Microstructure and friction properties of PVA/PVP hydrogels for articular cartilage repair as function of polymerization degree and polymer concentration

Abstract

Low friction coefficients are required for biomaterials in order for them to be used in the repair of articular cartilage. In the present study, polyvinyl alcohol (PVA)/polyvinylpyrrolidone (PVP) hydrogels were synthesized with different degrees of polymerization in the PVA and different polymer concentrations. A repeated freezing-thawing method was used to prepare them. The microstructures of the synthesized compositions gave an insight into the effects of the preparation processes and properties of the hydrogel. The influences of PVA polymerization and polymer concentration on the swelling behavior of PVA/PVP hydrogels in phosphate-buffered saline (PBS) were examined. The friction coefficients of PVA/PVP hydrogels against stainless steel were measured using a rotating ball-on-plate tribometer. The testing variables were: (a) polymerization degree of PVA, (b) polymer concentration, (c) lubrication condition (dry, physiological saline, and bovine serum), (d) sliding speed and (e) load. With increasing polymer concentration and polymerization degree of PVA, the inner structures of the hydrogels tended to be denser. An effective drop in swelling ratio was observed for hydrogels in PBS. The friction coefficient increased with an increase in polymerization degree of PVA, while it decreased with an increase of polymer concentration in the low load region and under liquid lubrication. At long testing times, the friction coefficient of hydrogels under dry sliding conditions increased rapidly owing to a lack of the interstitial fluid, while the friction coefficient remained stable during the entire friction test when fluid lubricated. Biphasic lubrication is proposed to be the key reason for maintaining a low friction coefficient level for PVA/PVP hydrogel when sliding against stainless steel.

Introduction

Articular cartilage, which covers the ends of the long bones in the body, plays an important role in characterizing and maintaining the delicate balance of synovial joint [1]. An acute trauma, osteoarthritis and congenital or acquired joint diseases may cause cartilage damage. However, the damaged cartilage lacks the capability of self-recovery due to its avascular and aneural nature [2]. The surgical techniques, such as microfracture [3], autologous chondrocyte implantation (ACI) [4], osteochondral autograft transfer (OAT) [5], [6] etc., have been widely taken to ease pain and regenerate tissue function. Unfortunately, the technique for microfracture may induce the cartilage defect replaced by fibrocartilage instead of hyaline, so that new cartilage could not meet the requirements of natural cartilage [3], [7]. Both ACI and OAT procedures need two surgical steps to assume a two risk of injure, and the problem of the lack of suitable donors should not be neglected [8], [9], [10]. In general, all of these options cannot reconstruct natural function to articular cartilage. Therefore, there is a push to find a permanent implant to substitute damaged cartilage and to encourage tissue regeneration.

The primary function of articular cartilage is to support and redistribute the high level of mechanical loads. It is pivotal for artificial articular cartilage to possess both mechanical and friction properties to function and load-bearing in vivo [11]. Recent years, encouraging progress has been made in mechanical properties of articular cartilage whose compressive and tensile strength is close to the level of native tissue [12], [13]. Now researchers are committed to the development of friction property of articular cartilage. Various lubrication mechanisms, including weeping lubrication [14], [15], boosted lubrication [16], boundary lubrication [17], [18] and biphasic lubrication [19], [20], [21] have been proposed to explain the low friction level in synovial joints. A consensus is that no single theory can comprehensively explain all the joint lubrication characteristics under various physiological conditions, and a mixed lubrication regime may be more convincing.

A great variety of biomaterials have been proposed as articular cartilage material to replace the damaged tissue. Lopes et al. [22] developed an unmodified bacterial cellulose (BC) pellicle, where the tribological properties of BC against bovine articular cartilage were estimated, and low friction coefficient values (∼0.05) were achieved. Freeman et al. [23] investigated the friction behavior of synthesized poly(2-hydroxyethyl)methacrylate (polyHEMA) hydrogel using an oscillating contact device. The results showed that the average friction coefficients for different conditions ranged from a low value (0.05) to a high one (1.7). In particular, PVA hydrogel is one of the most widely used polymers due to its excellent weight-bearing properties and biocompatibility [24], [25], which has been subjected to intense studies in terms of their friction properties. Li et al. [26] found the friction coefficient of PVA hydrogel decreased from 0.178 to 0.076 in 60 min under cartilage-on-PVA hydrogel. Ma et al. [27] found that the addition of PVP to PVA hydrogel would improve both mechanical and tribological properties. Katta et al. [28] investigated the effects of polymer content, load and lubricant on the friction and wear behaviors of PVA/PVP hydrogels. The results showed that the wear of hydrogel samples was remarkably reduced at higher polymer content, and a transition of the lubricating mechanism happened at the critical 125 N load. Effects of PVP content, sliding speed, load and lubrication condition on the friction properties of PVA/PVP hydrogels were preliminary discussed in our previous work [29].

In the present study, PVA/PVP hydrogels with various polymerization degree of the PVA and polymer concentration were synthesized by a repeated freezing–thawing method. The primary objective of this investigation was to determine the friction properties of PVA/PVP hydrogels in response to stainless steel ball component under different experimental conditions. The effects of polymerization degree of the PVA, polymer concentration, lubrication condition, sliding speed, and load on the friction coefficients were explored by a rotating ball-on-plate tribometer. The effects of polymerization degree of PVA and polymer concentration on the microstructure and swelling behaviors were also investigated.