The authors would like to thank Joseph Muskin and Matthew Ragusa at the University of Illinois at Urbana-Champaign for providing cross-sectional images for 3D structures shown in Figure 1D.

1. Preparation of Prepolymer Solution
<ol>
	<li>Mix poly(ethylene glycol) diacrylate (PEG-DA) (average molecular weight ~575, Sigma-Aldrich) and poly(ethylene glycol) (PEG) (average molecular weight ~200, Sigma-Aldrich) at 1:2 weight ratio.</li>
	<li>Add 0.67 %wt. photo-initiator (Phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide, Sigma-Aldrich). The solution should be kept in dark environment from this point on.</li>
	<li>Add 0.05 %wt. photo-absorber (Sudan I, Sigma-Aldrich).</li>
	<li>Mix the solution for 24 hr at room temperature using a magnetic stirrer.</li>
</ol>
2. Setting Up a Desktop 3D Printer Using a Digital Data Projector
<ol>
	<li>Place a digital data projector on a flat and stable position, and connect it to a computer with Microsoft PowerPoint installed.</li>
	<li>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 cm 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.)</li>
	<li>Place a mirror after the convex lens on the beam path at 45&deg; angle to direct the beam straight down.</li>
	<li>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.</li>
	<li>Place a resin bath underneath the sample holder.</li>
</ol>
3. Design and Fabrication of Gel Tubes
<ol>
	<li>Determine diameter, wall thickness, and height of the gel tube to be fabricated.</li>
	<li>Draw cross-sectional images for the gel tube. The images should be in white with black background. Insert these image in Microsoft PowerPoint slides.</li>
	<li>Start slideshow in Microsoft PowerPoint and project any image. Place the sample holder at the focal plane by adjusting the vertical position using attached stage.</li>
	<li>Switch to a "dummy" black image so that there will be no unwanted polymerization while putting prepolymer solution.</li>
	<li>Pour prepolymer solution to resin bath. Fill the bath until the solution slightly covers the sample holder. Now it is ready for print 3D object.</li>
	<li>Switch to the slide containing the first cross-sectional image of the gel tube to polymerize the first layer. Keep projecting the image for 8 sec and switch back to "blackout" slide.</li>
	<li>Rotate the knob on the linear stage by &frac14; turn (~160 &mu;m) to lower the sample holder. Now fresh resin flows in to cover the polymerized first layer.</li>
	<li>Project the cross-sectional image again to polymerize the second layer on top of the proceeding one. Repeat steps 3.6-3.8 until the gel tube of the desired height is fabricated.</li>
	<li>Once all layers are complete, lift the sample holder out of the prepolymer solution, and retrieve the fabricated sample carefully using a razor blade.</li>
	<li>Rinse the sample in acetone for ~3 hr, and then allow it to dry for ~1 hr.</li>
</ol>
4. Swelling Experiment for Prescribed Pattern Formation by Elastic Instability
<ol>
	<li>Prepare water-oil dual layer liquid in a transparent glass dish.</li>
	<li>Attach the dry sample on a sample holder using super glue.</li>
	<li>Flip the sample holder so that the sample is upside down. Immerse the sample in the water-oil liquid bath. Approach the sample to water-oil interface from the oil layer. The sample begins to swell when the sample touches the water surface while the base substrate part on which the gel tube was fixed stayed in the top oil layer. In this way, water can diffuse into the tube wall allowing the sample swell before the constraining base relaxes by wetting. Monitor the pattern change as the gel tube swells using a digital camera.</li>
</ol>

