Bioengineering in Wound Healing

The following sections are included:

• Dedication
• Preface
• Contents

Wounds and wound healing are parts of every organism’s life. What is a wound, how does it heal and what can we do to speed the healing, but decrease scarring? All these fundamental questions occupy the minds of people through all known history. Wound healing is an extremely complex phenomenon, which involves both the organism and its environment. Moreover, chemical, biological, mechanical and electrical forces drive this phenomenon. This chapter attempts to combine various aspects of wound healing phenomena with an emphasis on scarless regeneration. The system approach to wound healing is needed to develop new therapies. This chapter starts with the general overview on what is a wound from the system perspective. Then it describes the highlevel events that take place during normal and abnormal wound healing. Next, it gives a historic perceptive on the history of wound healing in humans. Then it describes the fundamentals of scar biology, going to the details of known biochemical and cellular events involved in the process of scar formation. It then deals with the mechanical and electrical events that affect wound healing process. Next, complex events such as inflammation and wound-bacteria interaction are discussed, followed by descriptions of the in vitro and in vivo models of scarless wound healing. This chapter concludes with open questions and future perspectives of scarless wound healing.

A description of the histologic anatomy of the skin of the scalp begins with an overview of the entire skin specimen with specific structures indicated by arrows. The layers are identified. Then each layer is described according to its components and their significance. First, the epidermis is detailed along with its basement membrane zone. The dermal architecture as well as its connective tissue components are indicated. Their significance especially as far as involvement by either cutaneous disorders versus systemic diseases is reviewed. Lastly, the importance functionally and anatomically is elucidated. The vasculature of the skin is also described along with its importance in the role of inflammation of the skin. The different infiltrating cells in the skin are each enumerated, and their functions reviewed.

With this background, the morphologic features of wound healing are described and illustrated with both clinical images and appropriate microscopic features. The basic types of wound healing, both primary and secondary, are explained. The four phases of wound healing include: hemostasis, inflammation, proliferation, and remodeling. Each of these phases is described and illustrated. Examples of the different clinical conditions that lead to different type of ulcers are enumerated. Also, abnormal healing responses are described as well as their hypothetical causes. Emphasis is placed on the pathophysiology of wound healing with a diagram that illustrates the structures and the cytokines that affect them and produce healing.

Tissue damage that follows an injury results in a cascade of biological events that in ideal conditions result in activation of local and systemic responses that activate wound healing. Optimal wound healing becomes impaired in a variety of clinical conditions such as when other chronic illnesses are present (e.g. wounds in a patient with pre-existing diabetes) or when environmental factors negatively impact the healing response (e.g. wounds in a person with limited physical activity). In this chapter, we review a wide body of research on two models that have elucidated the mechanisms involved in impaired wound healing due to environmental conditions. The two conditions examined, termed deprived environment (DE) and enriched environments (EE) are reviewed by examining how the systemic response to sensory inputs in these two conditions are altered. To examine the complex mechanisms involved in the wound healing response to these two conditions, we divide them into three components: (a) how the sensory regions of the brain are changed in response to these conditions, (b) how the sensory changes in the brain result in altered brain outputs to injured tissue during wound healing, and (c) how such outputs lead to impaired healing in the DE condition and improved healing in the EE condition. For each of these components of the wound healing response in DE and EE conditions, we identify abnormal (DE conditions) and normal responses (EE conditions) that can potentially be altered with therapy aimed at improving wound healing.

Delayed wound healing is a prevalent clinical problem with substantial economic burden on the healthcare system. This is due to an ageing population and increasing comorbidities such as diabetes and peripheral vascular disease. Several elements including chronic tissue ischemia or repeated episodes of ischemia reperfusion injury contribute to the development of skin ulcers and wound healing delay in the lower limbs. One of the challenges in studying wound healing is the lack of accurate models that fully recapitulate the human condition. Despite these differences, several models contributed to our understanding of the role of disease conditions associated with aberrant wound healing and provided a biological system for designing and testing novel therapies. In this chapter, we described the factors involved in wound healing impairment and the different experimental models which may simulate ischemic wounds in humans.

Cutaneous wound healing depends upon a series of events involving coordination of molecular pathways that regulate the function of specific skin cell types. Key questions in wound healing management involve the identification of the molecular mechanisms by which proteins interact to control wound repair and to contribute to scarless healing. Since there are no sources of stem cells in adult skin to be used for this reason, any attempt to create skin de novo or to treat wounds in a way to generate healing without scar formation will likely require the re-programming of other cell types. Thus, the fundamental understanding of the control mechanisms responsible for scarless skin repair is imperative.

A wound, by true definition, is a breakdown in the protective function of the skin; the loss of continuity of epithelium, with or without loss of underlying connective tissue (i.e. muscle, bone, nerves) following injury to the skin or underlying tissues/organs caused by surgery, a blow, a cut, chemicals, heat/cold, friction/shear force, pressure or as a result of disease, such as leg ulcers or carcinomas.

To improve our understanding of wound repair and develop novel therapeutic approaches, advanced tools to monitor and assess the healing process are needed. Optical coherence tomography is an optical imaging modality that provides images of the subsurface microstructure of biological tissue and has shown great potential for imaging the healing of skin-wounds. Polarization sensitive optical coherence tomography further analyzes the polarization states of the scattered light, and can provide additional insight into the arrangement and content of collagen in the tissue, providing important hallmarks of wound healing and tissue remodeling. This chapter explains the working principle of this imaging modality and discusses its prospects to help in research on wound healing.

