Advances in the Biofabrication of 3D Skin in vitro: Healthy and Pathological Models

Abstract

The relevance for in vitro three-dimensional (3D) tissue culture of skin has been present for almost a century. From using skin biopsies in organ culture, to vascularized organotypic full-thickness reconstructed human skin equivalents, in vitro tissue regeneration of 3D skin has reached a golden era. However, the reconstruction of 3D skin still has room to grow and develop. The need for reproducible methodology, physiological structures and tissue architecture, and perfusable vasculature are only recently becoming a reality, though the addition of more complex structures such as glands and tactile corpuscles require advanced technologies. In this review, we will discuss the current methodology for biofabrication of 3D skin models and highlight the advantages and disadvantages of the existing systems as well as emphasize how new techniques can aid in the production of a truly physiologically relevant skin construct for preclinical innovation.

Keywords: 3D tissue model; biofabrication; bioprinting; electrospinning; in vitro; preclinical testing; skin; skin disease.

Figure 1

Healthy skin structure: layers, sublayers, and appendages/macrostructures. The skin consists of three main strata, from bottom to top: hypodermis, dermis, and epidermis. The hypodermis, also known as the subcutaneous tissue, is comprised of adipocytes within a mesh of connective tissue through which nerves and blood vessels traverse. Above the subcutaneous tissue is the dermis, a thick fibrous layer partitioned into two sub-layers, the papillary dermis and the reticular dermis. The dermis is primarily comprised of a fibrous scaffold within which fibroblasts, dermal dendrocytes, mast cells, and histiocytes can be found. Additionally, blood vessels, lymphatic networks, glands, hair follicles, and tactile corpuscles reside here. Above the dermis, the outermost stratum is the epidermis. The epidermis consists of five substrata: the stratum basale, stratum spinosum, stratum granulosum, stratum lucidium, and stratum corneum from deepest to most superficial. The cellular constituents of the epidermis are predominantly keratinocytes, though Langerhans cells, Merkel cells, and melanocytes can also be found in this layer.

Figure 2

The interfaces at which biofabrication techniques enable the production of 3D physiological skin in vitro. Biofabrication techniques can function independently for a traditional purpose, but also in combination with other techniques to produce specialized structures. Bioprinting is specifically designed for the production of reproducible tissue constructs and precision positioning of cells. Electrospinning is designed for the production of functional tissues and use of biocompatible materials. New technologies should focus on the production of 3D microstructures. Where electrospinning and bioprinting meet we can create physiological extracellular matrix, while at the interface between electrospinning and new technologies is the ability to regulate tissue structure and mechanical properties as well as the production of vascular networks and adaptable microenvironments. The interface of bioprinting and new technologies would enable the production of 3D skin constructs with adaptable physiology for modeling disease and the possibility for preclinical testing of personalized medicines. Together the current technologies, bioprinting and electrospinning, as well as novel technologies can easily produce affordable 3D skin constructs that mimic physiology.