<ol>
	<li>Timoshenko, S. P., Gere, J. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Theory of Elastic Stability. , (1961).</li><li>Sharon, E., Marder, M., Swinney, H. L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Leaves+Flowers+and+Garbage+Bags:+Making+Waves.">Leaves Flowers and Garbage Bags: Making Waves.</a> American Scientist. 92, 254-261 (2004).</li><li>K&#252;cken, M., Newell, A. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Fingerprint+formation.">Fingerprint formation.</a> Journal of Theoretical Biology. 235, 71-83 (2005).</li><li>Liang, H., Mahadevan, L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+shape+of+a+long+leaf.">The shape of a long leaf.</a> Proceedings of the National Academy of Sciences. 106, 22049-22054 (2009).</li><li>Bayer, S. A., Altman, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> The human brain during the second trimester. , (2005).</li><li>Trujillo, V., Kim, J., Hayward, R. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Creasing+instability+of+surface-attached+hydrogels.">Creasing instability of surface-attached hydrogels.</a> Soft Matter. 4, 564 (2008).</li><li>Breid, D., Crosby, A. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Effect+of+stress+state+on+wrinkle+morphology.">Effect of stress state on wrinkle morphology.</a> Soft Matter. 7, 4490 (2011).</li><li>Mora, T., Boudaoud, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Buckling+of+swelling+gels.">Buckling of swelling gels.</a> The European Physical Journal E. 20, 119-124 (2006).</li><li>DuPont, S. J., Cates, R. S., Stroot, P. G., Toomey, R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Swelling-induced+instabilities+in+microscale,+surface-confined+poly(N-isopropylacryamide)+hydrogels.">Swelling-induced instabilities in microscale, surface-confined poly(N-isopropylacryamide) hydrogels.</a> Soft Matter. 6, 3876-3882 (2010).</li><li>Dervaux, J., Couder, Y., Guedeau-Boudeville, M. -. A., Ben Amar, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Shape+Transition+in+Artificial+Tumors:+From+Smooth+Buckles+to+Singular+Creases.">Shape Transition in Artificial Tumors: From Smooth Buckles to Singular Creases.</a> Physical Review Letters. 107, 018103 (2011).</li><li>Jang, J. -. H., Koh, C. Y., Bertoldi, K., Boyce, M. C., Thomas, E. L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Combining+Pattern+Instability+and+Shape-Memory+Hysteresis+for+Phononic+Switching.">Combining Pattern Instability and Shape-Memory Hysteresis for Phononic Switching.</a> Nano Letters. 9, 2113-2119 (2009).</li><li>Lee, H., Zhang, J., Jiang, H., Fang, N. X. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Prescribed+Pattern+Transformation+in+Swelling+Gel+Tubes+by+Elastic+Instability.">Prescribed Pattern Transformation in Swelling Gel Tubes by Elastic Instability.</a> Physical Review Letters. 108, 214304 (2012).</li><li>Sun, C., Fang, N., Wu, D. M., Zhang, X. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Projection+micro-stereolithography+using+digital+micro-mirror+dynamic+mask.">Projection micro-stereolithography using digital micro-mirror dynamic mask.</a> Sensors and Actuators A: Physical. 121, 113-120 (2005).</li><li>Muskin, J., Ragusa, M., Gelsthorpe, T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Three-Dimensional+Printing+Using+a+Photoinitiated+Polymer.">Three-Dimensional Printing Using a Photoinitiated Polymer.</a> Journal of Chemical Education. 87, 512-514 (2010).</li></ol>

No conflicts of interest declared.

In swelling of tubular hydrogel constrained on the substrate, stability depends only on t/h and buckling mode depends only on h/D12. Four groups of samples (I-IV) with different levels of normalized thickness t/h were fabricated, with group I being thicker and group IV being more slender. Each group consists of four samples (i-iv) with different levels of normalized height h/D, with the sample i being shorter and the sample iv being taller. Dimensions of the fabricated samples are presented in Table 1. Group I and II are designed to stay stable during swelling, whereas group III and IV are designed to buckle and transform upon swelling. For buckling samples, buckling mode should decrease with sample height. Figure 3A shows experimental result. As theory predicts, samples in group I and II were stable and remained circular on swelling, while samples in group III and IV all went through elastic instability and buckled. Also, samples with the same h/D displayed similar buckling mode. Figure 3B compares experimentally observed buckling modes of samples in group III and IV with theoretical prediction. We can see that samples with the same h/D pose the same post-buckling pattern regardless of the thickness and that experimental results agree well with the theory.
We present how to build up a simple desktop 3D printing system using a commercially available digital data projector. The proposed approach relies on photocuring of polymer to construct 3D structures, and therefore, any photocurable polymers can be also used in general, as far as the photoinitiator has appropriate absorbance in visible wavelength range. Note that many commercially available photoinitiators are designed for ultra-violet (UV) wavelengths, but the photoinitiator used here has relatively higher absorbance at wavelengths longer than 400 nm. Offering an easy and rapid way to fabricate 3D objects, this method will find many applications in various fields including soft materials mechanics as demonstrated here.

