Next generation human skin constructs as advanced tools for drug development

Abstract

Many diseases, as well as side effects of drugs, manifest themselves through skin symptoms. Skin is a complex tissue that hosts various specialized cell types and performs many roles including physical barrier, immune and sensory functions. Therefore, modeling skin in vitro presents technical challenges for tissue engineering. Since the first attempts at engineering human epidermis in 1970s, there has been a growing interest in generating full-thickness skin constructs mimicking physiological functions by incorporating various skin components, such as vasculature and melanocytes for pigmentation. Development of biomimetic in vitro human skin models with these physiological functions provides a new tool for drug discovery, disease modeling, regenerative medicine and basic research for skin biology. This goal, however, has long been delayed by the limited availability of different cell types, the challenges in establishing co-culture conditions, and the ability to recapitulate the 3D anatomy of the skin. Recent breakthroughs in induced pluripotent stem cell (iPSC) technology and microfabrication techniques such as 3D-printing have allowed for building more reliable and complex in vitro skin models for pharmaceutical screening. In this review, we focus on the current developments and prevailing challenges in generating skin constructs with vasculature, skin appendages such as hair follicles, pigmentation, immune response, innervation, and hypodermis. Furthermore, we discuss the promising advances that iPSC technology offers in order to generate in vitro models of genetic skin diseases, such as epidermolysis bullosa and psoriasis. We also discuss how future integration of the next generation human skin constructs onto microfluidic platforms along with other tissues could revolutionize the early stages of drug development by creating reliable evaluation of patient-specific effects of pharmaceutical agents. Impact statement Skin is a complex tissue that hosts various specialized cell types and performs many roles including barrier, immune, and sensory functions. For human-relevant drug testing, there has been a growing interest in building more physiological skin constructs by incorporating different skin components, such as vasculature, appendages, pigment, innervation, and adipose tissue. This paper provides an overview of the strategies to build complex human skin constructs that can faithfully recapitulate human skin and thus can be used in drug development targeting skin diseases. In particular, we discuss recent developments and remaining challenges in incorporating various skin components, availability of iPSC-derived skin cell types and in vitro skin disease models. In addition, we provide insights on the future integration of these complex skin models with other organs on microfluidic platforms as well as potential readout technologies for high-throughput drug screening.

Progress in melanoma modelling in vitro

“…Integration of microfluidics and electrical sensing modality in 3D tumor microenvironment may provide a powerful platform to accurately and rapidly monitor the response of cancer cells to a series of drugs . In addition, the on‐a‐chip approach at small‐scale makes this platform also considerably cost‐effective for drug screening applications . Mori and co‐workers in a perfusable skin equivalent model with vascular channels coated with endothelial cells were able to measure the cell density and distribution following perfusion and the amount of drug absorbed into the vascular channel .…”

“…Scale bar = 100 μm at small-scale makes this platform also considerably cost-effective for drug screening applications. [101] Mori and co-workers in a perfusable skin equivalent model with vascular channels coated with endothelial cells were able to measure the cell density and distribution following perfusion and the amount of drug absorbed into the vascular channel. [102] Abaci and co-workers using a skin-ona-chip platform with a unique capability to recirculate the medium demonstrated that the cancer drug, doxorubicin, may have direct toxic effects on keratinocyte proliferation and differentiation.…”

Engineering tissue‐specific blood vessels

“…Topically applied drugs must permeate the epidermal barrier, whereas systemically delivered drugs are controlled by the endothelial barrier. Incorporating vasculature into bioengineered skin is essential for improving the lifespan, graftability, and for studying the systemic delivery of drugs from/to the skin …”

Skin microbiota and human 3D skin models

“…Compared to the prevalent “classical” 3D skin models, the development of novel complex next‐generation skin models based on induced pluripotent stem cells (iPSCs) and microfluidic technology (“skin‐on‐chip”) or using 3D bioprinting techniques will offer promising tools for the study of the microbiota‐skin interplay. For example, the use of iPSC‐derived cells such as keratinocytes, fibroblasts and melanocytes has already led to the successful generation of a complex 3D skin model .…”

“…In this context, the use of immortalised N/TERT keratinocytes seems to be a valuable approach as N/TERT keratinocytes have similar characteristics to primary keratinocytes, and successful generation of 3D skin models using N/TERT keratinocytes has been reported. [25,26] Compared to the prevalent "classical" 3D skin models, the development of novel complex next-generation skin models based on induced pluripotent stem cells (iPSCs) and microfluidic technology ("skin-on-chip") [27,28] or using 3D bioprinting techniques [29] will offer promising tools for the study of the microbiota-skin interplay. For example, the use of iPSC-derived cells such as keratinocytes, fibroblasts and melanocytes has already led to the successful generation of a complex 3D skin model.…”