Creep of PVDF monofilament sutures: service performance prediction from short-term tests Joa ˜o F. Mano a,b, * ,1 , Joa ˜o L. Lopes c , Rui A. Silva c , Witold Brostow d,2 a Department of Polymer Engineering, University of Minho, Campus de Azure ´m, 4800-058 Guimara ˜es, Portugal b 3B’s Research Group—Biomaterials, Biodegradables and Biomimetics, University of Minho, Campus de Gualtar, 4710-057 Braga, Portugal c ISEP—Instituto Superior de Engenharia do Porto, CIEA, R Dr Anto ´nio Bernardino de Almeida, 431 4200-072 Porto, Portugal d Laboratory of Advanced Polymers and Optimized Materials (LAPOM), Department of Materials Science and Engineering, University of North Texas, Denton, TX 76203-5310, USA Received 23 April 2003; received in revised form 23 April 2003; accepted 30 April 2003 Abstract Isothermal short-term creep of poly (vinylidene fluoride) (PVDF) monofilament sutures was determined at several temperatures between 10 and 90 8C under the stress of 10 MPa. Long term service performance was predicted for 10 decades of time. The compliance master curve as a function of time fits a hyperbolic sine equation. The temperature shift factor as a function of the temperature a T ðT Þ is accurately represented by a general equation based on free volume. A simple relationship between the two parameters of the equation is explored. The viscoelasticity of PVDF is also seen in dynamic mechanical analysis performed at the frequency of 1 Hz. The origin of the viscoelastic character well present in the deformability of the PVDF in service is due to the occurrence of the a c relaxation that is active at ,50 8C(E 00 peak at 1 Hz). q 2003 Elsevier Science Ltd. All rights reserved. Keywords: Mechanical performance; relaxation behaviour; Biomedical applications 1. Introduction and scope The interest in fluorinated polymers is closely connected to a number of their useful properties, including resistance to harsh environment, unique pyro- and piezo-electric characteristics [1–3] and also the capability to lower friction [4] and increase scratch resistance of thermosets [5]. Among fluoropolymers, polyvinylidene fluoride (PVDF), processable as a thermoplastic has a wide applications range from electronics to medicine—as a homopolymer, copolymer or blended with other polymers [3,6]. The versatility of PVDF in a wide range of applications has rendered this material certain popularity. When an alternative suture material turned out necessary, especially in vascular surgery, PVDF was presented to the medical device industry as a logical substitute for polyester [7] and polypropylene (PP)-both synthetic non-absorbable sutures. The use and need for a suture should not be considered as a problem that needs solution, but rather as a solution that needs improvement. Dermatologic wounds can heal by themselves or—depending on the extension and depth—be closed by using a variety of methods. Notwithstanding the surgeon importance, the choice of the correct suture is fundamental for tissue healing and patient recovery. Usually, this choice takes into account the patient, the type of wound and tissue characteristics and also the anatomic region. The physical characteristics of a suture comprise the diameter, capillarity and hygroscopicity, tensile strength, knot strength, elasticity, plasticity, mem- ory, and configuration. The configuration is based on the number of strands used to produce the suture, i.e. either single-stranded (monofilament) or multi-stranded (multi- filament). For a long time, silk has been utilized as a non- absorbable suture (although it degrades within 24 months, when implanted), because of its softness and pliability. However, since it is a natural multi-filament, it can provide 0032-3861/03/$ - see front matter q 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0032-3861(03)00361-6 Polymer 44 (2003) 4293–4300 www.elsevier.com/locate/polymer 1 http://www.dep.uminho.pt/3bs. 2 http://www.unt.edu/LAPOM/. * Corresponding author. Address: Department of Polymer Engineering, University of Minho, Campus de Azure ´m, 4800-058 Guimara ˜es, Portugal. Fax: þ351-253510339. E-mail addresses: jmano@dep.uminho.pt (J.F. Mano), jll@isep.ipp.pt (J.L. Lopes), ras@isep.ipp.pt (R.A. Silva), brostow@unt.edu (W. Brostow).