<table border="1">
 <tbody>
 <tr>
 <td>Name of the reagent</td>
 <td>Company</td>
 <td>Catalogue number</td>
 <td>Comments (optional)</td>
 </tr>
 <tr>
 <td>Poly(ethylene glycol) diacrylate</td>
 <td>Sigma-Aldrich</td>
 <td>437441</td>
 <td>Mw~575</td>
 </tr>
 <tr>
 <td>Poly(ethylene glycol)</td>
 <td>Sigma-Aldrich</td>
 <td>P3015</td>
 <td>Mw~200</td>
 </tr>
 <tr>
 <td>phenylbis(2,4,6-trimethylbenzoyl) phosphine oxide</td>
 <td>Sigma-Aldrich</td>
 <td>511447</td>
 <td>Photo-initiator</td>
 </tr>
 <tr>
 <td>Sudan I</td>
 <td>Sigma-Aldrich</td>
 <td>103624</td>
 <td>Photo-absorber</td>
 </tr>
 <tr>
 <td>Digital data projector</td>
 <td>Viewsonic</td>
 <td>PJD6221</td>
 <td></td>
 </tr>
 <tr>
 <td>Convex lens</td>
 <td>Thorlabs</td>
 <td>LA1145</td>
 <td>f = 75.0 mm</td>
 </tr>
 <tr>
 <td>Mirror</td>
 <td></td>
 <td></td>
 <td>4" silicon wafer</td>
 </tr>
 <tr>
 <td>Manual stage</td>
 <td>Velmex</td>
 <td>A2506DE-S2.5</td>
 <td></td>
 </tr>
 </tbody>
</table>

micro 3d printing using a digital projector and its application in the study of soft materials mechanics

Buckling is a classical topic in mechanics. While buckling has long been studied as one of the major structural failure modes1, it has recently drawn new attention as a unique mechanism for pattern transformation. Nature is full of such examples where a wealth of exotic patterns are formed through mechanical instability2-5. Inspired by this elegant mechanism, many studies have demonstrated creation and transformation of patterns using soft materials such as elastomers and hydrogels6-11. Swelling gels are of particular interest because they can spontaneously trigger mechanical instability to create various patterns without the need of external force6-10. Recently, we have reported demonstration of full control over buckling pattern of micro-scaled tubular gels using projection micro-stereolithography (P&mu;SL), a three-dimensional (3D) manufacturing technology capable of rapidly converting computer generated 3D models into physical objects at high resolution12,13. Here we present a simple method to build up a simplified P&mu;SL system using a commercially available digital data projector to study swelling-induced buckling instability for controlled pattern transformation.
A simple desktop 3D printer is built using an off-the-shelf digital data projector and simple optical components such as a convex lens and a mirror14. Cross-sectional images extracted from a 3D solid model is projected on the photosensitive resin surface in sequence, polymerizing liquid resin into a desired 3D solid structure in a layer-by-layer fashion. Even with this simple configuration and easy process, arbitrary 3D objects can be readily fabricated with sub-100 &mu;m resolution.
This desktop 3D printer holds potential in the study of soft material mechanics by offering a great opportunity to explore various 3D geometries. We use this system to fabricate tubular shaped hydrogel structure with different dimensions. Fixed on the bottom to the substrate, the tubular gel develops inhomogeneous stress during swelling, which gives rise to buckling instability. Various wavy patterns appear along the circumference of the tube when the gel structures undergo buckling. Experiment shows that circumferential buckling of desired mode can be created in a controlled manner. Pattern transformation of three-dimensionally structured tubular gels has significant implication not only in mechanics and material science, but also in many other emerging fields such as tunable matamaterials.

