The fabrication and characterization of bioengineered ultra-high molecular weight polyethylene-collagen-hap hybrid bone-cartilage patch

Highlights

• A glue-free junction of layers enhances the structure of the bulk and porous bioinert ultra-high weight polyethylene (UHMWPE) through the biodegradable collagen derived layer maintaining strong adhesion.

• Attenuated Total Reflectance Fourier transform infrared (ATR FTIR) spectroscopy of collagen helps optimize the fabrication of the bone-cartilage patch.

Abstract

A layered hybrid implant was designed and fabricated for the surgical replacement of worn cartilage to meet the complex requirements of biocompatibility and mechanics. The natural hierarchical structure was purposefully mimicked to improve the implant performance and integration. The hybrid was fabricated using three components: processed collagen gel, hydroxyapatite (HAp) powder and ultra-high molecular weight polyethylene (UHMWPE) in bulk and sponge form. The fabrication included hot molding, sacrificial templating, infusion, and freeze-casting stages. The hybrid had a porous transition layer made of porous UHMWPE impregnated with collagen by means of infusion. SEM and FTIR analyses confirmed successful collagen impregnation of porous UHMWPE. The morphology of transition layer was selected and produced to provide the UHMWPE pore size distribution in a range ∼50–150 μm that is favorable for the osseointegration by osteoblast proliferation. Biocompatibility tests were carried out in vitro. The index of red blood cells hemolysis showed 0.6 % after 4 h of co-incubation that proved excellent biocompatibility of the fabricated UHMWPE-Collagen-HAp hybrid.

Introduction

Demanding reconstructive surgery applications call for continuous improvement of implant materials. Solutions based on metallic alloys, ceramics, polymers and their hybrids are being considered and optimized for safe bone and cartilage replacement. New biocompatible materials must meet a wide range of requirements in terms of different properties. In particular, suitable mechanical performance is required due to the static and dynamic stiffness and strength, fatigue durability and contact behaviour. In terms of biomedical performance, bioinertness and/or controlled biodegradability are sought [1]. Some of the studies focus on the synergy of components in order to develop new organic/inorganic hybrid materials, for instance, through nanosilicates being incorporated in polymer scaffolds [2].

Hip and knee joint replacement demands ensuring low friction and wear resistance, along with the properties listed above [3]. A complete joint replacement with stiff metal assembly and low friction polymer inserts is a widely commercialized engineering solution that delivers acceptable performance and durability [4], since failed implants require repair or replacements with traumatic surgical intervention to perform secondary operations that are particularly complicated for aged patients [5].

An artificial cartilage patch with bone integration offers an attractive alternative to reduce the instances of invasive surgical procedures. A multi-layered patch can be used locally to restore areas of damaged cartilage. The architecture of a hybrid involves the main load-bearing and low friction layer — bulk ultra-high molecular weight polyethylene (UHMWPE) — and two layers facilitating osseointegration and introducing a gradual change of mechanical properties.

UHMWPE is a popular prosthetic material for artificial hip joints [6] possessing low dry friction coefficient down to 0.05 [7] with excellent wear resistance [8]. Its tribological properties can be enhanced via the addition of carbon nanotubes [9], molecular chain orientation or radiation cross-linking [10]. It is highly bioinert, i.e. hardly interacts with the host tissues enhancing the stability of implant. Since bulk UHMWPE does not integrate with the host bone, it requires the creation of a porous structure to promote mechanical linking and “keying in” to establish a secure joint.

A bi-layered UHMWPE hybrid was fabricated in a one-stage hot molding process. The hybrid contains a low friction bulk layer and a porous layer that must have particular shape and size of open pores and the thickness of pore struts for optimal osseointegration [11,12]. The use of a water-soluble sacrificial filler allows creating a gradient structure [13]. The impregnation of bi-layered UHMWPE with antibiotics and growth protein factors [14] as well as the introduction of marrow-derived stem cells (MSC) can be used to facilitate faster osseointegration and new osteogenesis [15].

In the present study we pursue further development of the paradigm of multi-layered biomedical hybrids for faster osseointegration. Namely, porous UHMWPE impregnated with collagen gels and slurries can produce even better performance in reconstructive surgery. Collagen impregnation is based on the known phenomenon of stable adhesion of collagen layer to UHMWPE hybrid [16].

Collagen-based scaffolds have been investigated previously for artificial bone and cartilage [17], as well as for heart valve prostheses [18]. Collagen is a crucial fibrous component of mammalian tissues: bones, cartilage, tendons, and skin. It consists of a complex hierarchical system of protein fibers. Freeze-dried type I collagen has been reported in the literature as an extracellular matrix that promotes cell propagation and proliferation [19]. There is an extensive range of studies devoted to collagen as a base for fabricating bone replacement scaffolds [[20], [21], [22]], in combination with electrospun fibers and additives [23]. Scaffolds can be freeze-cast to create aligned open porosity, with crosslinking to improve the mechanical performance for osseointegration [18]. These core ideas underlie the concept of the multilayered patch presented herein.

Hydroxyapatite (HAp) is widely used as the main mineral component of bioactive materials as an accelerating additive for bone regeneration [24]. HAp is a natural created candidate for bone prostheses as it forms the mineral part of the mammalian bone. HAp powder has been successfully used in additive manufacturing to treat local injuries. 3D printing of HAp with burn-off binder for bone implants allows obtaining the required geometry [25,26]. However, bulk HAp possesses rather limited strength and toughness in the absence of tight integration with an organic fibrous scaffold such as collagen. We therefore concluded that the use of a Collagen-HAp composite offers a promising avenue for further exploration: the fibrous collagen scaffold reduces the brittleness of pure HAp grafts, and also protects from fast HAp resorption [27]. Freeze-casting has been proposed as a method of creating versatile HAp-Polyvinyl Alcohol (PVA) systems with complex organization for load-bearing biomaterials [28]. Subsequent osseointegration involves the re-deposition of biogenic HAp, resulting in the formation of stable bone tissue.

Table 1 shows that the material systems mentioned above were successfully used in fabricating cartilage and bone tissue scaffolds for reconstructive surgery.

In the present study, stepwise combination of hot molding, collagen impregnation and freeze-casting/drying was used to produce multilayered hybrids. UHMWPE-Collagen-HAp hybrids were characterized with SEM, FTIR, and Raman spectroscopy for comprehensive evaluation. Characterization of the properties of each layer within the hybrid was carried out. Due to the complexity of the hybrid hierarchy and layer organization, once the layers were characterized in terms of the general morphology and collagen spatial distribution, similar assessments were performed for the layers after joining. The presence of collagen was confirmed using spectroscopic methods. The mechanical integration by collagen bridging established strong glue-free bonding between layers within the layered hybrid. Mechanical tests revealed the principal phenomena taking place in the hybrid under loading.