One-step double network hydrogels of photocurable monomers and bacterial cellulose fibers

Highlights

• Photocurable biocompatible double network (DN) hybrid hydrogels in a single step

• Polyacrylic acid (PAA)/bacterial cellulose nanofibers (BCNFs) DN hydrogels are stiff but flexible.

• The DN hydrogels swell less when immersed in water than PAA hydrogels.

• PAA hydrogels with cellulose nanocrystals do not form a DN system.

• PAA/BCNFs hydrogels are potential candidates for vascular or cartilage soft-implants.

Abstract

Soft-tissue replacements are challenging due to the stringent compliance requirements for the implanted materials in terms of biocompatibility, durability, high wear resistance, low friction, and water content. Acrylate hydrogels are worth considering as soft tissue implants as they can be photocurable and sustain customized shapes through 3D bioprinting. However, acrylate-based hydrogels present weak mechanical properties and significant dimensional changes when immersed in liquids. To address these obstacles, we fabricated double network (DN) hydrogels composed of polyacrylic acid (PAA) and bacterial cellulose nanofibers (BCNFs) by one fast UV photopolymerization step. BCNFs/PAA hydrogels with a 0.5 wt% BCNFs content present an increased stiffness and a lower, non-pH-dependent swelling than PAA hydrogels or PAA hydrogels with cellulose nanocrystals. Besides, BCNFs/PAA hydrogels are biocompatible and can be frozen/thawed. Those characteristics endorse these hybrid hydrogels as potential candidates for vascular and cartilage tissue implants.

Introduction

Bioengineering soft tissue replacements such as skin, tendons, ligaments, cartilages, and blood vessels are extremely challenging with demanding characteristics that encompass biocompatibility, durability, high wear resistance, low friction, precise tuning of the water content, and differentiated mechanical performance for each specific type of implant (Zhao et al., 2020). For example, the mechanical properties of the arteries differ depending on their direction, presenting elastic modulus values ranging from 0.1 to 10 MPa and failure stress values from 0.3 to 5 MPa (Miramini et al., 2020). On the other hand, the articular cartilage is defined by the joint (location, age, fatigue). Therefore, it presents mechanical properties ranging from 1 to 170 MPa for the elastic modulus and 4–50 MPa for the stress failure (Riemenschneider et al., 2019; Wasyłeczko et al., 2020). Thus, although various scaffolds for soft tissue replacements are already approved for medical use, they are not fully optimized, and efforts are still needed to properly accomplish each type's sought-after requirements.

Hydrogels are materials containing a large amount of water (50 %–95 %) within a crosslinked polymeric three-dimension (3D) network. Their porous structure can act similarly to the extracellular matrix, helping the proliferation of the cells and the diffusion of the nutrients, whereas their mechanical properties can be tuned according to the damaged soft tissue to be replaced (Hunt et al., 2014). Among the different hydrogel polymeric networks, acrylate systems, i.e., polyacrylic acid (PAA) and polyacrylamides (PAAm) are pH-responsive hydrogels worth considering as soft tissue implants as they are already being exploited as wound dressings (Agarwal et al., 2012; Champeau et al., 2018) or drug delivery systems (Follmann et al., 2018; Kim et al., 2008; Tian et al., 2017; Zhang et al., 2014), in sensors and actuators (Breger et al., 2015; Lu et al., 2020), separation systems (Mathew et al., 2019; Safavi-Mirmahalleh et al., 2019; Takemori et al., 2020) and adhesives (Li et al., 2018; Su et al., 2020). Acrylic monomers are attractive because they can be photopolymerized at room temperature, while a crosslinking agent is usually added to control the mechanical properties of the hydrogel (Ma et al., 2020; B. Zhang et al., 2018). The possibility of quickly obtaining UV-photocurable acrylate hydrogels increases their potential to customize their shapes and use them in 3D bioprinting inks (Choi & Cha, 2019; Kam et al., 2021). Unfortunately, similar to other single network polyelectrolytes, PAA and PAAm hydrogels present weak mechanical properties and significant dimensional changes when immersed in different pH solutions. This significant swelling produces mechanical fragility and low tolerance to compression loads, compromising their use in patient-tailored printable implants.

To improve the stiffness and to control the swelling of the hydrogels, chemical grafting of molecules and the addition of other polymers, particles or ions have been explored (Fan et al., 2020; Follmann et al., 2018; Lu et al., 2020; Myung et al., 2008; Safavi-Mirmahalleh et al., 2019; Yang et al., 2018). Likewise, to address their poor mechanical performance, Gong et al. pioneered the field of double network (DN) hydrogels, widening the synthetic landscape of soft material's tunability (Gong, 2010). DN gels consist of two interpenetrating polymer networks with contrasting mechanical properties and polymerization mechanisms. The first network is highly stretched and densely crosslinked, making it stiff and brittle. The second network is flexible and sparsely crosslinked, making it soft and stretchable (Ducrot et al., 2014). The field has witnessed an increasing number of examples (Chen et al., 2015; Fuchs et al., 2020) displaying sophisticated properties such as phase transition from rubbery to glassy in PAA hydrogels/Ca-acetate (Nonoyama et al., 2020) or striking applications such as 3D printable PAA-graphene oxide-Ca²⁺ biosensors (Wang et al., 2019). Besides, DN systems with two bio-based polymers (such as cellulose/gelatin) (Azuma et al., 2007; Geng et al., 2021; Yasuda et al., 2005) as well as mixtures of a bio-based polymer with a synthetic one (Yu et al., 2021) are also being investigated. Surprisingly, a deterioration of the cellulose/gelatin hydrogel properties was identified after body implantation. In contrast, excellent tensile strength and good integration as a ligament substitute have been reported when a synthetic polymer, PAAm, is combined with bacterial cellulose (BC) (Azuma et al., 2007; Hagiwara et al., 2010; Hua et al., 2021). BC is a high purity and crystallinity cellulose, biosynthesized by bacteria that produce hydrophilic and interwoven nanofibers with a high-water uptake capability (approx. 90 % w/w) (Zeng et al., 2014) and high tensile strength (Wang, Jiang, et al., 2017). As aforementioned, Azuma et al. obtained BC/PAAm DN hydrogels using an as-prepared BC hydrogel pellicle. The main drawbacks of such fabrication approach reside in the lengthy (circa ten days) and multiple-step process and the impossibility of producing hydrogels with predesigned shapes. Besides, compressive deformations on those films could not be fully recovered after drying.

Here, we report a one-step fabrication of DN hydrogels by rapid photopolymerization (a few seconds) of low crosslinked PAA and bacterial cellulose nanofibers (BCNFs) motivated by the biocompatibility and endotoxin-free characteristics of BC (Anton-Sales, Roig-Sanchez, Sánchez-Guisado, Laromaine, & Roig, 2020) and the recently reported mechanical enhancement of PAAm while controlling their swelling response (Hua et al., 2021). The mechanical properties of BCNFs/PAA hydrogels with various compositions and their response at multiple pHs were investigated. The superior performance of the DN hydrogel is presented compared to hydrogels containing only PAA or PAA with cellulose nanocrystals (CNC). A cytotoxicity study of the BCNFs/PAA hydrogels is also reported.