A simple P&mu;SL system using an off-the-shelf digital data projector is shown in Figure 1. A convex lens with a focal length of 75 mm concentrates the beam into small illumination area of 2 cm by 2 cm. Resulting in-plane optical resolution is about 45 &mu;m. Vertical resolution is determined by the precision level of the linear stage. Layer thickness of the structures made for this study is 160 &mu;m. Each layer was polymerized for 8 sec light illumination. A representative 3D structure fabricated by the system is shown in Figure 1D. This objects consists of 58 layers of PEGDA.
We prepared photo-curable PEGDA hydrogel. Low crosslinking, therefore large swelling, of PEGDA hydrogel was achieved by adding non-crosslinking PEG into prepolymer solution. Length-wise swelling ratio of the resulting PEGDA hydrogel is 1.5, which corresponds to higher than 300% volumetric expansion.
A set of PEGDA hydrogel tubes were designed and fabricated based on our theory12. We placed a sample upside down and put in the bath with water covered with oil layer on top as illustrated in Figure 2A. Depending on the dimensional parameters, circular tubes either remained stable or transformed into a wavy pattern as shown in Figure 2B. The wide variety of swelling pattern of different samples was captured by a digital camera and presented in Figure 3A.
<img alt="Figure 1" src="/files/ftp_upload/4457/4457fig1.jpg" /> 
 Figure 1. A desktop projection micro-stereolithography system (a) schematic representation (b) actual system (c) close-up view of components (d) representative 3D structures. <a href="https://www.jove.com/files/ftp_upload/4457/4457fig1large.jpg" target="_blank"> Click here to view larger figure</a>.
<img alt="Figure 2" src="/files/ftp_upload/4457/4457fig2.jpg" /> 
 Figure 2. (a) Experimental setup for hydrogel tube swelling (b) constrained hydrogel tube transforms into different patterns. Scale bar indicates 5 mm.
<img alt="Figure 3" src="/files/ftp_upload/4457/4457fig3.jpg" /> 
 Figure 3. (a) Patterns formed in swelling experiment. Vertical axis indicates t/h (thus stability), and horizontal axis indicates h/D (thus buckling mode). Scale bar indicates 5 mm. (b) Buckling mode depends only on h/D. Experimental result agrees well with theoretical prediction. <a href="https://www.jove.com/files/ftp_upload/4457/4457fig3large.jpg" target="_blank"> Click here to view larger figure</a>.
<table border="1">
	<tbody>
		<tr>
			<td colspan="2">Sample</td>
			<td>D (&mu;m) </td>
			<td>t (&mu;m) </td>
			<td>h (&mu;m) </td>
		</tr>
		<tr>
			<td rowspan="4">I</td>
			<td>i</td>
			<td>9300 &plusmn; 420</td>
			<td>760 &plusmn; 40</td>
			<td>840 &plusmn; 40</td>
		</tr>
		<tr>
			<td>ii</td>
			<td>9700 &plusmn; 420</td>
			<td>1040 &plusmn; 40</td>
			<td>1060 &plusmn; 40</td>
		</tr>
		<tr>
			<td>iii</td>
			<td>9700 &plusmn; 420</td>
			<td>1210 &plusmn; 40</td>
			<td>1340 &plusmn; 40</td>
		</tr>
		<tr>
			<td>iv</td>
			<td>9700 &plusmn; 420</td>
			<td>1660 &plusmn; 40</td>
			<td>1680 &plusmn; 40</td>
		</tr>
		<tr>
			<td rowspan="4">II</td>
			<td>i</td>
			<td>9000 &plusmn; 420</td>
			<td>480 &plusmn; 40</td>
			<td>880 &plusmn; 40</td>
		</tr>
		<tr>
			<td>ii</td>
			<td>9000 &plusmn; 420</td>
			<td>660 &plusmn; 40</td>
			<td>1060 &plusmn; 40</td>
		</tr>
		<tr>
			<td>iii</td>
			<td>9500 &plusmn; 420</td>
			<td>740 &plusmn; 40</td>
			<td>1350 &plusmn; 40</td>
		</tr>
		<tr>
			<td>iv</td>
			<td>9200 &plusmn; 420</td>
			<td>970 &plusmn; 40</td>
			<td>1650 &plusmn; 40</td>
		</tr>
		<tr>
			<td rowspan="4">III</td>
			<td>i</td>
			<td>8900 &plusmn; 420</td>
			<td>160 &plusmn; 40</td>
			<td>790 &plusmn; 40</td>
		</tr>
		<tr>
			<td>ii</td>
			<td>8900 &plusmn; 420</td>
			<td>300 &plusmn; 40</td>
			<td>1020 &plusmn; 40</td>
		</tr>
		<tr>
			<td>iii</td>
			<td>9100 &plusmn; 420</td>
			<td>380 &plusmn; 40</td>
			<td>1330 &plusmn; 40</td>
		</tr>
		<tr>
			<td>iv</td>
			<td>9000 &plusmn; 420</td>
			<td>490 &plusmn; 40</td>
			<td>1630 &plusmn; 40</td>
		</tr>
		<tr>
			<td rowspan="4">IV</td>
			<td>i</td>
			<td>8900 &plusmn; 420</td>
			<td>140 &plusmn; 40</td>
			<td>780 &plusmn; 40</td>
		</tr>
		<tr>
			<td>ii</td>
			<td>8800 &plusmn; 420</td>
			<td>190 &plusmn; 40</td>
			<td>1010 &plusmn; 40</td>
		</tr>
		<tr>
			<td>iii</td>
			<td>9300 &plusmn; 420</td>
			<td>230 &plusmn; 40</td>
			<td>1340 &plusmn; 40</td>
		</tr>
		<tr>
			<td>iv</td>
			<td>8900 &plusmn; 420</td>
			<td>290 &plusmn; 40</td>
			<td>1650 &plusmn; 40</td>
		</tr>
	</tbody>
</table>
Table 1. Sample dimensions measured through optical microscope. Errors indicate measurement uncertainty.

