What is a composite?

Fermented biomaterials

Level: ADVANCED

Version: 8.10.20.1

Learning objectives: 

The student will gain knowledge and understanding of:

1. Types of biomaterial composites.
2. Benefits of fiber composites.
3. Mechanical testing of natural fiber composites.

 What is a composite?

Composite materials are composed of two or more materials providing a final material whose overall properties are improved over those of the individual materials used. Improvements can be mechanical, optical, thermal or even relate to other issues like sustainability and cost. Most biological materials are complex composites with many different materials assembled together in intricate ways – all the way down to the nano and molecular scales. Here are some examples of natural composites with amazing properties and why they are better together!

Wood

Wood is largely made up of cellulose, hemicellulose and lignin (see figure below). Cellulose is a fiber that gives wood strength in tension (imagine pulling on a strong rope). But you cannot push a rope! So cellulose generally is not good in compression. Lignin, however, is a non-fibrous matrix that is much better in compression. Hemicelluloses have a role in connecting the cellulose fibers to the lignin to allow improved stress transfer between the materials. Thus wood is a material that performs well in both compression and tension even though the constituent parts do not. This is why wood has been used as a building material for millennia. It also grows naturally using the energy of the sun, captures carbon and can be reclaimed by the environment. It is an ideal sustainable composite!

Spider silk

Spider silk is much different than wood. Wood is composed principally of polysaccharides (cellulose fibers and hemicelluloses) and polyphenols (lignin). It is strong, but not like spider silk! Spider silk, however is composed of proteins! Yes, just like your muscles. But silk proteins are much different. The silk protein is able to elongate and deform under stresses allowing the applied load to be redistributed over the cross section of the protein. It also is itself a composite of different forms of protein. The Alanine protein regions are hard crystalline protein domains embedded in an elastic Glycine matrix (see first figure below). Also, the spider spins and weaves a complex web thread using several silk spinnerets (see second figure below). Amazing!

[Image: Spider Silk Gland Spigots (gasteracantha Sp.) is a photograph by Dennis Kunkel Microscopy/science Photo Library which was uploaded on September 16th, 2018. From: https://fineartamerica.com/featured/2-spider-silk-gland-spigots-gasteracantha-sp-dennis-kunkel-microscopyscience-photo-library.html]

Limpet teeth

It would not be surprising if you have never heard of a Limpet! Limpets are aquatic snails that eat algae growing on rocks (see first figure below). They scrape the algae using a ribbon like tongue that contains rows of teeth (see second figure below). These teeth have evolved to be extremely strong to continually scrape rocks! It was once thought that spider silk was the strongest biological material, but now we know Limpet teeth are the strongest!

Limpet teeth's great strength comes from the structure of its composite composition. Like wood, it is a fiber composite (see third figure below), but a unique one. In this case, the hard fibers are goethite (iron(III) oxide-hydroxide) and the matrix is chitin (a polysaccharide). Can you believe that a snail can produce mineral fibers in its teeth so it can eat?! Once again, hard fibers embedded in a dense matrix is the key to improving the properties of the composite.

[Image: https://www.hakaimagazine.com/news/how-tiny-limpets-do-the-heavy-lifting-of-climate-resilience/]

[Image:https://www.reddit.com/r/pics/comments/48ij18/micro_of_limpet_teeth_on_radula_natures_strongest/]

[Image: Barber, Asa H., Dun Lu, and Nicola M. Pugno. 2015. "Extreme Strength Observed in Limpet Teeth." Journal of the Royal Society Interface 12 (105): 20141326.]

QUESTIONS:

What materials do you see in nature that you think have amazing properties?

What are they made of?

How do you think they have achieved those properties?

How can we use them or the knowledge they provide to design and build better sustainable composites?

Human made fiber composites

Humans have learned that natural fiber composites like wood offer superior performance so many such materials have been engineered for numerous applications including oriented strand board (OSB) for construction (wood fiber chips in a synthetic adhesive), plastic-fiber composite decking (wood fiber powder in a plastic matrix), fiber cement siding (wood fiber pulp in a cement matrix), fiberglass panels like those in boats (glass fibers in a plastic matrix) and countless others. Many of these products offer improvements over natural materials in terms of performance (stability, water resistance, maintenance, decay resistance, cost) but incorporate unsustainable materials that negatively impact our environment. There is a real need to develop sustainable composites that have improved properties but are compatible with our environment. This could be your career!

