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Following our previous blog on the potential first report of a plastic in Capri around 30 A.D., the recent publication of Walter Issacson's biography of Leonardo Da Vinci prompted us to query whether this prolific inventor and student of materials dabbled in plastic technology. We, of course, were not the first to think of this, and there is some evidence that Da Vinci did plastically dabble.

As originally reported by the Discovery Channel, the director of the Museo Ideale in the Tuscan town of Vinci, Da Vinci’s birthplace, believes that Leonardo’s copious notes in his Codices contain recipes for synthetic materials. Allessandro Vezzosi, the director and Da Vinci scholar, found experiments in Da Vinci’s notes where he combined glues derived from animal and vegetable sources with organic fibers, and coated these mixtures onto animal organs and the leaves of cabbages or lettuces.

When Vezzosi repeated these experiments, the hardened layer was removable from the vegetable matter or organ tissue, and resembled BakeLite, a phenol-formaldehyde based thermoset developed in the early 1900s and considered to be one of the first widespread synthetic plastics. Da Vinci’s notes further describe incorporating pigments into his mixtures, perhaps making one of the first master-batch compounded resins. Vezzosi suggested that the resulting materials were tough enough to resist breakage when dropped on the floor, and could have been used to make vases, cups, and knife handles, all items that incorporate modern plastics.

Sadly, none of Da Vinci’s plastic cups or utensils survive today, if indeed they were ever created. In addition to the historical significance, they would have presented a wonderful opportunity for a long-term shelf-aging study. Similarly, given Da Vinci’s interest in the human anatomy, one speculates what would have happened if Da Vinci had tried his hand at an artificial knee using his resin.

Given that today is Halloween, this blog entry started with the question of what is the most popular candy? For Massachusetts, the home of Cambridge Polymer Group, that candy is Starburst.[1] Starburst bears a loose association with taffy, which is a candy made by stretching a heated mass of sugar. The act of pulling taffy introduces small air bubbles into the molten sugar, which makes the mixture softer and improves the texture.

Do the Taffy Math

You have likely seen a taffy stretcher at a candy shop. I always thought it was for show, but it turns out that it is a necessary process to make the taffy. Pulling taffy by hand is challenging, given the temperature of the material, the viscoelasticity, and the number of required pulls. What I hadn’t also realized was the complex math that these confectioners were practicing in front of us. As discussed by Jean-Luc Thiffeault, from the University of Wisconsin, taffy pulling is modeled by topological dynamics, and is associated with the dilatation of pseudo-Anosov maps.[2]

I know; I was excited to hear about this association as well, but even more so when I heard that they give a nod to the special integer relationships of Golden and Silver ratios.  Any budding theoretical rheologist would grow faint. What this means, essentially, is that the taffy is stretched exponentially overtime. Thiffeault’s article touches on the interesting patent fights that occurred in the 1920s over taffy stretcher designs, going all the way up to the Supreme Court.

Mixing Polymer Melts

The relevance to our work at Cambridge Polymer Group is the mixing of viscoelastic materials, such as polymer solutions and melts, and pharmaceutical compounds. Standard mixing processes used for Newtonian solutions (think sugar mixing into water) do not apply to viscous materials, which requires a great deal more energy and benefit from a combination of shear and extensional flows. Optimization of mixing procedures is key, since polymers and pharmaceutical compounds can be sensitive to heat and mechanical deformation; finding the optimal mixing conditions can preserve the original properties.

So ponder those pseudo-Anosov maps next time you have a salt-water taffy, and consider how this popular candy, at least in Massachusetts, has benefited the plastics and pharma industry. Note: salt water is not a taffy ingredient. The name is allegedly a joke made by a Atlantic City candy shop owner whose taffy was soaked during a storm.

Biofouling, the adhesion of sea life to ships' hulls, is a serious problem for all marine vessels. The sticky stowaways significantly increase the hull's friction; the resulting drag decreases boat speed and increases fuel consumption by up to 40%. In the past, the most effective solution was to dry-dock the boat and manually remove the sea life, which is expensive and labor intensive. In addition to the financial toll of increased fuel usage and dry dock cleaning, marine fouling also contributes to the spread of invasive species.

Anti-Fouling Biocides vs. Non-Toxic Hydrogel

Many commercial anti-fouling agents are composed of toxic chemicals. The obvious drawbacks to a toxic approach are water pollution and the poisoning of non-fouling species. Additionally, anti-fouling biocides need to be replaced frequently, and do not always work well. 

The US Naval Coastal Systems Station contracted Cambridge Polymer Group to develop a slow-dissolving hydrogel formulation to clean ships' hulls underwater. Applied by divers using gun applicators, the hydrogel eliminates the need for dry dock manual removal. Simple dilution of the hydrogel makes it safe for marine life, providing a non-toxic, affordable solution for boat cleaning.

Mussel Attachment Issues

But what if the fouling itself could be prevented? Recently, Harvard researchers unveiled a lubricant-infused coating which stops mussels from attaching to underwater surfaces. Mussels are some of the worst biofouling offenders; they have evolved to stick under the severest of marine conditions. As part of that adaptation, mussels secrete adhesive filaments called byssal threads. These threads are tipped with adhesive plaques which remove water molecules from the wet surface, allowing the plaques to bind to it.

