Drawing and Hydrophobicity-patterning Long Polydimethylsiloxane Silicone Filaments
January 7th, 2019
Here, we present a protocol to produce long filaments of polydimethylsiloxane (PDMS) silicone by gravity-drawing through a furnace. Filaments are on the order of hundreds of micrometers in diameter and tens of centimeters in length and are hydrophobically patternable via an Arduino-controlled corona discharge system.
Polydimethylsiloxane is a versatile material that could not be formed into long filaments until now. While silicone filaments may have applications in weaving or additive manufacturing, we are particularly interested in them as a model foldamer. Scalable production is enabled by drawing already curing silicone through a tube furnace.
We also demonstrate a way to modify the surface of the silicone filaments using a computer controlled corona discharger. Timing in this procedure is crucial because the polymer is curing continuously. Keep a timer nearby, and be prepared for the next step in advance.
Without A Visual demonstration, the dynamic process of making filaments would not be easily reproduced. The process involves several time sensitive steps that require you to adapt to evolving conditions. To begin, connect 1/8th inch inner diameter high temperature silicone tubing to compressed air through a metering valve.
Connect the other end of the tubing to a brass extrusion adaptor with a 2.15 millimeter diameter aperture. Then, fix the cylindrical ceramic tube furnace vertically in a fume hood, two feet above the floor of the hood. Lay aluminum foil under the furnace to catch excess PDMS during extrusion.
Mount the extrusion adaptor above the tube furnace with the aperture centered on the furnace opening. Ensure that the furnace is not at an angle so that extruded filaments will not contact the furnace. Next, attach a die with a circular cut to an extruder and use a zip tie to connect high temperature silicone rubber tubing to the extruder bit.
Connect the other end of the tubing to the extrusion adaptor. Direct an infrared thermometer towards the furnace and then heat the furnace until the inner temperature is approximately 250 degrees Celsius. Then, move the furnace out from under the extrusion adapter so that the adaptor does not heat up before filament production begins.
Next, start preheating a disposable sample tube to 65 degrees celsius in a viscometer that can measure 200 to 10 thousand millipascal seconds. Check the balance, set the spinning rate to five RPM, and set the measurement rate to once per minute. While the sample tube heats, mix 18 grams of PDMS base with 1.8 grams of curing agent, in a weighing boat.
Degas the PDMS mixture in a room temperature vacuum desiccator for 15 minutes, or until no bubbles remain in the mixture. Periodically vent the desiccator to pop bubbles near the surface. Then remove the preheated sample tube from the viscometer.
Pour the degassed PDMS mixture into it, and put the tube back in the viscometer. Immediately start the measurement sequence. When the PDMS dynamic viscosity reaches four thousand millipascal seconds, note the time, and then use pliers to remove the sample tube from the viscometer.
Immediately pour the PDMS mixture into the room temperature extruder. Confirm that the furnace is at 250 degrees celsius and wait until the PDMS has been out of the viscometer for about four minutes. Then, move the furnace under the extrusion adaptor and align the inner needle of the adaptor with the tube furnace.
Twist the extruder screw by a half revolution and start collecting the filament on a wooden stick. Every three to five seconds, twist the extruder another half revolution to maintain the steady stream of PDMS. The viscosity window for drawing filaments is very narrow.
If the viscosity is too low, wait 30 seconds and try again. The PDMS will continue crosslinking at room temperature. Lay the drawn filaments across wooden racks to cure.
Let the filaments cure at room temperature for 12 hours when finished. To begin the patterning process, set up the patterning assembly in a fume hood, and send the desired pattern to the micro processor. Next, wash a cured PDMS filament with 1%sodium dodecyl sulfate.
And thoroughly rinse it with ultra pure water. Remove visible drops of water with compressed air and let the filament finish drying in the ambient air. Then, tie non conductive fishing line to the circular cut out of the non conductive filament tray.
Lay the dry PDMS filament over the central cut out of the tray and secure it in place with double sided tape. Insert the tray into a ventilated corona discharge box and ensure that it is level. Place a metal slab under the filament tray so that the filament is aligned along the edge of the slab.
Mount the corona discharge electrode about three millimeters above the filament, and connect the corona discharger to the micro controller. Fix the free end of the non conductive line to the spindle mounted on the stepper motor, ensuring that the line is taut. Run the micro controller program to pattern the filament with hydrophilic sections.
The PDMS filaments produced by this method were approximately 200 micrometers in diameter and up to 0.5 meters long. Removing the PDMS from heat before it's viscosity was high enough for drawing filaments, and letting it cross link at room temperature for four and a half minutes, extended the time window in which filaments can be spun to about four minutes. The success of corona patterning was confirmed via droplet contact angle measurements.
Water formed symmetric, barrel shaped droplets on the corona treated hydrophilic PDMS, and asymmetric, shell shaped droplets on the untreated hydrophobic PDMS. The key to this procedure is that you're manipulating the viscosity of PDMS with time and temperature. The viscosity increases with time as it crosslinks and curing accelerates at higher temperatures.
