Biomaterials for Cell Delivery

Vehicles in Regenerative Medicine

Aaron S. Goldstein, Aaron S. Goldstein

• Publisher: CRC Press
• Year: 2018
• ISBN:
  9781351647793
• Edition: 1
• Topic: Medicine
• Subtopic: Biotechnology in Medicine

The purpose of this book is to summarize key strategies and recent accomplishments in the area of developing cell/biomaterial constructs for regenerative medicine. The first section is a review of the state-of-the-art of biomaterial carriers and is divided into synthetic and natural materials. A subset of the latter are decellularized organs which retain the structure and some of the biological activities of the target organ. The bulk of the book is devoted to unique problems associated with key tissue and organ targets.

Key selling features:

• Describes developing cell/biomaterial constructs for regenerative medicine
• Reviews state-of-the-art of biomaterial carriers
• Summarizes the unique problems associated with key tissue and organ targets
• Discusses issues associated with clinical translation including quality control, manufacturing practices, nondestructive imaging, and animal models

1	Natural Materials for Cell-Based Therapies
	Marc Thompson and Mark Van Dyke

CONTENTS

1.1 Introduction

1.1.1 Hydrogels

1.1.2 Foam Biomaterials

1.1.3 Films

1.1.4 Cell Encapsulation

1.1.5 Composites

1.2 Protein-Based Biomaterials

1.2.1 Keratin

1.2.1.1 Structure

1.2.1.2 Fabrication

1.2.1.3 Applications

1.2.2 Fibrin

1.2.2.1 Structure

1.2.2.2 Fabrication

1.2.2.3 Applications

1.2.3 Collagen

1.2.3.1 Structure

1.2.3.2 Fabrication

1.2.3.3 Application

1.3 Polysaccharide Biomaterials

1.3.1 Alginates

1.3.1.1 Structure

1.3.1.2 Fabrication

1.3.1.3 Application

1.3.2 Chitosan

1.3.2.1 Structure

1.3.2.2 Fabrication

1.3.2.3 Application

1.3.3 Hyaluronic Acid/Hyaluronan

1.3.3.1 Structure

1.3.3.2 Fabrication

1.3.3.3 Application

1.4 Summary

References

1.1 INTRODUCTION

A biomaterial can be described as any material designed to interact in concert with a biological system. Biomaterials past and present have long stood as a symbol of scientists’ attempts to interact with, recreate and improve upon human anatomy and function. From surgical sutures to organ replacement, investigators have strived to better understand the details of human biology and apply this knowledge to numerous applications using a myriad of conventional and novel biomaterials. Today, that work has resulted in innovations correlating to innumerable lives improved and saved. Improvement of the synthesis and application of materials in particular have spurred the growth of an entire field of biomaterials research, into which this chapter will take an in-depth look, specifically, the impact of natural biomaterials used for delivering cells for tissue regeneration purposes. In terms of tissue repair and regeneration, biomaterials have increased in complexity and robustness while their ability to support the repair or replacement of damaged tissue has become more probable.

Today, clinically applied tissue regeneration procedures are in large part performed using either autografts (tissue originating from the same individual) or allografts (tissue originating from another human individual) (Khan et al. 2005). Autografting, which has been used extensively with satisfactory results, has several limitations, including patient pain, medical costs and a finite supply of suitable donor tissue (Oryan et al. 2014). Allografts, while more abundant than autograft tissue, carry the uncertainty of biocompatibility and the potential for disease transmission. The delivery of cells for tissue regeneration by means of natural biomaterial vehicles that are more accessible than autograft tissue, and elicit reduced complications compared with allograft tissue, hold great promise as a means to overcome current limitations in repairing and regenerating defective or damaged tissues (Kanbe et al. 2007; Naderi et al. 2011).

A prerequisite of classical tissue engineering approaches is a suitable biomaterial scaffold or substrate with an architectural design, chemical, mechanical and physical makeup comparable to that of the native tissue. Prior clinical practices of delivering cell suspensions directly into a defect site have proven to be problematic—if not ineffective—due to insufficient retention within the defect and subsequent flushing into the surrounding tissue (Endres et al. 2010). Consequently, the implementation of naturally derived vehicles, composed of more durable materials found readily in the body and more specifically within the tissue region of interest, are an attractive option for mimicking the native host tissue in the form of three-dimensional (3D) prefabricated scaffolds that retain the delivered cell population (Kretlow and Mikos 2007; Wang et al. 2015). Herein we focus on scaffolds derived from natural polymers (i.e., proteins, polysaccharides) that can then be processed into hydrogels, foams and films, as just a few examples. These materials are capable of conforming to irregular shapes at the site of a defect or injury for complete access and infiltration of the wound and of delivering cells and pharmaceutical agents to guide tissue regeneration.

