Cell Responses to Biomimetic Protein Scaffolds Used in Tissue Repair and Engineering

Robert A. Brown*; James B. Phillips†    * Tissue Regeneration & Engineering Center, Institute of Orthopedics, University College London, Stanmore Campus, London, HA7 4LP, United Kingdom
† Biological Sciences Department, The Open University, Walton Hall, Milton Keynes, MK7 6AA, United Kingdom

I Introduction

The field of research covered by this review represents and feeds into the basic science platform for a wide range of biotechnical activities commonly grouped under the umbrellas of tissue engineering and regenerative medicine. It lies at the intersection of cell biology–biomaterials–bioengineering–reconstructive surgery. The overall aim is to define and to develop experimental models for the control parameters that regulate new tissue (especially connective tissue) deposition, growth, and remodeling in three‐dimensional (3D) culture. The importance and utility of this generic (i.e., nontissue specific) knowledge lies in developing the capability for rational design of biomimetic tissues, model tissues, and their in vitro fabrication processes. We are presently at a stage which might best be visualized (perhaps through the eyes of A. V. Roe or Anthony Fokker) as comparable with aircraft wing design in 1910. We know the basic structures that can function, the simple building materials that sometimes suffice, and we have an outline concept—if ambitious—of what, where, and how fast we want to fly. The trouble is that outside the very narrow envelope of previous wing shapes that have ACTUALLY left the ground, our quantitative knowledge of the parameters predisposing to flight versus crash prevents us from being able to sit down and DESIGN a reasonably successful wing/tissue‐substitute.

This review discusses the use of biomimetic proteins and cellular therapies, both in the development of applied clinical products and of model platforms for scientific investigation. Clinical approaches currently used in the repair of specific tissues are described to identify some of the important parameters for research to target. This is followed by discussion of the biomimetic proteins being studied and manipulated to provide future strategies for technological advance in the field of tissue repair.

II Background

A Beyond Prosthetic Implants

Implantation science is presently at a crossroad in its development. Biomedical concepts of desirable implants are evolving from the prosthetic toward the bioartificial or biomimetic. At the same time, there are new opportunities to improve on whole organ transplantation using cell or engineered tissue implantation. More than ever before, this is dependent on new knowledge in cell biology and the spinout of this knowledge into biomimetic engineering. This is mirrored by a steady progression away from the idea that cell‐support scaffolds or implant materials where needed should be as inert (so inoffensive, or bioinvisible) and long‐lived as possible. Because prosthetic biomaterials (metals, polymers, ceramics) have become highly refined in terms of physical function and durability, even encouraging surface biointegration, a clear tension has developed between our aspirations and what is achievable. Incorporation of ever more biological function into our constructs means we must surrender some of the excellent physical properties and immediate functionality of conventional prosthetic and cadaveric implants. This is an inevitable stage in development of the field until it is possible to fabricate implants that resemble autologous graft tissue. Examples of this are in bone, cartilage, heart valve, and skin replacements. This aspiration implies a biofunctional incorporated to the level that the implanted construct participates in the normal tissue remodeling processes. However, in doing this, it is inevitable that such graftlike implants will gain and lose function in a quite different manner than conventional prosthetic implants, as shown in Fig. 1. The advantage of replacing lost or damaged skin with autologous skin grafts, for example, rather than inert biomaterial coverings is obvious. There is a tension, however, in that current stage skin constructs fall so far short of native tissue complexity and function that they in fact struggle to compete with present generation implants.

The fields of research involved in these next generation clinical strategies have a strong interdisciplinary base, described variously as tissue engineering, regenerative medicine, and cell/gene therapies. However, they are all underpinned by new understanding of the mechanisms which control interaction of cells with their immediate 3D support scaffolds. Such scaffolds are increasingly biomimetic, either as protein‐based connective tissue equivalents or coatings onto synthetic polymers.

There is a wide consensus that these new forms of clinical implant will only live up to their promise if they are cell‐seeded. Indeed this is the basis for rapid expansion of interest in technologies for cell acquisition (autologous, allogeneic, even xenogeneic), as adult and embryonic stem/progenitor cells. However, it is also increasingly clear that production of the cells alone is only part of the answer. Tissue architecture is the key to native function in solid 3D implants (Brown, 2002). This is particularly true in mammalian connective tissues, which almost always undergo repair with scarring, rather than functional regeneration (Brown, 2005; Tomasek et al., 2002). Inevitably, then, advanced, biomimetic 3D scaffolds are required to provide the range of controls needed for cell differentiation, migration, and extracellular matrix production and for tissue organization (Brown, 2000; Brown et al., 1997). Accumulating evidence now suggests these key cues most often need to be built into the structure of the scaffold (or extracellular matrix), as they are in nature. In other words, these are best considered as biomimetic cues, rather than supra‐physiological doses of single cytokines, growth factors, or pharmaceutical agents. The present review will aim to examine attempts to understand how these cues can be adapted and incorporated into 3D soft tissue biomimetic scaffolds. Because the actual extracellular matrix environment rapidly becomes protein‐based (either as molecular attachment elements or bulk ECM material), we shall consider aggregated protein, rather than synthetic polymer substrates.

B What Are Cell Support Scaffolds and Tissue Replacements?

A great deal of the drive to understand and develop improved cell support scaffolds is grounded in our need to understand more about how tissues organize in 3D. This in turn is a requirement of clinical need in replacing, reconstructing, regenerating, and repairing injured and diseased tissues. The vision suggests that if we understood how cell growth is controlled in 3D in vivo, it would become possible to solve many problems of disease and aging. It follows, for example, that a suitably controlled 3D cell culture would eventually produce a viable tissue (perhaps as a precursor or template), which could be implanted to replace or supplement lost function (e.g., skin, cartilage, bone, liver).

This evolution track of course is well known and represents classical tissue engineering rationale. Clinically targeted technologies such as this are well known and generally aim to produce useful implants, either very quickly after cell‐scaffold assembly (immediate implantation in Fig. 1A) or after extended periods in culture where resident cells are persuaded to fabricate neo‐tissues with at least some native function (Fig. 1B). Track [A] is inexpensive and rapid, but all the information for biocontrol of regeneration must be inbuilt at this very early stage, seemingly a high technical hurdle. The long culture period of track [B] at least allows close control of conditions as the construct develops, though at a cost. The...