Molecular dynamics simulation of hyaluronic acid hydrogels: Effect of water content on mechanical and tribological properties

Highlights

• The atomic models with different water content of hyaluronic acid hydrogel were established, and the mechanical parameters of each model were analyzed in detail.

• Under the condition of continuous compression, the atomic space distribution of the hydrogel model with different water content was analyzed in detail.

• The friction forces of different water content models under different sliding friction speeds and different loads are simulated in detail.

• In the way of simulation, the general trend of hydrogel mechanics and wear resistance with water content was obtained.

Abstract

Background and objective

Recently conducted biomedical studies have shown that the drug diffusivity of hyaluronic acid hydrogel plays an important role in the treatment of joint diseases. The drug diffusivity is closely related to the water content of hydrogel. In addition, different water content will not only affect its mechanical and tribological properties, but also change the effect of drug release.

Methods

In this work, a Molecular dynamics simulation was used to investigate the effect of water content on spatial distribution, tribological and mechanical properties of a hyaluronic acid hydrogel network. This paper focuses on the analysis and calculation of the radial distribution function of 20, 40, 60, and 80% water content model and the friction force and mechanical parameters under the influence of different load and friction speed.

Results

The results show that at 20 and 40% water content, the spatial distribution is loose and the intermolecular force is not strong, resulting in a major lack in tribological and mechanical properties; whereas at 60 and 80% water content, the spatial distribution becomes gradually compact and the intermolecular force is gradually increased. The tribological and mechanical properties manifest a marked improvement.

Conclusions

The calculations reveal that the hydrogel model has the best wear resistance, pressure resistance, and plastic deformation resistance at 80% water content. In the range of 20–80% water content, the mechanical properties and friction properties of hydrogels become better and better with the increase of water content.

Introduction

Hydrogels are water-containing materials with good biocompatibility, durability, and other unique properties after cross-linking with other substances, and have become more popular in tissue engineering [1], [2], [3], drug sustained-release [4], [5], [6], disease treatment [7,8], and many other fields. Natural hydrogels have attracted wide attention in simulating tissue structure [9], cell proliferation and differentiation [10], [11], [12] and substance transport [13,14]. Researchers are increasingly resorting to Hyaluronic acid (HA) hydrogel as a natural hydrogel with enormous potential. HA is a component of extracellular matrix and synovial fluid which are both abundant in articular cartilage and its biocompatibility and biodegradability render it very useful in tissue engineering applications [15], [16], [17], bone regeneration [18], [19], [20], and joint disease treatment [21,22].

Hydrophilic HA hydrogel can absorb substantial amounts of water [23], [24], [25]. The amount of water in the hydrogel has a direct influence on its performance. As a method for treating retinal ailments, Dromel et al. [26] examined how varying water content affects medication release from injectable HA hydrogels. The degree of drug molecular diffusion of drug-loaded hydrogels increases with water content, as per a study comparing drug molecular diffusion of drug-loaded hydrogels. Veskovic et al. [27] employed EPR to track drug release, swelling, and breakdown in various hydrogels. When Tran et al. [28] studied the mechanical properties of composite hydrogels with high water content. Young's modulus and tensile strength of hydrogels with different water content were tested to speculate the best water-bearing state.

Many researchers have studied extensively on the macroscopic mechanical properties of hydrogels. To study and analyze the mechanical and friction behavior of hydrogels with different water content, it is also very important to study the effect of water molecular weight on the properties of hydrogels at the molecular level. All-atomic molecular dynamics (MD) simulation technology has evolved into a potent scientific research tool for building, displaying, and analyzing structural models of molecules, solids, and surfaces, as well as studying their properties. It provides a clear grasp of polymer molecular structure. Micromechanics and macroscopic experimental data from molecular dynamics simulations are combined to provide a more comprehensive explanation of material properties [29], [30], [31], [32].

Sun et al. [29] used molecular dynamics simulations to study the effects of different water content on the network structure and oxygen permeability of poly(bis(trimethylsilyloxy)methylsilylpropyl glycerol methacrylate) (PSiMA) and poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC) hydrogels. The results show that an increase in hydrogel water content is closely related to a decrement in its oxygen permeability. Lee et al. [33] employed an all-atomic MD simulation to investigate the mechanical characteristics and equilibrium structure of block and random sequence models of Poly (N-vinyl-2-pyrrolidone-co-2-hydroxyethyl methacrylate) hydrogels under varied water content. MD simulations of gel self-healing behavior [31], drug molecular transport [34], molecular permeability [35], shock response [36], and hydrogel deformation [37] were also performed. Friction is both an ancient and a young science [44]. In current MD simulation-based research, there are only a few investigations on the mechanics and wear resistance of hydrogels with varied water content at the molecular structural level, and this subject remains largely untouched.

The influence of water content on the mechanical characteristics and wear resistance of hyaluronic acid hydrogels was investigated and analyzed in this paper. All-atomic molecular dynamics (MD) simulation was employed to simulate the network of hyaluronic acid hydrogels with varied water content. The goal of this work is to investigate and analyze the molecular space distribution after different compression strains, the mechanical properties affected by different water content, and the wear resistance of hydrogels affected by different friction environments under the influence of different water content. Molecules and atoms' microscopic states and influencing mechanisms are discussed in depth.