Sarah Isabelle Ksouri

Optical screw-wrench for interlocking 2PP-microstructures

Two-photon polymerization (2PP) has emerged as a powerful platform for processing three-dimensional microstructures with high resolution. Furthermore, by adding nanoparticles of different materials to the photopolymer the microstructures can be functionalized, e.g. magnetic or electric properties can be adjusted. However, to combine different functions within one microstructure or to manufacture complex microsystems, assembling techniques for multiple 2PP written building blocks are required. In this paper a qualitative approach for assembling microstructures utilizing optical forces is presented. Therefore, screw and nut shaped microstructures are produced by 2PP-technique and screwed together using a holographic optical tweezer (HOT). The interlocking structures are trapped and rotated into each other to cause connection. In this paper the used parameters and possible designs of the interlocking connection are discussed. These findings provide not only the assembling of building blocks to complex microstructures, rather different functionalized 2PP-microstructures can be combined by simply screwing them together with the use of optical forces.

Optical tweezers in microassembly

Integrated hybrid MEMS require new micromanipulation devices in assembly processes. Although absolute forces are restricted optical tweezers are promising tools with unique advantages. Recent developments in beam shaping allow the control of a large number of different particles. Optical manipulation can also be used to assemble tiny structures by a generative process. Any type of particle, primarily coated with high-affinity biomolecules, can be applied as building blocks to form complex structures. By moving the particle into the requested orientation by holographic optical tweezers complex parts become possible. Also, shape-complimentary preforms can be fabricated with 2-photon-polymerization (2PP) and utilized to assemble the desired structure. Finally, microvalves and motors in lab-on-a-chip systems can be optically fabricated and also driven by optical forces.

Optical screw-wrench for microassembly

For future micro- and nanotechnologies, the manufacturing of miniaturized, functionalized, and integrated devices is indispensable. In this paper, an assembly technique based on a bottom-up strategy that enables the manufacturing of complex microsystems using only optical methods is presented. A screw connection is transferred to the micrometer range and used to assemble screw- and nut-shaped microcomponents. Micro-stereolithography is performed by means of two-photon polymerization, and microstructures are fabricated and subsequently trapped, moved, and screwed together using optical forces in a holographic optical tweezer set-up. The design and construction of interlocking microcomponents and the verification of a stable and releasable joint form the main focus of this paper. The assembly technique is also applied to a microfluidic system to enable the pumping or intermixing of fluids on a microfluidic chip. This strategy not only enables the assembly of microcomponents but also the combination of different materials and features to form complex hybrid microsystems.

Optical tweezers as manufacturing and characterization tool in microfluidics

Pumping and mixing of small volumes of liquid samples are basic processes in microfluidic applications. Among the number of different principles for active transportation of the fluids microrotors have been investigated from the beginning. The main challenge in microrotors, however, has been the driving principle. In this work a new approach for a very simple magnetic driving principle has been realized. More precisely, we take advantage of optical grippers to fabricate various microrotors and introduce an optical force method to characterize the fluid flow generated by rotating the structures through magnetic actuation. The microrotors are built of silica and magnetic microspheres which are initially coated with Streptavidin or Biotin molecules. Holographic optical tweezers (HOT) are used to trap, to position, and to assemble the microspheres with the chemical interaction of the biomolecules leading to a stable binding. Using this technique, complex designs of microrotors can be realized. The magnetic response of the magnetic microspheres enables the rotation and control of the structures through an external magnetic field. The generated fluid flow around the microrotor is measured optically by inserting a probe particle next to the rotor. While the probe particle is trapped by optical forces the flow force leads to a displacement of the particle from the trapping position. This displacement is directly related to the flow velocity and can be measured and calibrated. Variations of the microrotor design and rotating speed lead to characteristic flow fields.

Development and characterization of a microsnap-fit for optical assembly

Snap-fits are classified as interlocking connections and commonly used to assemble two or more components in a fast and cost efficient way. The mechanism is simply based on mechanical flexibility. Therefore, the applications cover a broad field ranging from automotive engineering to mobile phone design. By scaling and transferring the snap-fit mechanism into micrometer scale, advantages can also be utilized to assemble complex microsystems. In this paper, a microsnap-fit based on a cantilever design is developed and investigated by means of optical techniques only. Two-photon polymerization as micro-stereolithography is utilized to manufacture the microcomponents and the mechanical flexibility is analyzed by optical forces in a holographic optical tweezer setup. The locking mechanism is theoretically and experimentally characterized, e.g, the flexibility of the polymer with regard to the design is studied. It can be demonstrated that assembling as well as disassembling of microcomponents is achievable. These findings provide fast and easy assembling of complex microsystems in the fields of microrobotics, -sensors, and -mechanics.