1.1.1 HYDROGELS

Hydrogels are a class of biomaterials with high water content and soft tissue-like mechanical properties (Matricardi et al. 2013, Dubbini et al. 2015) that make them attractive as scaffolds for a variety of tissue engineering applications. Moreover, highly moldable hydrogels can be injected into irregular-shaped defects, enabling targeted delivery of cells for in vivo tissue engineering (Park et al. 2012) with limited surgical invasion (Zhang et al. 2014). Further, injectable biomaterials may be designed to crosslink in situ to facilitate integration and tissue regeneration at the region of interest (Montanari et al. 2015). In situ crosslinking is generally accomplished by chemical polymerization, photopolymerization or thermal crosslinking (Berger et al. 2004; Hennink and van Nostrum 2012; Montero-Rama et al. 2015). Chemical polymerization and photopolymerization have been applied extensively for gelation of injectable hydrogels, although the use of chemical initiators risks cytotoxicity and introduces undesired complexity to the delivery systems (Elias et al. 2015). Thermal crosslinking is applicable for thermosensitive hydrogels that undergo phase transitions in response to temperature change; however, these systems are consequently limited to temperature ranges that are well tolerated by living cells.

1.1.2 FOAM BIOMATERIALS

Instances in which a solid or microporous hydrogel scaffold does not provide the desired level of nutrient, fluid or waste exchange may warrant the application of hydrogel foams or sponges. Implantation of interconnected, macroporous foams, commonly fabricated from alginate, collagen or chitosan (Anderson et al. 2014; Mi et al. 2001; Ranucchi et al. 2000), allows for more complete cellular invasion and improved mass transport of oxygen, nutrients and waste. Increases in porosity also provide greater cavity space for new tissue ingrowth and formation without having to compete with the implanted biomaterial, which may not degrade at the same rate that tissue is formed (Anderson et al. 2014; Prieto et al. 2014). These foams may be delivered prewetted; however, an added benefit of non-wetted foams is that improved fluid infiltration within the dry material may result in further adsorption of cells, nutrients and factors from the surrounding milieu that are beneficial to tissue regeneration. Depending on the intended use, foams have exhibited highly variable mechanical properties, with more elastic foams exhibiting Young’s moduli of 4 kPa and more rigid foams displaying compressive strengths of up to 200 kPa, supporting their use in soft tissue as well as bone regeneration, respectively (Karashima et al. 2009; Yu et al. 2013).

Macroporosity in a biomaterial foam is not naturally occurring, as in the microporousness of similar biomaterial constructs. Instead, pores of larger and more consistent diameter are produced using techniques such as gas foaming, porogen leaching and phase separation (Christenson et al. 2007; Moglia et al. 2011; Zhang et al. 2005). While successful material synthesis has been accomplished and tested clinically, incomplete leeching of the sacrificial pore-forming material risks cytotoxicity, and potentially deleterious consequences for the regeneration of native tissue (Prieto et al. 2014).

Foams also experience occasional difficulty during implantation. Foams are often cast to a specific size and shape that is not necessarily an exact fit for a defect of a particular size and shape, which can vary in cases such as broken bones. In addition, foams can undergo expansion or contraction during fabrication (Karashima et al. 2009). Consequently, foams may need to undergo a shaping step, which can lead to significant chipping and breaking, especially with harder and more brittle constructs.

1.1.3 FILMS

In line with improving transport through biomaterials, thin films or biomaterial sheets have been employed when the environment calls for rapid transport of materials. Previously, collagen and chitosan-based biomaterial films have been synthesized as corneal implants or wound dressings, respectively (Lee et al. 2001; Lahooti et al. 2016). In both instances, these biomaterials proved effective, allowing for sufficient oxygen, fluid and waste transport. In most cases, these films are easily biodegraded or removed (several uses include topical or superficial applications).

In scenarios in which films will not experience large mechanical strains (as is the case with corneal implants) where materials can only withstand tens of kilopascals in tensile forces at most, softer and more flexible films are preferred. On the other hand, stiffer and tougher materials have been processed into films for applications that experience thousands of kilopascals of tensile stress (Vrana et al. 2007; Kroustalli et al. 2013).