Watch this Scientific Journal Video about Micro 3D Printing Using a Digital Projector and its Application in the Study of Soft Materials Mechanics at JoVE.com

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.

Buckling is a classical topic in mechanics. While buckling has long been studied as one of the major structural failure modes1, it has recently drawn new ...

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25.1K Views.  Massachusetts Institute of Technology. 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.

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

Quantification of Hydrogen Concentrations in Surface and Interface Layers and Bulk Materials through Depth Profiling with Nuclear Reaction Analysis

Nuclear reaction analysis (NRA) via the resonant 1H(15N,&#945;&#947;)12C reaction is a highly effective method of depth profiling that quantitatively and non-destructively reveals the hydrogen density distribution at surfaces, at interfaces, and in the volume of solid materials with high depth resolution. The technique applies a 15N ion beam of 6.385 MeV provided by an electrostatic accelerator and specifically detects the 1H isotope in depths up to about 2 &#956;m from the target surface. Surface H coverages are measured with a sensitivity in the order of ~1013 cm-2 (~1% of a typical atomic monolayer density) and H volume concentrations with a detection limit of ~1018 cm-3 (~100 at. ppm). The near-surface depth resolution is 2-5 nm for surface-normal 15N ion incidence onto the target and can be enhanced to values below 1 nm for very flat targets by adopting a surface-grazing incidence geometry. The method is versatile and readily applied to any high vacuum compatible homogeneous material with a smooth surface (no pores). Electrically conductive targets usually tolerate the ion beam irradiation with negligible degradation. Hydrogen quantitation and correct depth analysis require knowledge of the elementary composition (besides hydrogen) and mass density of the target material. Especially in combination with ultra-high vacuum methods for in-situ target preparation and characterization, 1H(15N,&#945;&#947;)12C NRA is ideally suited for hydrogen analysis at atomically controlled surfaces and nanostructured interfaces. We exemplarily demonstrate here the application of 15N NRA at the MALT Tandem accelerator facility of the University of Tokyo to (1) quantitatively measure the surface coverage and the bulk concentration of hydrogen in the near-surface region of a H2 exposed Pd(110) single crystal, and (2) to determine the depth location and layer density of hydrogen near the interfaces of thin SiO2 films on Si(100).

We illustrate the application of 1H(15N,&#945;&#947;)12C resonant nuclear reaction analysis (NRA) to quantitatively evaluate the density of hydrogen atoms on the surface, in the volume, and at an interfacial layer of solid materials. The near-surface hydrogen depth profiling of a Pd(110) single crystal and of SiO2/Si(100) stacks is described.

Nuclear reaction analysis (NRA) via the resonant 1H(15N,&#945;&#947;)12C reaction is a highly effective method of depth profiling that quantitatively and ...