Many of the etiologies that contribute to delayed or impaired wound healing result in metabolic challenges to the wound site. The effect of underlying disease states as well as the natural changes in metabolic requirements during the different phases of wound healing have been characterized through a variety of imaging techniques. Positron emission tomography (PET), hyperspectral imaging, and two-photon excited autofluorescence imaging are three emerging modalities with applications in the field of wound healing. Each of these technologies is capable of providing real-time, quantitative metabolic information through non-invasive imaging that is sorely lacking in the wound healing field. In this chapter, we highlight recent advances in these imaging modalities and their applications in monitoring inflammatory and proliferative phases of wound healing. The continued development and validation of metabolic imaging biomarkers based on these modalities can serve an important role in guiding wound care in the clinic and product development in the laboratory.

In recent years, a number of skin substitutes have been developed for the treatment of acute and chronic cutaneous wounds in order to provide sterile wound coverage and stimulate tissue repair. Advancements in tissue engineering and biomaterials have led to an array of products designed to target several facets of the regeneration process. Some of these products have been rigorously evaluated in clinical settings, producing variable results. Cutting-edge fabrication techniques, stem cell technology, growth factor therapies, and genetic engineering are poised to overcome limitations associated with current products on the market. Due to the enormous financial burden cutaneous wound management places on healthcare systems worldwide, there is a significant demand for treatment strategies that are cost effective, easy to use, and result in accelerated and cosmetically pleasing wound closure.

Cutaneous wound healing is a highly complex and dynamic process involving the transient expression and function of various cytokines, growth factors, and cell types. Any disruption in this tightly controlled process can in principle cause impaired healing, turning a healing wound into a non-healing one that does not follow the normal process of wound repair. Non-healing wounds encompass two very different types of wounds: at one end of the spectrum there are hypertrophic scars, characterised by fibrosis and excessive deposition of fibroblast-derived collagen and other extracellular matrix (ECM) proteins; at the opposite side of the spectrum are chronic ulcers, open wounds that fail to close due to lack of collagen deposition and re-epithelialization. Cells from chronic wounds exhibit decreased proliferation, poor response to growth factors, diminished migratory capacity, and are in a persistent inflammatory state. The wound bed exhibits little ECM production, increased concentration of proteases, and impaired angiogenesis. Moreover, chronic wounds are often accompanied by persistent pain and bacterial infection, leading in many cases to lower limb amputations and a significant permanent burden for patients. Finding effective long-term treatment options are therefore necessary to improve patient outcomes.

Pharmaceutical-based approaches for treating wounds are attractive as they have the potential to abrogate or at least mitigate the cellular-level deficiencies within the wound tissue. These deficiencies include improper gene expression, protein expression, or signal activation/propagation. To date, many different pharmacological-based strategies have been explored to correct these changes, although with very limited success. One reason for this may be that the method of delivery has not been optimized for wound healing applications. In this chapter, various therapeutic options are discussed including small molecule drugs and biologics that are of interest for wound healing experiments, with a focus on non-healing ulcers. This discussion is followed by a basic introduction to controlled drug delivery systems and an overview of various strategies that have or can be employed for therapeutic intervention to promote wound closure.

Laser tissue welding (LTW) is a sutureless technique for sealing incised or wounded tissue and is an attractive alternative or supplement in various surgeries. In LTW, chromophores convert laser light to heat which ultimately results in tissue sealing. While LTW has had success without exogenous absorbers, introducing chromophores that absorb near-infrared light creates differential laser absorption and allows for a laser wavelength that minimizes tissue damage. Additionally, chromophore-embedded thin biopolymer films have been used to create robust tissue bonding in larger tissue geometries. Advances in LTW procedures and materials (chromophores and composites) can reduce treatment complexity, facilitate welding of larger tissue, and result in higher tensile strengths compared to suturing in many cases. However, the elevated temperatures reached due to laser light absorption and concomitant heat generation can cause significant tissue damage in some cases, and procedural and material improvements to LTW are underway to optimize the trade-off between tissue damage and tissue seal strength.

Bioprinting technology has emerged in recent years as an attractive method for biofabrication of 3D tissues and organs. The widespread utility of this promising technology has resulted in its exploration and implementation in a variety of regenerative medicine applications, including the production of skin or treatment of wounds for patients with skin wounds or burns. Bioprinting can take the shape of several printing modalities, including inkjet printing, extrusion-based printing, and laser-induced forward transfer printing, each of which has their own pros and cons, and lend themselves to different approaches and applications. Using these technologies, researchers can leverage engineered biomaterials and potent biological components such as stem cells or cytokines to produce effective therapies to treat patients. This chapter discusses skin regeneration through bioprinting, and the multiple technologies employed for skin regeneration, including the development bioprinter device and their general requirements, biological components that are printed, and the biomaterials and matrices that are integral in facilitating integration between the hardware, biologicals, and the patient.

Controlling the process of wound healing is necessary to prevent abnormal healing processes such as chronic wounds and scarring. In this chapter, we discuss the use of electroporation, and it’s various applications treating wounds. Electroporation is an increase of cell membrane permeability in a temporary or permanent way by exposing the cells to a high voltage pulsed electric field. Temporary increase of cell membrane permeability by external pulsed electric fields is known as reversible electroporation and permanent increase in the permeability, which leads to cell death, is known as irreversible electroporation. Both modes of electroporation have been used for the wound healing applications. We start the chapter with the basic explanation of electroporation phenomena. Next, we discuss the use of reversible electroporation in the wound healing for DNA delivery to wounds. We continue with examples of the use of irreversible electroporation for burn wounds disinfection. Next, we show the use of both reversible and irreversible electroporation in scar treatment. We conclude with future direction that could lead this non-thermal, chemical free intervention by pulsed electric fields described for practical clinical applications in wound management.

The following section is included:

• Index