[Images taken from: https://evstudio.com/staples-or-nails/   https://www.harlowfencing.com/product/osb-3-board/  https://www.jameshardie.com/?loc=refresh  https://en.wikipedia.org/wiki/Homebuilt_aircraft https://www.boatingmagz.com/author/asad-ahsan/

home depot composite decking search - https://www.homedepot.com/b/Lumber-Composites-Decking-Deck-Boards-Composite-Decking-Boards/N-5yc1vZc5mb?NCNI-5&searchRedirect=composite+decking&semanticToken=d00r20r001220000_202010071416547968792107902_us-east4-lh38+d00r20r001220000+%3E++cnn%3A%7B0%3A0%7D+cnp%3A%7B10%3A0%7D+cnd%3A%7B4%3A0%7D+cne%3A%7B8%3A0%7D+cns%3A%7B5%3A0%7D+cnx%3A%7B3%3A0%7D+cnq%3A%7B0%3A0%7D+cnw%3A%7B0%3A0%7D+cnv%3A%7B1%3A4%7D+st%3A%7Bcomposite+decking%7D%3Ast+lca%3A%7B567155%7D+tgr%3A%7BNo+stage+info%7D+qu%3A%7Bcomposite+decking%7D%3Aqu ] 

Why are fiber composites superior? Let's discuss the concept of mechanical percolation!

Fiber composites are superior because they bring together the best properties of the fiber and the matrix material. But how much fiber should be used? Typically enough to achieve mechanical percolation. Mechanical percolation is achieved when there is enough fiber in the composite so that the fibers can touch each other and form longer load transfer paths. This makes the final composite behave mechanically more like the fiber than the weaker matrix. A schematic illustration of this is shown below. A significant improvement in mechanical properties occurs when enough fiber is added to reach the percolation threshold. Do you see above how there are enough goethite fibers to form connected clusters in the chitin matrix? This makes the Limpet teeth strong!

Time to be a scientist!

[Image taken from: https://www.blackenterprise.com/9-black-women-in-stem-you-need-to-know/]

Let's make an improved hydrogel: a fiber-hydrogel composite!

What do we want to learn? 

In the first experiment, you learned how to make a hydrogel from gellan that has been shown to be potentially useful in wound care treatment [Ferris, Cameron J., Kerry J. Gilmore, Gordon G. Wallace, and Marc in het Panhuis. 2013. "Modified Gellan Gum Hydrogels for Tissue Engineering Applications." Soft Matter 9 (14): 3705-3711.]. However, wound care is only one biomedical application. What about applications like hernia repair, pelvic floor reconstruction or even cosmetic surgery? There are many applications where the biomedical material needs to be much stronger then the hydrogel you made previously. But how can we make them stronger and still have them be natural and biocompatible? We can make a natural composite using natural materials like natural fibers! Cotton is cellulose like the cellulose found in wood except cotton is cellulose produced by the cotton plant. Adding cotton fiber to gellan hydrogels will improve their mechanical properties while preserving the other properties of the hydrogel. In this exercise you will learn how to make a gellan hydrogel and will test its mechanical properties. Specifically, you will do a simple shear test to see how the added cotton fiber impacts the gellan's shear strength.

What you will need

You will need the following materials and instruments to do this experiment:

• 500mL water
• Wood or silicone stir
• Scale
• Rice, oats or something used to apply weight
• 5 teaspoons Gellan gum, ONLY High Acyl, LT-100 (available from the  Modernist Pantry and other sources)

• Teaspoon

• Stove or hot plate to boil water

• High speed blender

• 1L shallow metal pan

• Silicone Butter Mold 4 Cavities Rectangle Large Collins Ice Cube Trays (5.25x1.25x1.25 inches cubes)

• 2 thick nails (16d penny common nails work best) or thick toothpicks.

• Wire

• Plastic dish or cup

• Lab support base and clamp

• Ruler

• Timer

Process for making your hydrogels

Step 1:   Bring 500mL of water to a rolling boil in the 1L pan.

Step 2:   Pour water into blender. Immediately add 15g or 5.5 teaspoons of gellan gum powder. This will create a ~3% solution.

Step 3:   Blend on high for 30 seconds.

Step 4:   Immediately fill 2 of the 4 molds with the gellan foam

Step 5:   Pour the remaining solution back into the 1L pan or beaker and simmer on medium just so the solution is barely boiling. Stir gently and continuously for 4-5 minutes allowing the air bubbles to escape. The gellan will form a film on the stir but you can scrape off and add the gellan back into the pan as it will dissolve again.

Step 6: Fill the remaining 2 molds with gellan solution

Step 7: Allow to cool (can be overnight or in a refrigerator).

You will end up with a material that looks like this when you remove it from the mold (shown on a wooden cutting board):

Setup for testing your hydrogels

 In this experiment we will measure the shear strength of your gellan hydrogel material. Your final setup should look like the figure below. The nails and loops holding them should be level as should the cup. The cup will hold the rice or other material you use to test how much weight your hydrogel can hold. You should bend the wire so that the loops are positioned close to the hydrogel and so that the system is stable under the applied load.