1.1.4 CELL ENCAPSULATION

As an emerging methodology in tissue engineering, complete cell encapsulation has attracted increasing interest in recent years (Hunt and Grover 2010; Man et al. 2012; Uludag et al. 2000). It promotes tissue regeneration by the localized retention of cells and delivery of therapeutic trophic factors over a desirable period of time. In addition, the biomaterial encapsulant can mitigate the risks associated with direct cell injection such as apoptosis within the first few days of implantation (Abbah et al. 2006).

To date, a broad variety of multipotent progenitor cell types have been examined for the regeneration of musculoskeletal, nerve, hepatic, and cardiac tissues, including mesenchymal stem cells (MSCs) (Niemeyer et al. 2007), bone marrow progenitor cells (Steadman et al. 2002), adipose tissue stromal cells (ATSCs) (Cowan et al. 2004) and muscle-derived stem cells (Prigozhina et al. 2008). These primary cell types exhibit a variety of favorable properties for potential delivery. For example, MSCs from bone marrow are negative for immunologically relevant surface markers and inhibit proliferation of allogenic T-cells in vitro (Hennink and van Nostrum 2012) Although concerns exist that MSCs from bone marrow may lose their immunosuppressive potential after expansion in vitro or implantation into allogenic recipients (Prigozhina et al. 2008). Alternatively, ATSCs are easily harvestable with lower donor site morbidity when compared with other pluripotent stem cell sources (Cowan et al. 2004). Additionally, reports indicate that ATSCs possess phenotypic and functional characteristics similar to MSCs derived from bone marrow, and attach and proliferate easily in culture, making them potentially available on a large scale.

1.1.5 COMPOSITES

To date, a wide variety of natural biomaterials have been identified and employed for tissue engineering applications. Individually, these materials exhibit similar properties to a wide range of biological tissues. However, in most cases, no singular biomaterial perfectly mimics the biological system it is meant to recreate. As a result, composite materials at varying concentrations and ratios are often employed to modulate factors such as degradation rate and better mimic the chemical, biochemical, and mechanical properties of the system of interest. In this chapter, the focus is mainly on individual natural biomaterials for cell delivery; however, in many cases composites are necessary to better achieve and maintain tissue regeneration.

Within biological systems, three major classes of natural polymers exist: polysaccharides (e.g., cellulose, alginate), polynucleic acids (e.g., DNA, RNA), and proteins. However, in reality, proteoglycan and glycoprotein hybrids also exist. In the following sections, we describe a few of the most popular and promising natural proteins and polysaccharides for cell delivery and tissue regeneration.

1.2 PROTEIN-BASED BIOMATERIALS

1.2.1 KERATIN

Keratins are a family of structural proteins, and can be found within a broad variety of animal tissues including: skin, hair, claws, horns, hooves, whale baleen, and bird feathers. The toughness and hardness of these proteinaceous materials derive from the organization and extensive crosslinking of the individual proteins.

1.2.1.1 Structure

Two types of mammalian α-keratins have been characterized and denoted as the “soft” α-keratins found in skin (epidermal keratins) and the “hard” α-keratins (trichocytic keratins) stemming from epidermal appendages, such as hair, claws, and quills. Both forms of keratin are stabilized around the time of cell death by disulfide bonds between pairs of cysteine residues. The number of disulfide bonds in trichocytic keratin is greater than in epidermal keratin. Electron micrographs of cross-sections reveal that both types of keratin exhibit a filament and matrix-like texture (Bragulla and Homberger 2009). In the case of the epidermal keratins, the matrix is believed to comprise some or all of the N- and C-terminal domains of the molecules. The cores of the filaments are formed from rod domains that are approximately 40–50 nm in length. A comparable structure exists in the keratins of birds and reptiles except for the fact that the secondary structure of the filaments is predominantly the β-sheet (Wang et al. 2016). A major difference between epidermal and trichocytic keratins is that the matrix in trichocytic keratins contains considerable amounts of sulfur-rich and glycine–tyrosine-rich proteins. Also, during the transition from the reduced to the oxidized state, the framework of the intermediate filaments undergoes both molecular slippage and compaction, a process that is much more apparent under chemically induced processes, but has been shown to occur in natural settings as well (Grune et al. 1997; Thiele et al. 1999). This brings the cysteine residues of neighboring molecular segments into axial alignment and enables the formation of disulfide linkages (Hill et al. 2010). By compa...