Characterization of Hexsyn, a Polyolefin Rubber
Authors: Mcmillin Carl R. 
Published: January 2008

Hexsyn is the Goodyear Tire and Rubber Company tradename for a polyolefin rubber synthesized from 1-hexene with 3-5% methylhexadiene as the source of residual double bonds for vulcanization. Under license from Goodyear, this same polymer has been manufactured by Lord Corporation for the hinge portion of finger joint prostheses using the tradename Bion. This rubber is currently licensed to the University of Akron and to the Cleveland Clinic Foundation for use in biomedical applications, and is being used primarily for biocompatible and highly fatigue resistant rubber components in ventricular assist and artificial heart systems. Results are presented from the physical, mechanical, and biological characterization of Hexsyn.

Procedures are described for the synthesis, compounding, and post-molding extraction for Hexsyn. The physical testing of Hexsyn reported includes determinations of its density at 23 and 37°C, initial hardness and hardness after aging in oxygen, blood, pseudoextracellular fluid and polyethylene glycol 600, typical molecular weights determined by gel permeation chromatography/low angle laser light scattering and intrinsic viscosity, thermal analyses by differential scanning calorimetry of Hexsyn gum, and vulcanized Hexsyn after exposure to blood and blood/fatigue conditions. Also reported are results of differential thermal analyses, thermomechanical analyses of virgin and annealed samples, and thermogravimetric analyses conducted in helium and in air. Dynamic mechanical analyses of Hexsyn include Clash-Berg and Rheovibron tests. Swelling was conducted to determine lot-to-lot and sheet-to-sheet variation for quality control and also a number of solvents were used so that the polymer-solvent interaction parameters could be determined. The permeability of Hexsyn to water, water vapor, and a variety of gases is reported. The wettability by contact angle measurements, refractive index, residual solvent analyses, migration of blood components into Hexsyn, melt rheology by Monsanto Rheometer, resistance to acids, and typical mold shrinkage for Hexsyn are reported.

Mechanical testing of Hexsyn includes tensile strength, elongation, and tensile stress (modulus) at 23 and 37°C and after conditions including exposure to blood, pseudoextracellular fluid, polyethylene glycol, oxygen, 100% relative humidity, and fatigue testing. Stress/strain calibration curves, flexural rigidity after aging in blood, tension set, compression set, stress relaxation, and the effect of repeated cycling on the elastic modulus are presented along with the results of Pico abrasion, skid resistance tests on wet concrete. Tear strength and tear energy have been determined by Grave, Crescent, and trouser tear tests (initial and after up to one year in blood). Peel strength is reported of Hexsyn comolded to butyl rubber and adhesively bonded to Ti and a Cr-Mo alloy followed by exposure to polyethylene glycol 300 at 70°C. The hysteresis characteristics of Hexsyn were determined at frequencies of 4 to 35 Hz, and at a near zero rate, and the dynamic resilience was determined at 45°C. Uniaxial and biaxial creep and creep to failure behavior of Hexsyn is shown for six different temperatures and a variety of stresses. Uniaxial increasing strain level fatigue tests of Hexsyn were conducted in blood, saline, air, nitrogen, 95% oxygen, and ozone at 37°C, and in saline at 4 and 60°C. Similar tests with a one mm razor blade cut were conducted in blood, saline, air, nitrogen, 95% oxygen, ozone, distilled water, and albumin at 37°C. The effect of more rapid acceleration and differing cut sized on the fatigue life of Hexsyn was determined. Longer-term fatigue life tests at a single strain level are reported in air and saline, along with results from in vivo and in vitro testing of Hexsyn blood pump diaphragms. Flex fatigue test results are presented from pierced and non-pierced DeMattia tests in air, a tuned fluid oscillator test in saline, and a rolling fatigue test in polyethylene glycol and a pseudoextracellular fluid. A plunger type of biaxial fatigue test was conducted at 24 and 37°C in air at 3 Hz, while disks of Hexsyn were biaxially tested in saline at 33 Hz and 37°C, and at 3 Hz and 23°C. The type of initiation points of fatigue failure were evaluated and compared with other elastomers.

The reported biological testing of Hexsyn includes agar overlay cytotoxicity using mouse and human cell types, rabbit muscle and intracutaneous implants, a hemolysis test, a mouse systemic toxicity test, and a cell growth inhi. bition test. Mutagenicity testing was conducted using both direct application and spot tests of water, F-12 culture media, dimethyl sulfoxide, serum, and ethanol extracts onto TA98 and TA100 strains of Salmonella with and without induced S-9 rat liver enzyme activation.