

1.1. Introduction

Micro- and nanotechnologies are, in theory, very attractive. Theoretical models predict incredible properties for nano- and microstructures. In practice, however, researchers and inventors face an unknown puzzle: there is no analogy in the macroworld that can prepare operators for the unpredictability and delicacy of the microscopic world. Existing knowledge and know-how are experimentally insufficient. Exploration is the most delicate and unavoidable task for micromanipulation, microfabrication and microassembly processes. In biology, the manipulation and exploration of single cells or protein properties is a critical challenge [ZHA 08a, MEH 99]. This can only be performed by an experienced user. These procedures are highly time-consuming and uneconomical.

Well-designed user interfaces and force-feedback teleoperation increase the achievable complexity of operations and decrease their duration [HOK 06]. Several works have considered the coupling of existing micromanipulators to commercial or prototype haptic devices [SIT 03, KHA 09, WES 07] with little success. Indeed, the feedback is dependent on the degrees of freedom of the platform, the range of scaling and the type of interaction to render. Compared to other techniques, optical tweezers (OTs) [ASH 86] seem to be more promising for the integration of the robotic technique of force feedback teleoperation (see Figure 1.1). OTs are a very versatile tool and the quality of possible force feedbacks can be improved by the techniques discussed in this chapter. The chapter also highlights a new approach: to rethink the design of micromanipulators in order to reliably and usefully render the interaction through the user interface.

Figure 1.1. Dextrous use of a micromanipulation platform. An optical trap is teleoperated with an interface that allows force measurement feedback to be haptically experienced. A 3D reconstruction of the scene can also increase user immersion. The haptic interface presented is the OmegaTM from Force Dimension. For a color version of this figure, see www.iste.co.uk/ni/tweezers.zip

Appropriate techniques to get the user a high-quality force feedback with OTs are discussed. Better dexterity is achieved and tasks of higher complexity are performed with little knowledge and implementation of the haptic teleoperation methods. The principles for interactive micromanipulation systems and the advantageous properties of OTs are summarized in section 1.2. The different existing components and techniques of optical trapping are summarized and their drawbacks for haptic purposes are highlighted in section 1.3. As a consequence, new designs specific to haptic applications are discussed in detail in section 1.4, based on the most recent experiments described in the literature. Finally, the prospects brought by this new approach are carefully highlighted in section 1.5 in order to encourage further developments in this domain.

1.2. A dexterous experimental platform

Everyday interactions and manipulations are possible because of our remarkable sensors (our eyes and proprioceptive systems) and effectors (our hands and muscles). Traditional tools for visualizing and interacting with the microworld are not nimble or transparent. Microscopes and micromanipulators historically do not lend themselves to intuitive interaction or handling because human vision is two-dimensional (2D), force sensors are rare and the degrees of freedom are reduced. Intuitive interactions and control are especially complicated because of the particular phenomena of the microworld.

1.2.1. A dexterous micromanipulation technique

Micromanipulation experiments are often poorly repeatable, time-consuming and costly because of unique physical phenomena at this scale. Surface interactions become more significant than volume interactions for objects smaller than 500 µm. Particles tend to adhere and become bound to handling tools or substrates, or surface forces interact with low-inertia particles to produce huge accelerations which can damage or eject samples.

Table 1.1. Comparison of three individual techniques of micromanipulation (based on molecular study applications [NEU 08])

Many handling techniques have been designed to address the adhesion problem [CHA 10]. Current designs are focused on the development of miniaturized microtools with functionalized surfaces (atomic force microscope (AFM) probes [XIE 09], microgrippers [AND 09]) or potential field levitation and non-contact guiding (OTs [ASH 86], magnetics tweezers [GOS 02, VRI 05], electrophoresis [WAN 97], microfluidics [SQU 05], etc.).

In this chapter, we will only consider grasping phenomena that facilitate the manipulation of individual independent microscopic tools (electrophoretic, and microfluidics do not permit isolation of a single effector). AFM and microgrippers make possible independent displacement and application of high-amplitude forces (10 - 104pN). However, the effectors are large and therefore adhesion, inertia and visual obstruction limit their performance. Electromagnetic techniques have a localized magnetic field, but it is difficult to independently manipulate several robots [DIL 13]. Also, electromagnetic techniques can only be considered as an independent tool when the properties of the trap probe are very different from the sample nature, such as proteins, cells or non-magnetic microassembly parts.

OTs [ASH 86, NEU 04] avoid many of the limitations of competing techniques (see Table 1.1 for comparisons). Compared to other micromanipulation techniques, OTs offer greater versatility. Optical trapping relies on an immaterial electromagnetic field produced by highly focused laser light. This produces optical force (<100 pN) which is effective for the manipulation of particles between 100 microns [SHV 10] and atomic scale [ASH 00]. A highly focused laser produces a localized three-dimensional (3D) electromagnetic field that stably traps spherical dielectric microtools. This probe is then easily actuated by deflecting or defocusing the laser. The optical forces are easily modeled and 3D trap stiffness can be estimated experimentally [ROH 02, ASH 92, BER 04]. Particle tracking makes it possible for the force on the probe to be measured (see section 1.3.3). There are many experimental setups that use high-speed actuation (1 GHz bandwidth [RUH 11]) and high-precision force measurement (femtoNewton [ROH 05a]). Time or spatial sharing of the laser power also offers parallelisms the possibility of trapping: experiments have shown that more than 200 parallel traps [CUR 02] or up to 9 parallel sensors [SPE 09] have been accomplished on a single system. These properties of OTs, i.e. high speed, high precision and the capability for multiple independent interactive probes, allow unprecedented opportunities for teleoperative control of microscopic systems. The techniques for efficient construction of the force feedback interfaces are detailed in the next section.

1.2.2. A dexterous user interaction for micromanipulation

Force feedback teleoperation techniques originate from nuclear energy plants, where maintenance tasks can only be performed from a distance [SHE 92]. The aim of these techniques is to recreate the bilateral interaction between the user and an unreachable environment; in other words, touch sensations can be recovered on a system where the mechanical linkage is disconnected. In our case, microscopic particles are mechanically disconnected from the user's hands. Teleoperation techniques can be extended to micromanipulation platforms in the following way. A master robot, also called the active joystick or the haptic user interface, is connected to a slave robot (in our case, the OT platform):

This is a bilateral process, named the "haptic coupling loop" (see Figures 1.2(b) and (d)). The characteristics of this kind of automatic scheme are well known [LAW 93]. Stability and transparency are the main issues for an accurate bilateral transmission. Stable systems do not diverge from equilibrium positions. For bilateral coupling, this state can be evaluated by the sufficient condition of passivity: the ability of a system not to add energy in the loop [HAN 02]. Transparency can be defined as the degrees of reliability and latency of the transmission, which is measured by the frequency bandwidth of the system. It is important to note that human temporal frequency bandwidth is estimated up to 1 kHz for force perception and over 10 kHz for textures [JON 06]. This means that human hands are able to perceive discontinuity of the signal under this sampling limit. It is critical that the bandwidth and sampling of all components and coupling of the system are over those thresholds.

The most realistic force feedback is obtained with a scheme called direct coupling because it is only composed of fixed homothetic scaling gains (see Figure 1.2(d)) [BOL 09]. Unlike passive coupling (see Figure 1.2(b)), direct coupling does not possess a filter to reduce information reliability and response time. It, therefore, has great fidelity and is referred to as a transparent coupling. However, it is not unconditionally stable, and the stability must be carefully controlled for safe use [BOL 09].

Direct coupling is even worse...