Wicking Tests for Unidirectional Fabrics: Measurements of Capillary Parameters to Evaluate Capillary Pressure in Liquid Composite Molding Processes

During impregnation of a fibrous reinforcement in liquid composite molding (LCM) processes, capillary effects have to be understood in order to identify their influence on void formation in composite parts. Wicking in a fibrous medium described by the Washburn equation was considered equivalent to a flow under the effect of capillary pressure according to the Darcy law. Experimental tests for the characterization of wicking were conducted with both carbon and flax fiber reinforcement. Quasi-unidirectional fabrics were then tested by means of a tensiometer to determine the morphological and wetting parameters along the fiber direction. The procedure was shown to be promising when the morphology of the fabric is unchanged during capillary wicking. In the case of carbon fabrics, the capillary pressure can be calculated. Flax fibers are sensitive to moisture sorption and swell in water. This phenomenon has to be taken into account to assess the wetting parameters. In order to make fibers less sensitive to water sorption, a thermal treatment was carried out on flax reinforcements. This treatment enhances fiber morphological stability and prevents swelling in water. It was shown that treated fabrics have a linear wicking trend similar to those found in carbon fabrics, allowing for the determination of capillary pressure.

An experimental method to measure geometrical parameters and the apparent advancing contact angles describing capillary wicking in unidirectional synthetic and natural fabrics is proposed. These parameters are mandatory for the determination of the capillary pressures that must be taken into account for liquid composite molding (LCM) applications.

During impregnation of a fibrous reinforcement in liquid composite molding (LCM) processes, capillary effects have to be understood in order to identify ...

Challenges in Rheological Characterization of Highly Concentrated Suspensions &#8212; A Case Study for Screen-printing Silver Pastes

A comprehensive rheological characterization of highly concentrated suspensions or pastes is mandatory for a targeted product development meeting the manifold requirements during processing and application of such complex fluids. In this investigation, measuring protocols for a conclusive assessment of different process relevant rheological parameters have been evaluated. This includes the determination of yield stress, viscosity, wall slip velocity, structural recovery after large deformation and elongation at break as well as tensile force during filament stretching.
The importance of concomitant video recordings during parallel-plate rotational rheometry for a significant determination of rheological quantities is demonstrated. The deformation profile and flow field at the sample edge can be determined using appropriate markers. Thus, measurement parameter settings and plate roughness values can be identified for which yield stress and viscosity measurements are possible. Slip velocity can be measured directly and measuring conditions at which plug flow, shear banding or sample spillover occur can be identified clearly. 
Video recordings further confirm that the change in shear moduli observed during three stage oscillatory shear tests with small deformation amplitude in stage I and III but large oscillation amplitude in stage II can be directly attributed to structural break down and recovery. For the pastes investigated here, the degree of irreversible, shear-induced structural change increases with increasing deformation amplitude in stage II until a saturation is reached at deformations corresponding to the crossover of G' and G'', but the irreversible damage is independent of the duration of large amplitude shear. 
A capillary breakup elongational rheometer and a tensile tester have been used to characterize deformation and breakup behavior of highly filled pastes in uniaxial elongation. Significant differences were observed in all experiments described above for two commercial screen-printing silver pastes used for front side metallization of Si-solar cells.

Challenges in Rheological Characterization of Highly Concentrated Suspensions &amp;#8212; A Case Study for Screen-printing Silver Pastes

A protocol for a robust and application relevant rheological characterization of highly concentrated suspensions is presented. Silver pastes used for screen-printing application in solar cell production are employed as model systems.

A comprehensive rheological characterization of highly concentrated suspensions or pastes is mandatory for a targeted product development meeting the ...

A Soft Tooling Process Chain for Injection Molding of a 3D Component with Micro Pillars

The purpose of this paper is to present the method of a soft tooling process chain employing Additive Manufacturing (AM) for fabrication of injection molding inserts with micro surface features. The Soft Tooling inserts are manufactured by Digital Light Processing (vat photo polymerization) using a photopolymer that can withstand relatively high temperaturea. The part manufactured here has four tines with an angle of 60&#176;. Micro pillars (&#216;200 &#181;m, aspect ratio of 1) are arranged on the surfaces by two rows. Polyethylene (PE) injection molding with the soft tooling inserts is used to fabricate the final parts. This method demonstrates that it is feasible to obtain injection-molded parts with microstructures on complex geometry by additive manufactured inserts. The machining time and cost is reduced significantly compared to conventional tooling processes based on computer numerical control (CNC) machining. The dimensions of the micro features are influenced by the applied additive manufacturing process. The lifetime of the inserts determines that this process is more suitable for pilot production. The precision of the inserts production is limited by the additive manufacturing process as well.

A protocol for fabricating injection molding inserts for complex geometry with micro features on surfaces employing Additive Manufacturing (AM) is presented.

The purpose of this paper is to present the method of a soft tooling process chain employing Additive Manufacturing (AM) for fabrication of injection ...