[Images: Many images taken from amazon]

Process for testing your hydrogels 

Step 1:   Weigh out 2.5g of cotton or shredded wheat or your choice of fiber. If you are using cotton balls, pull the cotton fibers apart to make well dispersed fiber fluff. In the end, this will make a composite with 20% fiber by weight of gellan.

Step 2:   Bring 500mL of water to a rolling boil in the 1L pan. BE CAREFUL!!!

Step 3:   Pour boiling water into blender with at least a 1L capacity. Immediately add 12.5g or 4.5 teaspoons of gellan gum powder. This will create a ~2.5% solution.

Step 4:   Blend on high for 20-25 seconds. IMPORTANT!!!: If the blender container is sealed, crack the lid every ~5 seconds to allow pressure to escape. 

Step 5:   Pour ½ of the solution back into the 1L pan and use the other half to fill 2 molds (about ¾ height).

Step 6:   Add the fiber to the pan and stir while boiling the solution. Stir for about a minute until the cotton fibers are fully mixed in the solution. The gellan may form a film on the stir but you can scrape off and add the gellan back into the pan as it will dissolve again.

Step 7:   Fill the remaining 2 molds with gellan solution (about ¾ height). It may be a bit lumpy but that is ok!

Step 8:   Allow to cool (can be overnight or in a refrigerator).

You will end up with a material that looks like this when you remove it from the mold (shown on a wooden cutting board):

Setup for testing your fiber hydrogel composites.

In this experiment we will measure the shear strength of your gellan hydrogel material. Your final setup should look like the figure below. The nails and loops holding them should be level as should the cup. The cup will hold the rice or other material you use to test how much weight your hydrogel can hold. You should bend the wire so that the loops are positioned close to the hydrogel and so that the system is stable under the applied load.

Process for testing your hydrogels 

Step 1: To test the gellan hydrogen shear strength as a function of density, you will first calculate the density (D) in (g/mm³). Using the scale, weigh each sample to determine the weight (W) in grams (g). Then carefully measure the length (l), width (w) and thickness (t) of each sample in millimeters (mm). The length and width will be determined by the dimension of the mold. Measure the thickness of the final sample. If the top is not flat, take an average of the edge thickness and the center thickness. Density is then given by:  D = W/l´w´t. Record the values on the worksheet. If you know the dimensions of the mold, you can use them for l and w, but you must measure t.

Step 2: Weigh the empty cup and wire fixture used in the experiment. Record the value on the worksheet.

Step 3: Measure the diameter of the nail (DN) or other object used to support the sample in mm.  Record the value on the worksheet.

Step 4: Insert the nails into the sample as shown above ~ 1 inch from the ends making sure they are parallel to the edge and place in the fixture as shown. The nail should penetrate through the thickness (t) of the sample. Specifically, the nail should be penetrating through the side of the sample whose thickness was measured to be t. This is the poured thickness of the sample in the mold after cooling.

Step 5: Assemble the rest of the fixture as shown.

Step 6: Pour 1/8 cup of rice into the cup and wait 20 seconds to see if the sample can support the weight. If it can, add another 1/8 cup of rice and wait another 20 seconds. Continue until the sample breaks. When the sample breaks, remove the last 1/8 cup of rice.

Step 7: Weigh the cup filled with rice and subtract the weight of the empty cup and wire. This is the maximum supported weight (Wmax). Record the values on the worksheet.

Step 8: Calculate the maximum shear stress Smax = Wmax/DN´t (g/mm²). To convert this value to Pascals (Pa) multiply by 9806. Here is an example:

           A 16d penny common nail is 4.1mm (DN=4.1mm). If your sample was 15mm thick, (t=15mm). If your sample could support 100g of rice thant would be 100 g of force (Wmax=100g). Thus Smax=100g/(4.1mm)(15mm) = 1.63g/mm² or Smax (Pa) = 1.63*9806 Pa or 15,983 Pa of 15.98kPa. This is a common number for a maximum shear stress (which is shear strength) for a gellan material.

Step 9: Do this measurement and calculation for all samples and complete the worksheet.

Step 10: Analyze the data. Did you see a trend with density? If not what could be the cause?

Worksheets (for print out)

pdf format

word format

Complete the worksheet. Be prepared to discuss. 

Competition! Who made the strongest fiber hydrogel composite? Who made the weakest? What was the difference in the processing or testing? Did anyone try a different fiber? Or a different amount of fiber added? Analyze!