The Project: Test the mechanical strength of drug-eluting, polymer fiber stents to determine their ability to be utilized in specific applications.

The Solution: Use benchtop tensile testing technology to verify the strength of the flexible stents.

A typical plastics testing lab will house a variety of benchtop materials testing systems, including tensile testing equipment, a plastics impact tester, melt indexers, plus software for automated testing.
By Harold E. Yohn
In any industry, quality assurance is crucial to the success or failure of a new product. But in the medical device industry, it can conceivably be a matter of life and death.

A defect in a medical product could have profound and devastating impact on disease management, treatment outcomes and, ultimately, even a patient’s well being. As such, materials testing is a critical component of medical device research and development, especially when a medical manufacturer comes up with a technological advancement that’s particularly innovative.

TissueGen Inc. is such a manufacturer. Founded in 2000 to develop and commercialize drug-releasing (or eluting) polymer fiber stents, the company has produced a revolutionary peripheral vascular stent made with biodegradable polymers that, when combined with second-generation drugs, promises breakthrough medical treatments.

“As our stents are 100% polymer, the strength of the material is key to their correct functionality,” says Kevin Nelson, PhD, TissueGen’s founder and chief scientific officer. “Therefore, we need a completely reliable and highly accurate testing method to assess the mechanical strength of the material.”

To ensure the requisite reliability and accuracy, TissueGen turned to Tinius Olsen, a leading designer and manufacturer of materials testing equipment. The company’s vast array of machines provides thorough testing regimens that determine the suitability of materials for specific applications. Typically used in research and quality control efforts, these machines accurately measure the strength and performance of diverse materials.
Polymer Stents
The technology especially suits TissueGen’s purpose. TissueGen has a proprietary manufacturing process that enables the company to incorporate biologically active agents (i.e., therapeutic proteins) into its devices.

Stents are tubes, usually made of metal, that a physician inserts into a narrowed artery or duct inside a patient’s body to maintain flow of fluids (i.e., blood). Stents are most often used to treat atherosclerosis, a condition that occurs when arteries become narrow or completely blocked. Typical drug-eluting stents are coated with a medication that helps prevent restenosis, or reclosure, of the coronary arteries and is left permanently in the artery. In contrast, TissueGen’s stent contains drugs that help the vessel heal. As that healing process takes place, the body no longer requires the stent, so it slowly degrades in the end, leaving only natural, healed tissue behind.

In addition to coronary artery disease, metal stents have been used to treat peripheral vascular disease, which involves blood vessels outside the heart and brain and often manifests as a narrowing of vessels that carry blood to the legs, arms, liver, or kidneys. However, the conventional metal stents have proven less successful for this condition, as they can lead to unacceptably high restenosis rates. Also, the metal stents can fracture inside the peripheral vascular system, causing further injury to the patient.

TissueGen’s all-polymer, drug-releasing, biodegradable stents represent a major advancement over metal stents, especially in treating peripheral vascular disease. TissueGen’s stents offer unique benefits over available technology. They can deliver either conventional anti-restenosis drugs or more powerful second-generation drugs such as growth factors or enzymes. The device will work like metal stents, but its polymer format is physically more flexible. Being biodegradable, it will gradually transfer dynamic loads back to the artery while delivering its drug over a longer time frame. Bioabsorption and second-generation drugs will work in concert with the body’s natural healing response, offering improved patient health and cost-effectiveness.

In addition to treatment of vascular disease, TissueGen’s technology could potentially address a broad variety of clinical applications including prostate and urinary disorders, solid tumor remediation, peripheral nerve healing, spinal cord repair, dermal wound healing, and treatment of retinal disease.

It is almost as if Tinius Olsen technology was tailor-made to support the development and subsequent production of such a device. The company’s light force test machines have proved valuable to the polymers and medical industries. Overall, the company’s complete line of testing technology has been lauded for its excellence by players in industries such as metals, plastics, textiles, automotive, aerospace, food, packaging, rubber, adhesives, composites, construction, and paper.
Tensile Testing
Tinius Olsen’s machines offer a wide series of tests in accordance with key international and industrial testing standards including ISO, ASTM, EN, and others. These tests include shear, compression, flex/bend, puncture/burst, tear, peel, melt flow, Charpy and Izod impact, friction, stiffness, heat distortion temperature, Vicat penetration, torsion, and tension. It is the last capability that most interested TissueGen.