Studying Large Amplitude Oscillatory Shear Response of Soft Materials

We investigate the sequence of physical processes exhibited during large amplitude oscillatory shearing (LAOS) of polyethylene oxide (PEO) in dimethyl sulfoxide (DMSO) and xanthan gum in water &#8212; two concentrated polymer solutions used as viscosifiers in foods, enhanced oil recovery, and soil remediation. Understanding the nonlinear rheological behavior of soft materials is important in the design and controlled manufacturing of many consumer products. It is shown how the response to LAOS of these polymer solutions can be interpreted in terms of a clear transition from linear viscoelasticity to viscoplastic deformation and back again during a period. The LAOS results are analyzed via the fully quantitative Sequence of Physical Processes (SPP) technique, using free MATLAB-based software. A detailed protocol of performing a LAOS measurement with a commercial rheometer, analyzing nonlinear stress responses with the freeware, and interpreting physical processes under LAOS is presented. It is further shown that, within the SPP framework, a LAOS response contains information regarding the linear viscoelasticity, the transient flow curves, and the critical strain responsible for the onset of nonlinearity.

We present a detailed protocol outlining how to perform nonlinear oscillatory shear rheology on soft materials, and how to run the SPP-LAOS analysis to understand the responses as a sequence of physical processes.

We investigate the sequence of physical processes exhibited during large amplitude oscillatory shearing (LAOS) of polyethylene oxide (PEO) in dimethyl ...

Multimodal 3D Printing of Phantoms to Simulate Biological Tissue

Biomedical optical imaging is playing an important role in diagnosis and treatment of various diseases. However, the accuracy and the reproducibility of an optical imaging device are greatly affected by the performance characteristics of its components, the test environment, and the operations. Therefore, it is necessary to calibrate these devices by traceable phantom standards. However, most of the currently available phantoms are homogeneous phantoms that cannot simulate multimodal and dynamic characteristics of biological tissue. Here, we show the fabrication of heterogeneous tissue-simulating phantoms using a production line integrating a spin coating module, a polyjet module, a fused deposition modeling (FDM) module, and an automatic control framework. The structural information and the optical parameters of a &#34;digital optical phantom&#34; are defined in a prototype file, imported to the production line, and fabricated layer-by-layer with sequential switch between different printing modalities. Technical capability of such a production line is exemplified by the automatic printing of skin-simulating phantoms that comprise the epidermis, dermis, subcutaneous tissue, and an embedded tumor.

Spin coating, polyjet printing, and fused deposition modeling are integrated to produce multilayered heterogeneous phantoms that simulate structural and functional properties of biological tissue.

Biomedical optical imaging is playing an important role in diagnosis and treatment of various diseases. However, the accuracy and the reproducibility of ...

In Situ Transmission Electron Microscopy with Biasing and Fabrication of Asymmetric Crossbars Based on Mixed-Phased a-VOx

Resistive switching crossbar architecture is highly desired in the field of digital memories due to low cost and high-density benefits. Different materials show variability in resistive switching properties due to the intrinsic nature of the material used, leading to discrepancies in the field because of underlying operation mechanisms. This highlights a need for a reliable technique to understand mechanisms using nanostructural observations. This protocol explains a detailed process and methodology of in&#160;situ nanostructural analysis as a result of electrical biasing using transmission electron microscopy (TEM). It provides visual and reliable evidence of underlying nanostructural changes in real time memory operations. Also included is the methodology of fabrication and electrical characterizations for asymmetric crossbar structures incorporating amorphous vanadium oxide. The protocol explained here for vanadium oxide films can be easily extended to any other materials in a metal-dielectric-metal sandwiched structure. Resistive switching crossbars are predicted to serve the programmable logic and neuromorphic circuits for next-generation memory devices, given the understanding of the operation mechanisms. This protocol reveals the switching mechanism in a reliable, timely, and cost-effective way in any type of resistive switching materials, and thereby predicts the device&#39;s applicability.

In Situ Transmission Electron Microscopy with Biasing and Fabrication of Asymmetric Crossbars Based on Mixed-Phased a-VOx

Presented here is a protocol for analyzing nanostructural changes during in&#160;situ biasing with transmission electron microscopy (TEM) for a stacked metal-insulator-metal structure. It has significant applications in resistive switching crossbars for the next generation of programmable logic circuits and neuromimicking hardware, to reveal their underlying operation mechanisms and practical applicability.

