Micro 3D Printing Using a Digital Projector and its Application in the Study of Soft Materials Mechanics

We demonstrate controlled pattern transformation of swelling gel tubes by elastic instability. A simple projection micro stereo-lithography setup is built using an off-the-shelf digital data projector to fabricate three-dimensional polymeric structures in a layer-by-layer fashion. Swelling hydrogel tubes under mechanical constraint display various circumferential buckling modes depending on dimension.

The overall goal of the following video is to demonstrate construction of a simple 3D gel microfabrication tool and its use in pattern transformation of swelling gel tubes by elastic instability. A simple micro 3D printer is built using an off-the-shelf digital data projector in order to fabricate tubular gel samples with different dimensions. Fabrication of the tubular gel samples is achieved by projecting a designed image onto the sample holder, which is immersed in a resin bath, containing a pre polymer solution with photo initiator and photo absorber.

Once a layer is formed by photo polymerization, the sample holder drops and the next layer is fabricated on top of the previous one. In this way, a 3D sample is fabricated in a layer by layer fashion. Next, each sample is brought into contact with water in order to trigger shape transformation by swelling induced elastic instability.

Results show that circular tubes transform into various wavy patterns with different wave numbers depending on the geometry of buckling gel. The main advantage of this fabrication technique over existing methods like phototherapy is that it offers a rapid 3D microfabrication tool for soft materials such as gels. As a result, various interesting three dig geometries that are difficult to make can now be easily realized into physical objects for experimental study.

To begin this procedure, prepare the pre polymer solution containing photo initiator and photo absorber as described in the written protocol. Following solution preparation, place a digital data projector in a flat and stable position and connect it to a computer with Microsoft PowerPoint installed. Place a convex lens right in front of the beam output lens of the digital projector.

Choose a convex lens to make the focal plane about 10 centimeters away from the projector. Optical resolution becomes smaller for a lens with shorter focal length, but one needs to reserve some space for optical components. Place a mirror after the convex lens on the beam path at a 45 degree angle to direct the beam straight down.

Then place a sample holder at the focal plane of the projected beam. The sample holder should be attached to a linear stage by which the vertical position of the sample holder is controlled. Finally, place a resin bath underneath the sample holder to design the gel tubes Project, an image with known pixel numbers onto the sample holder to measure the conversion ratio from a pixel to physical length.

In this particular case, an image of 135 pixels measured 5.8 millimeters, which corresponds to 43 microns per pixel. Based on this information, convert physical dimensions of the gel tube to fabricate diameter wall thickness and height into pixels. Next, draw cross sectional images for the gel tube.

The images should be in white with black background. Insert these images into Microsoft PowerPoint slides. Start slideshow in Microsoft PowerPoint and project any image.

Place the sample holder at the focal plane by adjusting the vertical position using the attached stage switch to a dummy black image so that there is no unwanted polymerization while adding in the pre polymer solution. Pour the pre polymer solution into the resin bath. Fill the bath until the solution slightly covers the sample holder using a pipette.

Now it is ready to print the 3D object. Switch to the slide containing the first cross-sectional image of the gel tube to polymerize the first layer. Keep projecting the image for eight seconds and then switch back to a blackout slide.

Rotate the knob on the linear stage by one quarter turn about 160 microns to lower the sample holder. Now fresh resin flows in to cover the polymerized first layer in case the liquid resin is too viscous to flow in. Move the stage further down to completely immerse the fabricated layer in the resin and locate the stage back to 160 microns below the surface.

Project the cross-sectional image again to polymerize the second layer on top of the preceding one. Repeat this process until the gel tube of the desired height is fabricated. Once all layers are complete, lift the sample holder out of the pre polymer solution and retrieve the fabricated sample.

Carefully using a razor blade, rinse the sample in acetone for approximately three hours and then allow it to dry for about one hour. To perform a swelling experiment, prepare a water oil dual layer liquid in a transparent Petri dish. Locate the water oil interface at the focal plane of the camera By adjusting the position of the Petri dish, attach the dry sample on a sample holder using super glue.

Flip the sample holder so that the is upside down. Immerse the sample in the water oil liquid bath. Approach the sample to the water oil interface from the oil layer.

The sample begins to swell when the sample touches the water surface, while the base substrate on which the gel tube is fixed stays in the top oil layer. In this way, water can diffuse into the tube wall, allowing the sample to swell before the constraining base relaxes by wet. Proceed to monitor the pattern change as the gel tube swells.

Using a digital camera, a simple projection micro stereolithography system using an off-the-shelf digital data projector is shown here. A convex lens with a focal length of 75 millimeters concentrates the beam into a small illumination area of two centimeters by two centimeters, resulting in plain optical resolution is about 45 microns. Vertical resolution is determined by the precision level of the linear stage layer.

Thickness of the structures made for this study is 160 microns. Each layer was polymerized with eight second light illumination. A representative 3D structure fabricated by the system is shown.

This object consists of 58 layers of peg da. A set of photo curable peg da hydrogel tubes was designed and fabricated to achieve low cross-linking and therefore large swelling as described in the written protocol, a sample was placed upside down in a water oil bath. As demonstrated in the video, depending on the dimensional parameters, circular tubes either remained stable or transformed into a wavy pattern.

The dimension of the gel tube determines the number of waves emerging during swelling. The wide variety of swelling patterns of different samples was captured by a digital camera. The vertical axis indicates stability as thickness over height or T over H, and the horizontal axis indicates buckling mode as height over diameter, or H over D.The white numbers indicate the buckling mode number, which is the number of waves along the circumference as shown here.

The buckling mode of the unstable samples depends only on HD, where the experimental result agrees well with theoretical prediction. We use this method in this video as a useful experimental tool for soft material mechanics, but we'll also find many applications in other fields of science and engineering, including soft robotics and biomedical engineering. Also, it is very simple and affordable.

Anyone can build up their own micro 3D printer in the lab by following the protocol presented in this video.