Assembly of multicomponent 3D microstructures using optical tweezers

The short review article is based on the original article entitled “Assembly of multicomponent structures from hundreds of micron-scale building blocks using optical tweezers” by Jeffrey E. Melzer and Euan McLeod published in Microsystems & Nanoengineering Journal (June 2021).

The authors employed a pressure-driven flow controller in combination with the novel microassembly fabrication technique using optical tweezers to perform their study and test the effect of various parameters on the microfabricated assembled structure.

Abstract

The production of three-dimensional (3D) microscale structures has found a growing interest over the past decades for many applications such as biomedical, robotics, and photonic applications. Yet, complex multicomponent integration with micronsized features remains challenging. Therefore, this work proposes an optical positioning and linking (OPAL) platform based on optical tweezers to accurately generate 3D microstructures from two kinds of micron-scale building blocks linked by biochemical interactions. 

A computer-controlled interface with rapid on-the-fly automated recalibration routines maintains accuracy even after placing many building blocks. The proposed technique achieves a 60-nm positional accuracy by optimizing the molecular functionalization and laser power. A two-component structure consisting of 448 1-µm building blocks is built. Up to now, the latter represents the largest number of building blocks used in 3D optical tweezer microassembly.

Introduction to optical tweezers microassembly

Recurrent challenges in photonics, electronics and fluidics reside in miniaturization of 3D structures with multiple material components with resolution down to the micron. To overcome those challenges, various techniques were developed over the years comprising grayscale lithography, self-assembly, direct ink printing and direct laser writing methods such as two-photon polymerization. [1 – 9]

Those techniques, despite being innovative and partially solving some of the problems encountered, present some drawbacks such as limitations regarding the complexity of 3D structures, the resolution of such geometries, the generation of multicomponent/multimaterial structures, or even the stability of the final structure due to potential shrinkage. 

Thus, there is a clear need to find new microfabrication structures that allow the operator to pattern in 3D using multiple components in a single platform, various simple and complex structures. 

Optical tweezers have received an increasing interest over the years for the assembly of small multicomponent structures. Automation became facilitated by the development of multiplexed optical trapping (OT) in combination with image processing and path-planning algorithms. [10 – 12] Yet, no large-scale multicomponent, 3D structure assembly has been reported using this technique.  

Therefore, this work aims to employ optical positioning and linking platform (OPAL), based on optical trapping with a biochemical binding mechanism to connect objects, to push further the microassembly capabilities by studying specifically the object placement precision, yield and structure resolution.

Aim & objectives

The primary objectives of this work are the following: 

• Investigate the use of  OPAL platform for microassembly.
• Study the impact of laser power and biochemical functionalization on position accuracy.
• Create proof of concept 3D multicomponent structures fabricated by the present technique

Materials & methods

Sample chamber preparation, building blocks, and microfluidics

The sample chamber employed for structure assembly consists of three layers (Figure 1). For more information about the various layers composing the sample chamber and the solutions employed in this study, please refer to the original paper available here. 

Two different microfluidic methods were employed in this work. For the assembly of the large sodium chloride lattice, ~1 µL of each bead dilution is flowed into its respective inlet port and only a couple millimeters into the chip. This accurate loading method ensures that the components do not preemptively mix and bind in the central chamber. 

The building blocks are introduced into the chamber during assembly using a microfluidic pressure pump (Elveflow, OB1 MK3+) while the chamber is in the OPAL setup. By keeping the chip unsealed, a nearly limitless supply of building blocks is available.

Figure 1: Schematic representation of the pressure-driven flow controlled microfluidic design (a) and chamber design (c,d). An example of a building block in one of the sample loading channels during assembly is given in (b). Courtesy of the authors. CC. by 4.0.

Figure 2: Pictures of the pressure-driven flow controlled microfluidic design and chamber design. Courtesy of the authors. CC. by 4.0.

Key findings

The effects of the laser power and biochemical functionalization was analysed on the positional accuracy. The authors established critical experimental parameters for optimal performance, and achieved a positional accuracy of ∼60 nm. Find below a video to illustrate the resulting multicomponent microassembly. 

A 3D pyramid with multiple building block sizes was also assembled, as depicted in Figure 4, to illustrate the platform flexibility to fabricate  heterogeneous structures. Both building blocks were made of polystyrene, with the larger (2 µm) spheres being fluorescent with a biotin coating and the smaller (1 µm) spheres with an additional streptavidin coating to allow object binding.

Figure 3 Experimental results obtained for large-scale microassembly using optical tweezers. Courtesy of the authors. CC. by 4.0.

Based on those results, 3D multiple component structures were considered as proof of concept for 3D material variation, culminating in the assembly of the largest 3D, multimaterial microstructure fabricated by any OT-based assembly platform to date, with 448 1-µm building blocks. 

This structure consists of several hundred 1 µm diameter spheres, which are smaller building blocks than those that have been used in most previous Optical Trapping microassembly studies.

Figure 4: Experimental results obtained for microassembly of multisized building blocks using optical tweezers. Courtesy of the authors. CC. by 4.0.

Conclusions

The present work showed that an Optical Trapping Assembly platform is effective for large 3D, multicomponent structures fabrication made of hundred building blocks. 

The effects of the silane concentration and laser trapping power was studied with the help of Elveflow’s pressure-driven flow controller, on the assembly positional accuracy, establishing minimum desired values of approximately 1 mg/mL and 5 mW, respectively, for an ~60 nm accuracy. Such results obtained with the help of pressure-driven flow controlled microfluidics allowed the authors to obtain high precision calibration and solution loading through the sample chamber. 

Next challenges remain and reside in the fabrication of even larger structures at faster rates with less error by multiplexed optical trapping and full system automation. Another interesting aspect to further investigate in the near future is the potential multiplexing of such systems to provide a further efficiency boost.

References