Researchers from Stanford University have developed a new high-resolution resin 3D printing process. This novel approach removes the risk of over-curing resin in negative spaces, such as channels or voids, making it ideal for 3D printing microfluidic devices.
Joseph M. DeSimone, Co-founder and former CEO of California-based 3D printer manufacturer Carbon, co-authored the paper.
Now a board member at the company, DeSimone played a key role in developing Carbon’s patented continuous liquid interface production (CLIP) technology. The Stanford team leveraged a modified version of CLIP, called injection CLIP (iCLIP), in their research.
Alongside project leads Ian A. Coates and Gabriel Lipkowitz, DeSimone is listed as an inventor on a patent pending application for methods involving negative space preservation using iCLIP. The CLIP and iCLIP patents and patent applications are being licensed to a new vaccine and drug delivery company called PinPrint, co-founded by DeSimone.
The Stanford team’s iCLIP approach continuously feeds a stream of fresh polymerizable resin through the negative space during 3D printing. This displaces resin at risk of over-curing, allowing channels to be produced with significantly smaller heights and diameters.
Posting on X, Andrew Sink, Staff Applications Engineer at Carbon, called this injection-based resin 3D printing process “the new leap forward in additive.” According to Sink, “it’s going to enable incredible things in the photopolymer space.”
According to the researchers, iCLIP unlocks improved design and material freedom for high-resolution microsystem devices such as vascular beds and microfluidic-backed microneedles.
The study, titled “High-resolution stereolithography: Negative spaces enabled by control of fluid mechanics,” was published in the Proceedings of the National Academy of Sciences (PNAS) journal.
How to prevent over-curing in resin 3D printing
Negative spaces are critical for microfluidic devices, biomedical devices, vascular networks, separation media, and electronic circuits. They facilitate precise control of fluid flow, improved sensor accuracy and enhanced separation efficiency.
Additive manufacturing methods have been increasingly adopted to produce these microsystems. Stereolithography 3D printing, including digital light processing (DLP), has become particularly popular in this field. DLP 3D printers use two-dimensional projections of UV light to cure layers of photopolymerizable resin layer-by-layer.
Although related to DLP, CLIP 3D printing instead relies on resin renewal at the build surface. The process employs an oxygen-permeable window, which creates a polymerization-free area at the bottom of the resin vat. This ‘dead zone’ prevents liquid resin from curing and sticking to the projection window, enabling shorter 3D print times and the creation of more fragile green parts.
In stereolithography, high-resolution optics are used to precisely direct the UV light and accurately cure each layer of resin on the XY plane. However, it is more challenging to achieve high resolution on the Z-axis (vertically).
Here, it can be difficult to keep the light confined to a single layer, with UV leaking into the preceding 3D printed layers. This leads to lower part resolution, with resin being over-cured in previously created negative spaces.
Current efforts to overcome this include incorporating UV light-attenuating additives into the resins which control layer thickness to improve 3D printing accuracy. However, these additives require stronger light to harden the resin, slowing the 3D printing process. They also often possess toxic properties, making them unsuitable for medical or life science applications.
Therefore, the researchers turned to iCLIP 3D printing. The team continuously pumped naturally oxygenated (inhibited) resin through the build platform, flushing out any residual resin that could become over-cured in the 3D printed channels. This method allowed the team to successfully 3D print high-resolution negative spaces with various materials.
iCLIP 3D prints high-resolution, microfluidic channels
To test their hypothesis, the Stanford team initially 3D printed 200 μm-diameter microchannels at angles ranging from 0° to 90°.
Using conventional stereolithography 3D printing, the 90° channel would be highly susceptible to over-curing. When the channels were fabricated using iCLIP, optical micrograph images indicated that all angles were 3D printed to a high resolution.
Next, the team 3D printed a microfluidic network at a 30° angle, with the channel diameter varying between 50 μm and 200 μm. Both imaging and electron microscopy confirmed an accurate resolution throughout the negative spaces when the iCLIP process was used.
The researchers also investigated how the injection rate of fresh resin impacted channel resolution during iCLIP 3D printing. They created a “turnover number” (Tu) to measure the ratio between how fast fresh resin was injected and how fast the negative space (or channel) was printed.
When no resin was injected, the 3D printed channels were over-cured and improperly formed. As the Tu increased, and more resin was injected, the channels more closely matched the intended design. However, increasing the flow rates too much could cause the channels to widen or crack.
The relationship between Tu and the resin’s penetration depth (Dp), the distance UV light can travel into the resin before becoming ineffective, was also assessed. The team found that as the Dp increases, so does the Tu required to achieve accurate channel resolution. This ensures that fresh resin replaces the old resin before it receives too much UV exposure, maintaining proper layer formation during 3D printing.
Looking to the future, the researchers believe iCLIP 3D printing offers significant value for personalized medical devices and microelectromechanical applications.
To demonstrate this, they 3D printed a range of iCLIP-enabled microsystems, including a microneedle patch, vascular networks for blood transport systems, conductive gallium elements, and a porous perfusion network.
Given the efforts made by DeSimone to patent this technology for his new biomedical firm, it may not be long before devices like this hit the commercial market.
3D printing microfluidic devices
Additive manufacturing is being increasingly leveraged for microfluidic applications. Last year, researchers from Queensland University of Technology evaluated resin 3D printing for the production of microfluidic components for cell-based applications.
MOIIN High Temp and MOIIN Tech Clear resins from DMG Digital Enterprises were used in conjunction with ASIGA UV Max X27 DLP 3D printers to fabricate common microfluidic designs. These included 2D monolayer culture devices, pillar arrays, and constricting channels for droplet generators.
The study concluded that MOIIN High Temp and MOIIN Tech Clear resins are effective at 3D printing microfluidic channels for cell-based applications. Both materials were confirmed to be biocompatible, and visible through imaging platforms such as microscopes.
Elsewhere, Massachusetts Institute of Technology (MIT) researchers recently developed 3D printed microfluidic devices that are self-heating. Requiring approximately $2 worth of materials, the devices can be manufactured as low-cost disease detection tools.
The MIT team utilized multi-material extrusion 3D printing, incorporating a biodegradable polymer (polylactic acid or PLA) and a modified version infused with copper nanoparticles. When transformed into a resistor, this modified PLA becomes conductive. This allows electrical currents to be dissipated as heat, resulting in a self-heating microfluidic device that can be 3D printed in a single step.
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Featured image shows a schematic of the iCLIP process and the resulting resolved negative structures. Image via PNAS.