Resistive switching crossbar architecture is highly desired in the field of digital memories due to low cost and high-density benefits. Different ...

3D Printing - Evaluating Particle Emissions of a 3D Printing Pen

Three-dimensional (3D) printing as a type of additive manufacturing shows continuing increase in application and consumer popularity. The fused filament fabrication (FFF) is an inexpensive method used most frequently by consumers. Studies with 3D printers have shown that during the printing process particulate and volatile substances are released. Handheld 3D printing pens also use the FFF method but the consumer&#8217;s proximity to the 3D pens gives reason to higher exposure compared to a 3D printer. At the same time, 3D printing pens are often marketed for children who could be more sensitive to the printing emission. The aim of this study was to implement a low cost method to analyze the emissions of 3D printing pens. Polylactide (PLA) and acrylonitrile butadiene styrene (ABS) filaments of different colors were tested. In addition, filaments containing metal and carbon nanotubes (CNTs) were analyzed. An 18.5 L chamber and sampling close to the emission source was used to characterize emissions and concentrations near the breathing zone of the user.
Particle emissions and particle size distributions were measured and the potential release of metal particles and CNTs investigated. Particle number concentrations were found in a range of 105 - 106 particles/cm3, which is comparable to previous reports from 3D printers. Transmission electron microscopy (TEM) analysis showed nanoparticles of the different thermoplastic materials as well as of metal particles and CNTs. High contents of metal were observed by inductively coupled plasma mass spectrometry (ICP-MS).
These results call for a cautious use of 3D pens due to potential risk to the consumers.

This protocol presents a method to analyze the emission of 3D printing pens. Particle concentration and particle size distribution of the released particle is measured. Released particles are further analyzed with transmission electron microscopy (TEM). Metal content in filaments is quantified by inductively coupled plasma mass spectrometry (ICP-MS).

Three-dimensional (3D) printing as a type of additive manufacturing shows continuing increase in application and consumer popularity. The fused filament ...

Pneumatically Driven Microfluidic Platform for Micro-Particle Concentration

The present article introduces a method for fabricating and operating a pneumatic valve to control particle concentration using a microfluidic platform. This platform has a three-dimensional (3D) network with curved fluid channels and three pneumatic valves, which create networks, channels, and spaces through duplex replication with polydimethylsiloxane (PDMS). The device operates based on the transient response of a fluid flow rate controlled by a pneumatic valve in the following order: (1) sample loading, (2) sample blocking, (3) sample concentration, and (4) sample release. The particles are blocked by thin diaphragm layer deformation of the sieve valve (Vs) plate and accumulate in the curved microfluidic channel. The working fluid is discharged by the actuation of two on/off valves. As a result of the operation, all particles of various magnifications were successfully intercepted and disengaged. When this technology is applied, the operating pressure, the time required for concentration, and the concentration rate may vary depending on the device dimensions and particle size magnification.

The present protocol describes a pneumatic microfluidic platform that can be used for efficient microparticle concentration.

The present article introduces a method for fabricating and operating a pneumatic valve to control particle concentration using a microfluidic platform. ...

Microfluidic Acoustophoresis for Flowthrough Separation of Gram-Negative Bacteria using Aptamer Affinity Beads

This article describes the fabrication and operation of microfluidic acoustophoretic chips using a microfluidic acoustophoresis technique and aptamer-modified microbeads that can be used for the fast, efficient isolation of Gram-negative bacteria from a medium. This method enhances the separation efficiency using a mix of long, square microchannels. In this system, the sample and buffer are injected into the inlet port through a flow controller. For bead centering and sample separation, AC power is applied to the piezoelectric transducer via a function generator with a power amplifier to generate acoustic radiation force in the microchannel. There is a bifurcated channel at both the inlet and outlet, enabling simultaneous separation, purification, and concentration. The device has a recovery rate of &gt;98% and purity of 97.8% up to a 10x dose concentration. This study has demonstrated a recovery rate and purity higher than the existing methods for separating bacteria, suggesting that the device can separate bacteria efficiently.

This paper describesthe fabrication and operation of microfluidic acoustophoretic chips using the microfluidic acoustophoresis technique and aptamer-modified microbeads that can be used for fast, efficient isolation of Gram-negative bacteria from a medium.