SPUS: The Secret Behind the Success of the SynCardia TAH

Learn what makes segmented polyurethane solution — the primary ingredient used to manufacture the SynCardia temporary Total Artificial Heart — so special.

Segmented polyurethane solution, or SPUS, is a highly durable and versatile chemical compound used to manufacture the SynCardia Total Artificial Heart (TAH). Because of its high degree of fatigue resistance, strength and biocompatibility, SPUS is ideally suited for use in the flexible and blood-contacting components of the TAH, which includes the housings, diaphragms and connectors. SynCardia has been manufacturing SPUS in house since 2011, and is the world’s sole owner and producer of this proprietary formulation.

Manufacturing SPUS

The original formulation of SPUS was developed for use in the textile industry — a close relative is Spandex (also known as Lycra or elastane). Because of its excellent physical and mechanical properties and biocompatibility, its potential medical applications were discovered — most notably, by Dr. Willem Kolff and his team — and the compound started to be used in early artificial hearts.

Today, a highly-refined, proprietary formulation of SPUS is used exclusively in the SynCardia TAH. The substance is manufactured in SynCardia’s SPUS lab at its headquarters in Tucson, Arizona. The process primarily takes place inside of an 11-foot tall reactor — a highly controlled environment in which critical factors like temperature, mixing speed and humidity can be controlled according to the exact specifications of the formula.

Testing Process

After the manufacturing process is complete, every batch of SPUS undergoes comprehensive testing before it is approved for use in TAH manufacturing. Some of these tests are conducted internally, while others are conducted by certified third-party labs.

Internal Testing:

  • Viscosity: Ensures SPUS thickness is uniform and to specification.
  • Gardner Color Test: Analyzes the final color to ensure purity.
  • Percent Solids: Measures the exact volume of each substance in the SPUS, ensuring that all of the requisite compounds made it in and out of the reactor.
  • Free Amine Titration: During the manufacturing process, amines are added to extend and terminate the polymer — this test measures how many amines remained in the final solution.

External Testing:

  • Fourier Transform-Infrared Spectroscopy (FTIR): Analyzes the basic chemical structure of the compound — i.e., determines that the end-product has all of the correct functional groups.
  • Mechanical (multiple tests):
    • Tear: The amount of force (lbs/inch) required to rupture SPUS under controlled conditions — must be capable of withstanding ≥ 350 lbs/inch.
    • Tensile: The capacity of SPUS to withstand a force (psi) causing it to stretch or elongate (tested until failure) — must be capable of withstanding ≥ 5000 psi (typical: 8000 psi).
    • Modulus: The measure of stiffness (psi) — must be capable of withstanding 462-704 psi.
    • Yield: The point of elongation (% of original length) at which a specified amount of SPUS breaks or fails — must be capable of withstanding ≥ 500% elongation (typical: 650%).
  • Gel Permeation Chromatography (GPC): Determines the molecular weight of the polymer.
  • Cytotoxicity: Ensures that the end-product is entirely comprised of bio-inert materials, meaning that the materials will not cause a reaction when introduced to biological tissue.

Material Properties

The testing SPUS undergoes requires the compound to withstand forces and pressures significantly greater than anything it will actually experience in the body. During patient use, the SPUS diaphragms typically experience between 2 and 6 psi (100 to 300 mmHg). This is well below the plastic deformation point of SPUS, which is typically between 100% and 150% elongation (approximately 500 to 750 psi).

The TAH can provide reliable, long-term support for patients, some of whom have been supported for more than four years. Assuming the TAH is set at a typical beat rate of 130 bpm, over a four-year period, the diaphragms will beat in excess of 273.3 million times.

By utilizing a compound capable of withstanding pressures and environments far more extreme than those found within the human body, SynCardia is able to offer patients a heart replacement device capable of providing the long-term, reliable support they need while they await their donor heart transplant.

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