PDMS Microfluidics Molding Techniques

This is a really interesting example of a microfluidic circuit molded in PDMS.

PDMS microfluidic molding basics

Here’s a video that demonstrates the concept of molding a microfluidic circuit.

First let’s look at a top-level overview of what molding a microfluidic circuit means. Later in this article you will learn a couple of detailed ways of applying this broad principle to making microfluidic circuits for yourself.

After this you can connect your tubes to the fluid channels and start using the circuit.

Advantages of PDMS molded microfluidic circuits

1. Repeatable process – once you dial in your prototyping process you can continue to make circuits without too many modifications to the process
2. You can embed electronics such as heating elements into your circuit
3. With the right mold design you can create 3D channel geometry
4. You can design active components like valves right into the circuit (example)

Disadvantages of PDMS molded microfluidics

1. The main disadvantage is that molding PDMS circuits is a very manual process and will require you to spend time making each individual circuit
2. You will also want access to equipment (such as 3D printers and ovens) if you’d like to make these quicker and with a fine channel resolution

What is PDMS?

PDMS is a 2 part resin. You mix the base with the curing agent, pour it into your mold, and allow it to cure and take on the designed shape.

Application note for PDMS: A 10:1 base:curing agent ratio by weight is a good starting point for PDMS (Sylgard 184) – add less curing agent for a softer circuit and more for a harder circuit.

Why PDMS?

• Very flexible – which makes it easy to seal and more durable
• Optically transparent – also allows you to add accessories like cameras onto the circuit to help you analyze the flow
• Reasonably biocompatible
• Takes on fine details very well
• Can be bonded with glass
• PDMS flexibility can be used to integrate active components such as valves right into the circuit (example)
• Tuneable gas permeablity – the gas permeability of PDMS can be tuned by varying the thickness and base:curing agent ratio during mixing
• Well studied for microfluidic applications – this means you have access to a variety of research papers that will show you techniques that allow you to modify the surface properties and bulk properties of PDMS

How to make PDMS microfluidics

1. 3D Printed Mold to Cast a PDMS Microfluidic Circuit
2. Puffy Paint Mold to Cast a PDMS Circuit
3. Duplicating an Existing Microfluidic Circuit By Casting PDMS
4. Cutting ABS Sheets to Mold PDMS Circuits
5. Dissolving 3D Printed ABS to Cast a PDMS Circuit

3D Printed Mold to Cast a PDMS Microfluidic Circuit

Overview

This is one of the fundamental techniques of prototyping microfluidics. Here you use a 3D printed mold and cast PDMS into it to create the microfluidic channels. This is then bonded to a glass slide and sealed to form the complete microfluidic circuit. This is an accurate and repeatable technique but the drawback is it is not easily scaleable as you need to put in the time to make every single circuit. It is a great technique for lab-scale prototyping and if you have access to all the equipment, you can design and prototype your circuits within the same day.

Metal Micromilling to Form a PMMA Master

Overview

In micromachining, a CNC machine is used to remove material from a substrate. The substrate for a metal master can be made of brass, aluminum, or copper. The micromachines are capable of achieving positional and repetition accuracy of ±1 µm with tooling bits ranging from 50 to 500 µm.

Technique

Steps:

1. The substrate is CNC-ed to create a negative metal master.
2. The metal master and the PMMA disk are placed in a hydraulic press with an integrated vacuum chamber.
3. The metal master is pressed on to the PMMA disk for 5 minutes at 155°C.
4. The PMMA mold is then cleaned and sprayed with mold release.
5. PDMS is cast into this mold to make microfluidic chips.

Puffy Paint Mold to Cast a PDMS Circuit

Overview

This is an ingenious concept from Wisconsin Materials Research Science and Engineering Center.

A mold is created by using puffy paint to draw the circuit onto a petri dish. This is then cast with PDMS to create microfluidic channels. This PDMS component is then sandwiched between two sheets of acrylic to complete the circuit.

Duplicating an Existing Microfluidic Circuit By Casting PDMS

(aka moldception)

Overview

The main aim of this technique is to duplicate a microfluidic circuit that you already have. It’s an extremely capable technique and can replicate features in the 5µm range. This is how it’s done:

You use an existing microfluidic circuit to make a mold. You then cast PDMS into that mold and create a PDMS copy of that circuit.

Now let’s talk about how to make a mold using an existing circuit.

First, place the circuit on a flat surface in a container (called the template mold) and cast resin into it. Once the resin cures, it can be used as a duplication mold to reproduce the circuit you started out with. Just cast PDMS into the duplication mold to make a copy of your original circuit.

As an aside: the way to make a template mold is also by casting. You can use scrap material lying around to make a mold for your template mold. The dimensions aren’t very critical. Alternatively you can 3D print a mold using this file we’ve prepared and cast resin into it to make your template mold. You will really only need 1 template mold but it’s always a good idea to make backups in case your first one gets damaged or if you want to duplicate multiple circuits in parallel.

Materials

The materials used to make the molds are important because you want a duplication mold that’s made of epoxy so that you can cast PDMS into it. Here’s a material reference chart to make it easier.

There are many modifications that could be made to this technique so feel free to contact us if you need help modifying this to suit your lab.

Note: After you use this technique to make your circuit you need to bond it to a glass slide. You can learn how to do that using a corona oxidizer and a hotplate by reading through this technique. It links to a video by Hashimoto Lab at the Singapore University of Technology and Design and the bonding demonstration starts at 9 mins 15s.

Cutting ABS Sheets to Mold PDMS Circuits

Overview

Cut the microfluidic channel geometry out of a sheet of ABS 0.20mm to 0.50mm thick.

Place the channel geometry in a petri dish and cast PDMS into the petri dish.

Place the circuit from the previous step in an acetone bath overnight to dissolve the ABS.

Dissolving 3D Printed ABS to Cast a PDMS Circuit

aka ESCARGOT (Embedded SCAffold RemovinG Open Technology)

Overview

Credit to Dr. Vittorio Saggiomo and Dr. Aldrik Velders for coming up with this technique.

In this technique, you will first 3D print the required channel geometry in ABS. You then cast PDMS into a petri dish and suspend the 3D printed part in the uncured resin.

Once the resin cures, you can submerge it in acetone overnight to dissolve the ABS, leaving you with the final microfluidic circuit.

This technique is great for microfluidic circuits with 3D channel geometry or circuits with embedded electronics like heating elements to heat up the fluid in the channels.