New 3D printer creates detailed structures from multiple materials

(Nanowerk News) Researchers have developed a new method for 3D printing precision microstructures with multiple materials. The new approach is poised to enable fabrication of structures with complex functions for consumer products and medical devices.
“Our 3D printing method could be useful for making a variety of devices, such as microcircuits, photonic devices and microfluidics,” said Wen Qiao, a member of the research team from Soochow University in China. “For example, the ability to 3D print with multiple materials that have different properties could be used to make micro-robots with sophisticated, integrated functions.”
In The Optical Society (OSA) journal Optics Express ("Roll-to-plate additive manufacturing"), the researchers demonstrate their new technique, which is called roll-to-plate projection micro stereolithography (PµSL). They show that it can be used to create a wide range of detailed structures and objects such as a miniature ear prosthesis, a tiny replica of China’s Great Wall and a multi-material butterfly with bendable wings.

Making 3D printing more flexible

PµSL, which is also known as digital light processing, is an established additive manufacturing technique that typically involves placing a liquid photosensitive resin in a tank with a transparent bottom. A digital projector flashes a light pattern that hardens — or polymerizes — all the UV-exposed points in a resin layer simultaneously. This is repeated layer by layer until the entire object is formed in the resin.
Although PµSL is a powerful tool for micropatterning, it cannot be used to make complicated microstructures with multiple materials, nor can it effectively print with resins that are highly viscous because they take too long to settle after polymerization. Using multiple resins with different viscosities is desirable for creating structures comprised of different materials with discrete parts that have different degrees of flexibility.
To overcome these limitations, the researchers designed a printer that uses a flexible membrane to hold the photosensitive resin while it is transferred and photopolymerized. “Using a membrane for transport greatly expands the materials that can be used and allows rapid printing with high viscosity materials,” said Qiao. “We also added multiple nozzles to supply different materials and allow high-resolution printing.”
The new printer uses a nozzle to dispense a few drops of photopolymer onto a flexible membrane. As the membrane moves across a guide roller, a blade spreads the photopolymer evenly. Adjusting the distance between the blade and the membrane allows the thickness of the photopolymer layer to be varied from 1 to 200 microns. The guide roller keeps the photopolymer-coated membrane slightly stretched and transfers the membrane to the area where polymerization occurs.
After polymerization, the printed structured is peeled off the flexible membrane. The surface energy of the membrane is carefully engineered so that the solidified resin adheres to the sample while the uncured photopolymer residue remains on the flexible membrane and can be recycled. This process is repeated to build a structure layer by layer.

Creating tiny, complex objects

With roll-to-plate PµSL, each layer can be made from a different material with unique physical or chemical properties. Using the flexible membranes to transfer various curable polymers eliminates the need for material exchange or chemical cleaning procedures when changing materials.
The researchers used the new printer to print a variety of complex structures. For example, they made a miniature version of the Great Wall of China that measured 0.86 × 2.34 × 1.11 cubic millimeters using layers that were 10 microns thick. They also created hollow cylinders
470 microns tall with a wall thickness of 10 microns, with no residue remaining inside. A butterfly with wings that can be bent up to 45 degrees was fabricated by using materials with different viscosities to create a flexible joint and a rigid region.
Because the thickness of each layer can be tuned over a wide range, the fabrication rate can be increased by using thicker layers for parts of a structure that are less detailed and thinner layers for areas that require higher-resolution printing. For example, the hollow channels of a microfluidic devices could be printed with high precision while inlets and outlets were printed at high throughput.
Now that the researchers have shown the feasibility of roll-to-plate PµSL, they plan to take it even further by integrating a light-based process called grayscale photolithography to eliminate the need to print in layers.
Source: The Optical Society
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