"Pick and Place" Assembly of Parts Using PDMS - Amy Lim
What is PDMS?
Polydimethylsiloxane (PDMS) is a silicone elastomer, being the simplest kind of silicone polymer at that. PDMS is made up of a monomer and crosslinker curing agent. Often combined at a weight ratio of 10:1, the PDMS mixture undergoes a process of degassing and curing to harden. Changing the ratios of monomer and curing agent offers control over different properties, such as mechanical strength, optical transparency, and permeability. PDMS has incredible chemical and physical properties that allow for ease of microfluidic device creation. Such properties include being chemically inert (unreactive), thermodynamically inert, easy to handle and manipulate, and less expensive than pure silicone. PDMS has elastomeric properties that allow for its use in soft lithography for techniques such as micro-contact printing, replica molding, and micro-transfer molding just to name a few. PDMS is especially useful in the development of biomedical devices, due to its inherent optical transparency, nontoxicity, and biocompatibility.
Microfludic Device Assembly Methods
Traditional Microfluidic Device Assembly
Microfluidic devices are traditionally assembled one at a time through a wide variety of approaches. These approaches can be integrated, modular, or hybrid. The integrated method approaches assembly as a single, integrated process for the whole device. Although integrated methods are initially well-organized, their component parts cannot be individually perfected for use in different, specific applications. Alternatively, modular approaches offer more control over individual component parts of devices. Modular assembly is well known in the processing of microfluidic circuit boards and for Lab-on-Chip technologies. Modular devices may be assembled through the stacking of microscale layers, the embedding of components, and the use of cartridges. Modular assembly leads way for another method of microfluidic device assembly called "Pick and Place."
"Pick and Place" Assembly
Conventional "Pick and Place" (PNP) assembly is often seen in the assembly of electronic parts through robotic assistance. PNP assembly is exactly how it sounds: premade components are placed in predetermined ways, oftentimes through automation, to increase manufacturing efficiency. PNP machines are often employed to assemble many devices at once. The PNP assembly method is very similar to conveyer belt manufacturing at a large factory. Many PNP machines have vacuum-assisted parts that are used to "pick" individual components and "place" them in their correct space and orientation. PNP assembly is much more efficient compared to simple modular assembly, as it automates an existing assembly process into a much faster and more efficient one. PDMS can also be used in PNP assembly for especially fragile component parts, as PNP assembly with PDMS allows for the control of the interfacial adhesive properties of the PDMS stamp and part.
"Pick and Place" Microfluidic Device Assembly
PDMS-based Pick and Place device assembly is greatly inspired by the controllable attachment and detachment capabilities of the fibrillar structures on gecko feet. Geckos have the ability to selectively adhere to both smooth and rough surfaces by changing the contact area between their feet and surfaces. Deforming the fibrillar structures changes the contact area between structures and the substrate. The stability of attachment is due to van der Waals interactions of the surface but is not dependent on surface chemistry. The incredible effectiveness of gecko foot adhesion has been used as a model for PDMS-based PNP assembly.
The "Pick and Place" Process
With the help of PDMS-based transfer printing and stamp-making, PNP assembly with PDMS has been achieved before with several hundred microscale PDMS pillars of 25-35 µm in diameter and 90 µm in height. Due to its ability to carefully "pick" up tiny component parts in substrate and "place" them in specific locations without much prior handling, PDMS-based assembly is a great alternative to vacuum-based part assembly. Experimentally, the several hundred PDMS PNP pillars were able to "pick and place" silicon platelets with the dimensions of 100 x 100 x 3 µm3. The mechanism of "pick and place" assembly can be thought of as a very selective magnet. The intermolecular force interactions between the tip of the PDMS pillar and the surface of the part play a large role in their adhesion and release, with adhesion and contact area having a directly proportional relationship. As a result, to "pick" parts, vertical compression is applied to the tip of the pillar to maximize the contact area between the tip and part, which allows for the secure hold of the part that withstanding sudden impacts and disturbances. On the other hand, to release the part, the contact area of the pillar is greatly reduced via deformation, reducing the overall adhesion of the pillar to the part. With PDMS, different ratios of monomer and crosslinker agents are used to adhere to different components with different surface properties.
Applications of "Pick and Place" Assembly
As described before, PDMS-based "Pick and Place" assembly has a wide variety of applications, including those of biological simulation and nanoscale materials.
Methods for "Pick and Place" assembly and transfer of PDMS membranes Organ-on-Chip have been developed for reliability and ease. Although the aforementioned method of Organ-on-Chip manufacturing is not inherently "Pick and Place," the transfer process designed for Organ-on-Chip is easily reproducible for "Pick and Place" automation because surfaces need only be brought into close contact. Instead of the PDMS tip adhering to the device parts, the surfaces of each Organ-on-Chip part adhere to the PDMS tip. Organ-on-Chip devices simulate the microstructures of various organs in replicated in vivo conditions, which can be used to better understand bodily processes and replace animals in drug development. Such devices are made of PDMS and are fabricated through 3D printing and soft lithography. A well-established method of fabrication is described as follows: PDMS is used for its permeability and strong structural characteristics in Organ-on-Chip devices. Membranes as small as a few microns must be transferred reliably and without damage. To transfer Organ-on-Chip devices, porous PDMS membrane layers were developed on the device itself and the surface of the carrier wafer. The PDMS portions are treated with polyacrylic acid and oxygen plasma to bond at constant pressure. The bonding region properties of the two component parts can be changed accordingly for each device made. Lastly, water allows the Organ-on-Chip devices to be removed from the carriers with ease, dissolving the polyacrylic acid layer that served as the "glue." The use of "Pick and Place" assembly is sure to greatly improve the efficiency of future Organ-on-Chip manufacturing and experimentation.
As the name suggests, Nanowires are tiny wires that exist on the nanoscale. Nanowires service a variety of applications, including their use as microfluidic channels. Due to their incredible electrical and optical properties, nanowires have a wide range of applications in electrodes, nanophotonic devices, and Transmission Electron Microscopy (TEM). Although nanowires have promising uses in the aforementioned applications, they have not yet been reliably integrated for microfluidic device assembly. Currently, a stamp transfer technique, as described in other sections, has the best potential for nanowire transfer and placement mechanisms. A lot of the time, nanowire "pick" up is not very precise, as the stamping technique adheres to more nanowires than desired. However, techniques of more precise pick-up are now being developed through intricate understandings of nanomechanical vibrations and field coupling. Complete automation of microfluidic device manufacturing will soon be possible with the further development of universal "Pick and Place" assembly for nanowires.
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