Optical assembly of microsnap-fits fabricated by two-photon polymerization

Abstract. To respond to current demands of nano- and microtechnologies, e.g., miniaturization and integration, different bottom-up strategies have been developed. These strategies are based on picking, placing, and assembly of multiple components to produce microsystems with desired features. This paper covers the fabrication of arbitrary-shaped microcomponents by two-photon polymerization and the trapping, moving, and aligning of these structures by the use of a holographic optical tweezer. The main focus is on the assembly technique based on a cantilever microsnap-fit. More precisely, mechanical properties are characterized by optical forces and a suitable geometry of the snap-fit is designed. As a result of these investigations, a fast and simple assembly technique is developed. Furthermore, disassembly is provided by an optimized design. These findings suggest that the microsnap-fit is suitable for the assembly of miniaturized systems and could broaden the application opportunities of bottom-up strategies.

Laser-based assembler and microfluidic applications

Microfluidic chips enable complete laboratory analysis and diagnostics from smallest volumes of liquids on dimensions not exceeding a matchbox. In the process, these systems are based on integration and miniaturization of pumps, mixers, and devices to control the fluid flow. Light plays an important role for manufacturing and actuation of these microdevices and can be applied for further improving microsystems with regard to new capabilities and higher performance density. In the following paragraphs, an introduction to microfluidics, optofluidics, and assembling techniques is given.

Holographic optical tweezers: microassembling of shape-complementary 2PP building blocks

Based on an ongoing trend in miniaturization and due to the increased complexity in MEMS-technology new methods of assembly need to be developed. Recent developments show that particularly optical forces are suitable to meet the requirements. The unique advantages of optical tweezers (OT) are attractive due to their contactless and precise manipulation forces. Spherical as well as non-spherical shaped pre-forms can already be assembled arbitrarily by using appropriate beam profiles generated by a spatial light modulator (SLM), resulting in a so called holographic optical tweezer (HOT) setup. For the fabrication of shape-complementary pre-forms, a two-photon-polymerization (2PP) process is implemented. The purpose of the process combination of 2PP and HOT is the development of an optical microprocessing platform for assembling arbitrary building blocks. Here, the optimization of the 2PP and HOT processes is described in order to allow the fabrication and 3D assembling of interlocking components. Results include the analysis of the dependence of low and high qualities of 2PP microstructures and their manufacturing accuracy for further HOT assembling processes. Besides, the applied detachable interlocking connections of the 2PP building blocks are visualized by an application example. In the long-term a full optical assembly method without applying any mechanical forces can thus be realized.

Resolution and aspect ratio in two-photon lithography of positive photoresist

In this work, the authors report on investigations of two-photon lithography of positive photoresist. The dependency of the pattern linewidth on variation in the processing parameters, like the laser patterning velocity or power of the femtosecond laser oscillator, is presented. The influence of the scan velocity between 0.38 and 1.90 mm/s on the resolution is discussed for a layer thickness of 3.5 μm. By using a commercial positive photoresist, an aspect ratio of 5 has been realized for grid structures and the qualities of the produced structures are discussed.

Optical micro-assembling of non-spherical particles

Holographic optical tweezers have been developed for the manipulation of polymeric microparticles or biological cells with almost circular shape. As is well known, spherical particles can be trapped and controlled by optical tweezers and assembled with an additional light modulator application. Complementary building blocks, which are used in the following experiments, are generated by a two-photon-polymerization process in micrometer range and are not equipped with spherical trapping points. The possibilities of manufacturing arbitrary building blocks within the 2PP process and the potential of HOTs lead to the idea of combining manufacturing techniques with manipulation processes in a bottomup operation. In this work we present an experimental setup with an integrated fiber laser for holographic optical trapping of non-spherical building blocks. Furthermore experimental requirements which permit trapping will be illustrated.