Technology

Polymer Processing and Material Attributes

TephaFLEX® Polymer

The TephaFLEX® polymer is a semicrystalline material with a melting point of approximately 60°C and is suitable for processing by all methods used for thermoplastic polymers. The most experience gained to date has been in the production of fibers through conventional melt spinning processes. Both monofilament and multifilament materials have been produced. These fibers can then be further processed using conventional textile processes such as braiding, knitting and weaving. Injection molding and film extrusion have also been used. Typical mechanical properties are shown in the table below. Tepha’s polymer is ductile and does not exhibit brittle fracture characteristics.

Property

Compression Molded Film

Oriented Fiber

Tensile Strength           MPapsi

50

7,500

580 – 800

85,000

Tensile Modulus           MPaPsi

70

10,000

670

110,000

Elongation at Break        %

~ 1000

~ 25-90

Hardness                     Shore D

53

ND

Typical mechanical properties of TephaFLEX® P4HB

P4HB is also soluble in a number of polar organic solvents such as chloroform, methylene chloride, tetrahydrofuran, and dioxane and so processes involving these solvents may also be used. Processes have been developed that result in very low residual solvent levels. P4HB is not soluble in water, hexanes, ethanol or methanol.

Ethylene oxide sterilization is the preferred method of sterilization and has been shown to be effective with low levels of residuals. γ sterilization has been investigated. In common with most polymers the use of γ sterilization leads to a reduction in molecular weight and a reduction in strength. If this can be accommodated by the device design then this form of sterilization could be an option.

The properties of monofilament suture produced from the P4HB polymer are shown in the table below with comparative data shown for polypropylene and polydioxanone (PDO, the chemical name for PDS®) sutures. This data demonstrates the superior strength characteristics of the P4HB monofilament, with it being 35% stronger than polydioxanone and 19% stronger than polypropylene suture.

 

Fiber

Diameter (mm)

Load at Break (Kgf)

Stress (MPa)

Elongation at Break (%)

Tensile Modulus (GPa)

TephaFLEX® suture

0.154

1.4

728

35*

1.6

PDS II®

0.166

1.2

538

39

1.6

Prolene®

0.143

1.0

610

31

2.9

Comparative properties of monofilament suture

Degradation

Numerous studies have been completed in which P4HB materials have been implanted either subcutaneously or intramuscularly and P4HB has been shown to degrade with loss of molecular weight, mechanical properties and mass as a function of time. The time to complete degradation and resorption varies with processing, i.e. orientation of the polymer and size but in general complete resorption occurs between 12 to 18 months. Orientation prolongs resorption time and thin, non oriented films may resorb in less than 12 months. For example, a 20 µm thick P4HB coating on a metal stent is nearly completely degraded and absorbed within 3 months. While the mechanism of degradation is primarily hydrolytic there is an enzymatic component of the process that leads to some surface erosion and pitting of devices. A consequence of this is that for products where there is a high ratio of surface area to volume such as in small diameter fibers (<300µm), strength retention will be influenced by fiber diameter.

The degradation of monofilament sutures following subcutaneous implantation has been investigated in a number of studies. The trend in loss of molecular weight shows that it approaches zero by approximately 52 weeks. At that timepoint fragments of fiber can still be seen, generally of insufficient quantity to be retrieved for testing. Additionally, the data shows that the Mw loss over time is similar for all suture sizes tested and is independent of size from approximately 0.15 µm diameter to approximately 0.8 µm diameter. These data are shown in the plot below for different sizes of suture.

Reduction of molecular weight of P4HB monofilament sutures as a function of implantation time

 

Strength Retention

The higher initial strength and prolonged in vivo strength retention of TephaFLEX® suture compared to polydioaxanone is shown in the figure below for size 2 suture.

And is shown below for size 5-0 suture.

Compared to PDS® monofilament, TephaFLEX® monofilament sutures possess both higher initial strength and prolonged strength retention.


Prolonged strength retention of TephaFLEX® fiber is also present in knitted constructs such as a surgical mesh.

Biocompatibility

For any medical device ISO 10993 biocompatibility testing has to be performed on the final product. Confidence in the safety of devices fabricated from TephaFLEX® polymers can be gained from the numerous sets of ISO 10993 testing that have been completed on the purified polymer as well as Tepha’s finished sterile devices.

*note: Tepha fiber can be manufactured in a wide range of elongations