Biological Properties of Plastics

The performance of a medical device is dependent on the behavior of the material that it is composed of, under in vivo conditions. Since many devices are either fully or partly composed of plastics, the biological properties of plastic are crucial in its use in medical applications. The manner in which a plastic material interacts with the human body is crucial to determining the use of the plastic in medical applications. In this context, biocompatibility and biostability are vital aspects in determining the biological properties of plastic.

Biocompatibility can be defined as the response of the body to the material that it comes in contact with. The adverse reaction, if any, of the implanted plastic on bodily fluids and tissues, can be determined by both in vitro and in vivo techniques.

Another important aspect that needs to be considered for longer term implants is the stability of the plastic material with respect to the environmental conditions within the body. Biostabilty can be defined as the resistance of the material to the degradative forces originating from the body environment.

In some ways, one can term biocompatibility as the reaction of the body to an implanted device whereas biostability can be said to be the reaction of the implanted device to the environment in the body. Biocompatibility is of relevance to all medical devices whether those that come in short term contact with the body or those which are permanently implanted. Biostability, on the other hand, is of greater importance as the device implantation duration increases.

There are a number of tests designed to evaluate the biocompatibility of a device. These tests depend on the nature of the device and its contact with bodily fluids.
These tests can be short term in vitro such as cytotoxicity or long term in vivo such as implantation studies. The tests detailed out in the ISO 10993 series of standards which are often used as the starting point to define the tests required for any device.

ISO stands for International Organization for Standardization, an international standards development organization [1]. ISO 10993 defines the various tests that regulate the evaluation of a device for biological compatibility [2]. ISO 10993, as of November 2007, is divided into a number of parts detailing the testing, elaborated below:

• Part 1: Evaluation and testing

• Part 2: Animal welfare requirements

• Part 3: Tests for genotoxicity, carcinogenicity and reproductive toxicity

• Part 4: Selection of tests for interaction with blood

• Part 5: Tests for cytotoxicity: in vitro methods

• Part 6: Tests for local effect after implantation

• Part 7: Ethylene oxide sterilization residuals

• Part 8: (withdrawn)

• Part 9: Framework for the identification of potential degradation products

• Part 10: Tests for irritation and sensitization

• Part 11: Tests for systemic toxicity

• Part 12: Sample preparation and reference materials

• Part 13: Identification and quantification of degradation products from polymers

• Part 14: Identification and quantification of degradation products from ceramics

• Part 15: Identification and quantification of degradation products from metals and alloys

• Part 16: Toxico-kinetic study design for degradation products and leachables

• Part 17: Establishment of allowable limits for leachable substances

• Part 18: Chemical characterization of materials

• Part 19: Physico-chemical, mechanical and morphological characterization

• Part 20: Principles and methods for immune toxicology testing of medical devices

There are no standardized tests for biostability. These tests are often based on the application of the device and the particular environment the device encounters in service. The wide variety of in vitro and animal tests used for biostability leave the results open to interpretation and the relevance of the results is debated as the results often do not reflect conclusions obtained from actual human clinical data [3, 4]. Nevertheless, the biostability tests can act as an important screening exercise and pointers to material performance in vivo.

Before use as a medical device, the material and the device have to undergo sterilization to render it effectively aseptic for the sterile environment of the body [5, 6]. Even though sterilization is not strictly a biological property of the material, the behavior of the material in the sterilization process is an important pointer to the suitability of the plastic in its application within the medical sector.

1. ISO. ISO - International Organization for Standardization. [online] Available at: https://www.iso.org/home.html [Accessed 24 Jul. 2016].

2. Gad, S. (2013). Standards and methods for assessing the safety and biocompatibility of biomaterials. Characterization of Biomaterials, pp.285-306.

3. Braun, U., Lorenz, E. and Maskos, M. (2011). Investigation of the durability of poly(ether urethane) in water and air. IJAO, 34(2), pp.129-133.

4. Coury, A., Slaikeu, P., Cahalan, P., Stokes, K. and Hobot, C. (1988). Factors and Interactions Affecting the Performance of Polyurethane Elastomers in Medical Devices. Journal of Biomaterials Applications, 3(2), pp.130-179.

5. Lerouge, S. and Simmons, A. (2012). Sterilisation of biomaterials and medical devices. Cambridge: Woodhead Pub.

6. Booth, A. (1999). Sterilization of medical devices. Buffalo Grove, Ill.: Interpharm Press.

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Author
Ajay D Padsalgikar, (Ph.D. California,USA)
Trainer, Polymerupdate Academy