The approach of partially curing a polymer and drawing it through a tube oven, may be adapted to other temperature cured polymers. These filaments are flexible enough to adopt to many conformations, and the hydrophobic regions affectively adhere together in water. We are studying how the hydrophobicity pattern of the filaments affects their folding pathways.
PDMS can drip onto the furnace and produce a small flame. Carefully align the extruder with the furnace and quickly clear any debris from the furnace with a wooden dowel.
Overview
This article presents a protocol for producing long filaments of polydimethylsiloxane (PDMS) silicone using a gravity-drawing technique through a furnace. The filaments, measuring hundreds of micrometers in diameter and up to half a meter in length, can be hydrophobically patterned using an Arduino-controlled corona discharge system.
Key Study Components
Area of Science
- Materials Science
- Polymer Engineering
- Surface Modification
Background
- Polydimethylsiloxane (PDMS) is a versatile silicone material.
- Long filament production has been challenging until now.
- Filaments have potential applications in weaving and additive manufacturing.
- Surface modification techniques can enhance functionality.
Purpose of Study
- To develop a scalable method for producing PDMS filaments.
- To explore the surface patterning of PDMS for specific applications.
- To investigate the effects of hydrophobicity on filament folding pathways.
Methods Used
- Connecting silicone tubing to an extruder and furnace setup.
- Mixing PDMS base with a curing agent and degassing the mixture.
- Controlling temperature and viscosity during filament drawing.
- Using corona discharge for surface patterning of filaments.
Main Results
- Filaments produced were approximately 200 micrometers in diameter and up to 0.5 meters long.
- Successful surface patterning was confirmed through droplet contact angle measurements.
- Viscosity manipulation was key to filament production.
- The method may be adapted for other temperature-cured polymers.
Conclusions
- The developed protocol allows for the production of long PDMS filaments.
- Hydrophobicity patterns significantly influence folding pathways.
- This approach opens avenues for further research in polymer applications.
Frequently Asked Questions
Executive Industry Relevance
This method enables scalable production of long, patternable PDMS filaments, addressing a key materials limitation in biomimetic and microengineered systems. By providing a route to tunable hydrophobicity along filament backbones, it supports de-risking of mechanistic studies in peptide-like folding and self-assembly. The approach offers a reproducible platform for generating silicone-based components with controlled surface properties for early-stage target validation and assay development.
Strategic Applications in Biopharma R&D
Early Discovery & Target Validation
- Scientific Value: Enables interrogation of amphiphilic sequence effects on conformational dynamics in model foldamer systems.
- Operational Value: Provides standardized, long filaments with patterned surface properties for reproducible binding or assembly assays.
Screening & Assay Development
- Scientific Value: Generates consistent, gas-permeable silicone filaments suitable for microfluidic sensor interfaces or cellular microenvironment studies.
- Operational Value: Supports fabrication of reusable, hydrophilic/hydrophobic patterned elements for multiwell plate integration or organ-on-chip platforms.
Translational & Preclinical Research
- Scientific Value: Facilitates structure-function analysis of hydrophobicity patterns in guiding peptide-mimetic folding pathways relevant to target engagement.
- Operational Value: Enables production of long, flexible filaments for preclinical prototyping of implantable or wearable sensor components requiring gas permeability.
Pipeline & Workflow Integration
The method fits within early discovery workflows where controlled surface chemistry and mechanical flexibility are needed to model biomolecular interactions or screen environmental responses.
- Discovery Biology: Supports hypothesis testing on how surface patterning influences self-assembly or binding kinetics in aqueous environments.
- Screening: Delivers assay-ready filaments with quantitative hydrophobicity contrast for screening compound effects on filament stability or aggregation.
- Analytics: Enables contact angle measurements as a quantitative readout for surface modification success and batch consistency.
- Translational Research: Connects filament folding behavior to preclinical models of peptide aggregation or membrane interaction.
- Enterprise Reuse: Establishes a reusable platform for generating silicone filaments across multiple projects requiring tunable surface properties.
Operational & Enterprise Impact
- Scientific Value: Improves predictive confidence in modeling amphiphilic-driven folding and target-like interactions.
- Operational Value: Ensures reproducibility through viscosity-controlled drawing and automated corona patterning.
- Strategic Value: Reduces biological de-risking timelines by enabling rapid iteration on hydrophobicity patterns in foldamer analogs.
- Portfolio Impact: Supports go/no-go decisions in early-stage projects exploring silicone-based biomimetic materials or sensor interfaces.
Implementation Considerations
- Requires expertise in polymer rheology and viscosity monitoring during crosslinking.
- Needs access to a tube furnace, viscometer, extruder setup, and corona discharge system with microcontroller control.
- Demands standardization of timing protocols due to continuous curing and narrow viscosity windows.
- Involves adaptation considerations when transferring the method to other temperature-cured polymers with different crosslinking kinetics.
- Includes practical limitations such as filament diameter constraints and sensitivity to environmental moisture during patterning.
