Rubber plunger surface texturing for friction reduction in medical syringes

Friction https://doi.org/10.1007/s40544-018-0227-5 ISSN 2223-7690 CN 10-1237/TH RESEARCH ARTICLE Rubber plunger surface texturing for friction reduction in medical syringes Haytam KASEM1,2,*, Harel SHRIKI1, Lihi GANON1, Michael MIZRAHI3, Kareem ABD-RBO1,3, Abraham J. DOMB3 1 Department of Mechanical Engineering, Azrieli College of Engineering, Jerusalem 9103501, Israel 2 Tribology Laboratory, Department of Mechanical Engineering, Technion, Haifa 32000, Israel 3 School of Pharmacy, Faculty of Medicine, the Hebrew University, Jerusalem 9103501, Israel Received: 16 November 2017 / Revised: 27 March 2018 / Accepted: 21 June 2018 © The author(s) 2018. This article is published with open access at Springerlink.com Abstract: Friction is a genuine issue in the use of many medical devices involving rubbery materials such as plungers in medical syringes. This paper presents a new direction for the reduction of friction in medical syringes based on surface texturing of the rubber plunger. The specimens were prepared by casting poly(vinylsiloxane) (PVS) rubber into a pre-fabricated negative template obtained by 3D printing. Friction tests were performed on a home-made test-rig. It was clearly shown that friction resistance can be considerably manipulated when using textured plungers. Keywords: syringe; rubber; plunger; surface texturing; friction; medical syringes 1 Introduction Low-friction combined with smooth movement is vital characteristic for drug-delivery devices particularly in medical syringes. Current materials and design, however, do not optimize these features. Indeed, many medical syringes use a rubber plunger for adequate sealing and compliance. In addition, it has been well established that the laws of metallic friction do not apply to rubber, since contacting asperities are primarily in the plastic loading range in the friction of metallic materials [1, 2]. In contrast, rubber asperities in contact remain generally in the elastic range [3]. Thus, appropriate friction mechanisms should be considered when investigating friction of rubber materials. Relevant findings of many research projects on the friction of rubber lead to the current acceptance of three distinct friction mechanisms that develop, when rubber slides against harder counterface: adhesion [4], bulk deformation and hysteresis [5], and wear [6]. An early study published by Roth et al. indicates that the rubber coefficient of friction decreases as the applied normal load increases [7]. Another study published by Schallamach reports that, analogously to metals, the friction force generated by rubber is proportional to the real contact area of asperities contacting the antagonist surface [8]. It has also been reported that at higher speeds the compressed rubber surface in front of the asperity undergoes a buckling that produces detachment waves known as Schallamach waves [9]. In the light of the above, the complexity of rubber friction mechanisms indicates that friction is a practical issue for many uses of medical devices involving rubber, such as syringes, injection pens, and auto-injectors. For instance, a toohigh friction value between plunger and barrel in a medical syringe may influence the delivery of a drug, resulting in the patient receiving a spurt of the drug or a smaller than expected dose. Known solutions to the problem often deal with plunger design. These solutions are generally protected by patents [10, 11]. However, no attempts based on tribological considerations have yet been reported to resolve the problem of reducing friction between rubber * Corresponding author: Haytam KASEM, E-mail: Friction 2 plunger and barrel in medical syringes. Rubber surface texturing was considered for increasing friction in a wet contact condition. Kown et al. studied the friction of a micro-patterned elastomer in an attempt to enhance adhesion on a small intestinal surface for the development of friction-based clamping mechanisms for robotic endoscopic microcapsules [12]. Similarly, Tsipenyuk and Varenberg used rubber with a biomimetic hexagonal textured surface to increase friction against lubricated skin for shaving applications [13]. Even though rubber surface texturing has already been taken into account for increasing friction in a wet condition, it can be considered for decreasing friction in a wet condition like in medical syringes, as well. Thus, the present work aims at investigating, for the first time, the possibility of decreasing friction between rubber plunger and barrel in medical syringes by introducing surface texturing in the plunger. The concept is based on using a micro-surface structure in the form of micro-dimples obtained by casting rubber into a pre-fabricated negative template. The surface texturing in the form of micro dimples has been developed during the recent decades and was proved as an effective technology for reducing friction between mechanical components undergoing relative motion under lubricated condition [14]. It consists of the application of texturing to one of the two mating surfaces under lubricated contact. Due to relative motion, a hydrodynamic pressure is produced tending to separate the mating surfaces, thus reducing the friction between them. Indeed, each micro-dimple acts as a micro-hydrodynamic bearing to enhance hydrodynamic lubrication. The load carrying capacity provided by each micro-dimple is due to an asymmetric hydrodynamic pressure distribution over the dimple [15]. While this technology was essentially developed for hard materials such as sealing and cylinder liners [16, 17], in which both of the mating surfaces were rigid. Surface texturing, however, can also be effective in tribological contacts involving mating surfaces of which one is soft (Young’s modulus around 10 MPa) [18, 19]. In this case, the soft material may be deformed under the influence of the hydrodynamic pressure and viscous shear. Such cases are known as soft elastohydrodynamic lubrication (SEHL). Typical SEHL applications can be found in elastomeric seals [20–23], e.g., elastomer bearings [24], and metering size press [25]. In the present study we intend to apply this concept to textured rubber plungers (micro-dimples on the surface) and investigate by experiments the impact on the dynamic friction. To this end, three plungers having different textured surfaces but made of the same poly(vinylsiloxane) (PVS) rubber material have been prepared by casting. They were tested in medical syringes with three different fluids: water, alcohol (ethanol) and glycerol. Friction tests were performed at different sliding velocities. 2 Experimental 2.1 Materials Poly(vinylsiloxane) (PVS) was obtained from Coltène Whaledent AG, Altstätten, Switzerland. This material has been used for the manufacturing of bionic textured surfaces for enhanced friction and adhesion [26, 27]. 2.2 Smooth and textured plungers The purpose of the present work is to investigate experimentally th (...truncated)