The researchers' lubricant-infused polymer coating fools the mussel into sensing the hull's surface as too soft for attachment. This trick discourages the mussel from secreting its adhesive filaments, preventing attachment of the mussel's foot. Even if a mussel attempts to deploy its byssal threads, this Slippery Liquid Infused Porous Surface (SLIPS) stops the threads from binding. Co-first author Shahrouz Amini speculates the liquid overlayer of the lubricant-infused surfaces resists displacement by mussels' adhesive proteins.

The amazing adhesive ability of mussels does have positive applications. UC Santa Barbara researchers have developed stronger, more durable dental fillings using catechols, the same chemical groups used by the mussel to promote adhesion on wet surfaces (wet surfaces except for SLIPS, that is). Purdue researchers have inserted catechol into a biomimetic polymer, creating an underwater adhesive that outperforms many commercial adhesives.

 Thursday, October 12, 2:10 p.m. EST

Adam Kozak, a senior research scientist at Cambridge Polymer Group, presents "Leachable Studies of Medical Devices in Complex Biological Environments" at Eurofins Lancaster Laboratories' Extractables and Leachables Symposium for Drugs and Devices in Pennsylvania.

Detailed Extractables/Leachables Studies

Extractables and leachables studies usually follow a two-step program. In the first step, an exaggerated extraction is conducted using simple solvent conditions more aggressive than those anticipated to be realized in a clinical setting in order to determine to determine the complete extraction profile and to identify the potential extraction compounds, desired or undesired. In the second step, a leaching study is conducted that attempts to simulate the clinical environment of the target application. The simulated leaching environment often comprises a more complex biological matrix than those used for extraction, which in turn complicates the chemical analysis assays used to identify and quantify the leaching materials. In this presentation, we show examples of studies that required a more detailed testing assay to identify and quantify compounds coming from implanted medical devices.

Biography

Adam Kozak specializes in the chemical and mechanical analysis of polymer materials and medical devices. He has substantial experience in the use of gas chromatography-mass spectroscopy (GC-MS) for leachables and extractables analysis, trace impurity/contaminant analysis, residual monomer content, unknown compound identification, migration levels, deformulation, and odor analysis by headspace GC-MS and other chromatographic techniques.

Figure 1: On left, flax seeds, the source of linseed oil. On right, a representative triglyceride found in a linseed oil.

Linseed oil is derived from flax seeds (Fig 1; typically via pressing and solvent extraction methods). In the presence of oxygen, it polymerizes to form a rigid and hydrophobic solid through a highly exothermic oxidation reaction. This is often misleadingly referred to as “drying”; the process does not depend on evaporation of water but rather on the presence of oxygen to form crosslinks between adjacent triglycerides at points of fatty acid unsaturation (Fig 2). The resulting material is an excellent example of a naturally derived, crosslinked polymer whose properties arise from a complex mixture of its constituents and the processing conditions under which it is formed.

Figure 2: Polymerized and hydrogenated linseed oil. Crosslinks between triglycerides (formerly double bonds) are indicated with arrows.

Due to its easy workability and advantageous “dry” properties, linseed oil is widely used in applications ranging from nutritional supplements (it is rich in Omega-3 fatty acids), as a binder in paint, and in linoleum. In fact, the name “Linoleum” derives from the Latin words for flax (“linum”) and oil (“oleum”). In its native state, linseed oil is a liquid composed of triglycerides (Fig 1) consisting of various combinations of fatty acids—the most common being linoleic acid, alpha-linoleic acid, palmitic acid, stearic acid, and oleic acid.

Figure 3: Example separation of 37 fatty acids by gas chromatography for subsequent quantitation.

The fatty acid fingerprint of a triglyceride-based oil (Fig 3) is a valuable analytical tool for material/compositional identification, correlation to functional properties, good/bad analysis, and quantitative analysis. CPG has a variety of methods which may be leveraged for the analysis of oils—this includes techniques such as gas chromatography (which may be coupled to mass spectrometry; GCMS) which can be used to determine the fatty acid distribution for a sample. 

Note that due to the highly exothermic nature of the polymerization process, linseed oil should be handled with care and is often treated as a fire hazard. Over the years many fires have been attributed to the spontaneous combustion of oil soaked rags, often abandoned after a painting job. Such rags are especially dangerous because they present a very high surface area and accelerate the oxidation reaction. If left balled up, temperatures may rise sufficiently high to ignite the entire mass. The Massachusetts Office of Public Safety and Security offers clear guidelines for the safe disposal of such materials.

Working with natural oils? Looking to better understand differences between performance of naturally derived materials? Contact us to see how our team may be able to assist in your project.

Cambridge Polymer Group has received notification of the award of their patent “Thiolated PEG-PVA Hydrogels” (14/328,176).  This patent describes a new way of creating hydrogels from a conventional biomaterial poly(vinyl alcohol).  The resulting hydrogels cure under physiological conditions with no toxic crosslinkers or bi-products and can be tuned to degrade over periods of weeks. 

They are therefore likely to see value in drug release, temporary tissue bulking or scaffold applications, and although they can be cured in vitro, they appear particularly well suited to in vivo applications.  In fact, we are already working to commercialize this material through the recent award of an NIH Phase 1 SBIR grant in the field of retinal detachment.