The melt indexer (extrusion plastometer) can be used for manual (Procedure A) or automatically timed (Procedure B) testing.
“For our peripheral vascular polymer stents, we required tensile testing,” comments Nelson. “It’s pretty straightforward, as it involves stress relaxation in both the axial and radial directions.”

Indeed, tensile testing is regarded by many as the easiest and most widely used test to best determine a material’s mechanical properties. Test results greatly influence product design across many manufacturing sectors, as many structural components function under tension.

One of TissueGen’s primary goals is to make its polymers stronger and stronger, and the key to achieving that is the company’s ability to track performance and productivity, says Nelson. “The Tinius Olsen technology makes it possible to ensure that we are doing just that, to see if we are on the right track,” he says. “It’s an optimal form of quality assurance. It provides us with a target to shoot for, and it helps us identify the modifications we need to make to achieve the target.”
Benchtop Testers
In opting for the technology, TissueGen looked to the company’s benchtop materials testing machines. Tinius Olsen’s line of benchtop equipment is designed to test a wide spectrum of materials including metals, plastics, rubber, textiles, paper, yarn, wire, adhesives, ceramics, timber, foils, composites, finished components, and, of course, polymers in tension, compression, flexure, or shear.

Specifically, TissueGen chose the H5kT model, a single-column tester that is part of Tinius Olsen’s T series, which the company introduced in 2001.

“I was very impressed with Tinius Olsen because they make very large tensile testers,” recalls Nelson. “I knew that they were the recognized leader when it came to big-size testing machines, so when I learned about their benchtop testing machines, that was sufficient for me to make a decision.”

The T series builds on the technology and quality engineering used in Tinius Olsen’s S Series to provide optimal durability, convenience, and high accuracy. The H5kT has the same capabilities as equipment in the S Series, but at a reduced cost. Moreover, the T-Series was designed by specialist materials testing engineers experienced in using leading edge technology to provide practical, high quality solutions.

The T Series range of tension and compression materials testing machines have been specially developed to meet the complex technical requirements of research or quality control engineers operating in laboratory, factory, and educational environments. With the series’ reliability and accuracy, customers are guaranteed force accuracy of 0.5% of the applied force, extension resolution of 0.0001 mm, and speed accuracy of 0.005%.

The equipment can perform a range of tests to check, prove, and monitor the quality of incoming raw materials and outgoing finished products, assuring the delivery of guaranteed quality goods.

The T Series is compatible with more than 1,000 test specimen holders, grippers, and attachments, enabling the machines to perform more than 1,300 certified and traceable mechanical and physical tests across a broad range of materials.
The T series is designed for direct connection to a PC running Windows-based software. For all of its products, Tinius Olsen offers a comprehensive range of software; each designed to make testing simple, precise, and efficient, no matter what the material being tested.

The company provides several software features common to all programs, but Tinius Olsen feels the most important feature is the ability to further customize testing parameters used to collect and document testing data and, in many cases, control the machine as well. Standard software features include powerful databases, customized report generation, standard SPC programs, recall and reanalysis of raw data, and user-configurable machine parameters and control settings. The system automatically saves test parameters and results, ensuring no data loss if the PC crashes or PC power fails.

Another aspect of Tinius Olsen technology is its user-friendliness. Despite the complex and important tasks Tinius Olsen machines perform, the technology doesn’t overwhelm. Machines are supplied with all necessary tools, components, and manuals for easy installation and easy use. Further, customers find it very easy to train new operators.

“The equipment is very intuitive,” comments Nelson.

Overall, TissueGen has been quite pleased with its choice. “We’ve been using our H5kT for two years now, and we’ve been very happy with the quality it provides,” says Nelson.
For additional information on the technologies and products discussed in this article, see the following websites:Harold E. Yohn is a product application specialist at Tinius Olsen, 1065 Easton Rd., Horsham, PA 19044. He is responsible specifically for all forms of plastics testing. He can be reached at 215-675-7100 or