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CPG researchers Yuri Svirkin, Adam Kozak, and Gavin Braithwaite will be presenting their work on thiol-modified PVA hydrogels at the Fall Materials Research Society Meeting in Boston at 4:45 pm on December 3rd (Paper E5.09).

Abstract

Poly(vinyl alcohol) (PVA) is a well-respected biomaterial and forms highly hydrated hydrogels. It has been used in a number of applications, such as tissue bulking and nerve-guides, but is not intrinsically biodegradable, nor substantially mucoadhesive. These features can be built in to the molecule through complex co-polymerizations, but here we describe a simpler route involving modification of existing off-the-shelf materials.
Biodegradable hydrogels based on PVA modified with thiol groups (TPVA) were prepared and characterized. The TPVA was synthesized by an esterification reaction of PVA with 3-mercaptopropionic acid and characterized by 1H NMR. The TPVA produced contained pendant chains with ester bonds linking the thiol groups to the PVA backbone. Further, hydrogels were synthesized from this TPVA in a reaction between the TPVA and acrylate derivatives of poly(ethylene glycol) using Michael-type addition. The gelation reaction between the TPVA and PEG-acrylate proceeded under physiological conditions in aqueous environment without radical initiators or irradiation. The kinetics of gelation, including gelation time and dynamic modulus, were determined by rheology. The properties of the final hydrogels and cure characteristics were investigated as a function of pH, polymer concentration, molecular weight, degree of PVA and PEG chemical modifications and their ratio in the composition. The gelation time varied from seconds to 30 minutes and the equilibrium elastic modulus (G’) was in the range 500 Pa to 10 kPa.
The hydrogels were rendered degradable by the presence of the ester groups, which are easily hydrolysable and do not require the presence of enzymes for degradation to occur. In addition, the thiol-functional groups impart mucoadhesive properties to the PVA hydrogels, as has been reported elsewhere. The level of mucoadhesion was controlled by the amount of free thiol functionalities remaining uncrosslinked after the hydrogel formation reaction between the TPVA and PEG-acrylate molecules.
The degradability and swellability of these PVA-PEG hydrogels was tested in 1xPBS under ambient conditions. The hydrogels at 3 wt % polymer solids started losing mechanical integrity after 18 days and completely dissolved after 35 days. In addition, a specialized peel test was developed to measure the mucoadhesive properties of the hydrated TPVA hydrogel.

More information.

Density, or the ratio of the mass of an object to its volume, is a commonly reported material parameter. Density is influenced by the chemical composition of the material, crystallinity, and porosity. The chemical composition depends on the elements that make up the material. Plastics are normally composed of hydrocarbons (carbon and hydrogen), which have lower atomic masses and tend to have densities less than 1 g/ml. Metals, which have higher atomic masses, tend to have densities in excess of 1 g/ml. Water at room temperature has a density around 1 g/ml, which is why plastics tend to float, and metals tend to sink in water.

There are two common methods of measuring the density of solid materials. The first involves a density column, which is a tall cylinder containing a mixture of fluids, often alcohol and water, that establishes a gradient in density, with the most dense fluid at the bottom, and smoothly decreases in density as one moves up the column. Floats of known density are placed in the column, where they settle at a height in the column where the surrounding fluid is equal to their density. An unknown piece of a sample is then placed in the column, where it will descend until it reaches a point in the column where its density matches the column density. By interpolating its position relative to the calibration floats, the density of the sample can be determined. This method is described in ASTM D1505.

In the second method, an analytical balance is required. A piece of an unknown sample is weighed, providing its mass. The sample is then immersed in a fluid of known density, such as water or alcohol, where its mass is counteracted by buoyancy forces (see How to Measure Volume), allowing one to measure the volume. The density is then calculated from the ratio of mass to volume. This method is described in ASTM D792.

The medical device task group of ASTM met in Jacksonville, FL from November 12-15th. The meeting starting with a workshop on modularity and tapers in total joint replacement devices, and discussed potential issues with femoral head fretting.

The cleanliness task group worked on two draft standards. The first, a guideline for validation of medical device cleanlines, is now ready for balloting. The second, preparation of test soils for cleanliness testing, is making good progress, and will be ready for balloting soon. The task group also discussed starting a new standard for measuring cleanliness levels of plastic implants.

The PEEK task group heard a presentation about material properties similarities and differences between different grades of PEEK, and further discussed how to replace the test method for heavy metals as lead with a more current assay.

The UHMWPE task group discussed the up-coming round robin study of small punch on UHMWPE (samples are being sent). Net ash on consolidated UHMWPE will be removed from ASTM F648, and new draft standards for electron spin resonance characterization and fatigue crack propagation are in consideration.

In the bone cement group, the preliminary results of a round robin study on the titration method for benzoyl peroxide quantification was presented and discussed.

For the May 2014 meeting, a workshop on Additives in Biomedical Plastics will be held. A soliciation will go out shortly, but all those interested in presenting can also contact Stephen Spiegelberg from Cambridge Polymer Group, who is one of the chairs.

The molecular weight distribution of polymers strongly influence their properties, such as tensile strength, crack resistance, and solubility. Gel permeation chromatography is a commonly used technique to measure molecular weight distribution, but relies on the ability of the polymers to be dissolved in a solvent that is readily usable in a GPC column. Additionally, the molecular weight distribution is inferred from polymer standards, which should have a similar, if not identical, repeat unit to the unknown sample for the most reliable results.

Melt rheometry can be be used to determine molecular weight distributions, using the fact that the viscosity of a polymer is strongly dependent on the molecular weight and distribution. For instance, at low shear rates, the zero shear viscosity of a polymer melt scales with the weight average molecular weight to the 3.4 power. Similarly, the dynamic viscoelastic properties of polymer melts can be used to determine the overall molecular weight distribution. To do so, a frequency sweep is conducted on the sample, providing the G' and G" as a function of frequency. Using theoretical models by Mead or Thimm, the molecular weight distribution can then be determined, assuming that the model parameters are known for the polymer in question.

Contact CPG for more information on this technique.

CPG recently purchased a new impact tester, a CEAST 9050 (Instron). With both V-notching and blade notching capabilities, we can perform Izod impact testing on materials in compliance with ASTM D256, as well as impact testing on UHMWPE per ASTM F648. The pneumatic release option on the impact tester allows very reproducible results.

CPG researcher Dr. Stephen Spiegelberg was recently elected to the post of Recording Secretary for ASTM Committee F04 “Medical and Surgical Materials and Devices”.  Click here for more information on the committee. The committee's 160 members meet biannually, attending over two days of technical meetings capped by a symposium or workshop on relevant topics in the medical/surgical materials and device industry.