Construction of Bone Hypoxic Microenvironment Based on Bone-on-a-Chip Platforms

Abstract

The normal physiological activities and functions of bone cells cannot be separated from the balance of the oxygenation level, and the physiological activities of bone cells are different under different oxygenation levels. At present, in vitro cell cultures are generally performed in a normoxic environment, and the partial pressure of oxygen of a conventional incubator is generally set at 141 mmHg (18.6%, close to the 20.1% oxygen in ambient air). This value is higher than the mean value of the oxygen partial pressure in human bone tissue. Additionally, the further away from the endosteal sinusoids, the lower the oxygen content. It follows that the construction of a hypoxic microenvironment is the key point of in vitro experimental investigation. However, current methods of cellular research cannot realize precise control of oxygenation levels at the microscale, and the development of microfluidic platforms can overcome the inherent limitations of these methods. In addition to discussing the characteristics of the hypoxic microenvironment in bone tissue, this review will discuss various methods of constructing oxygen gradients in vitro and measuring oxygen tension from the microscale based on microfluidic technology. This integration of advantages and disadvantages to perfect the experimental study will help us to study the physiological responses of cells under more physiological-relevant conditions and provide a new strategy for future research on various in vitro cell biomedicines.

Keywords: bone disease bone-on-a-chip platform; bone marrow; cell culturing; hypoxic microenvironment.

Figure 1

(A) Blood vessel projection image from bone to bone marrow (Reprinted with permission from Ref. [2], Copyright 2014, Springer Nature). (B) In the hematopoietic stem cell niche, cells compete for scarce nutrients and oxygen (Reprinted with permission from Ref. [1], Copyright 2010, Elsevier). (C) HIF signaling pathway (Reprinted with permission from Ref. [11], Copyright 2020, Elsevier). (D) Hypoxic microenvironment in bone marrow. * = Estimated conversion to pO₂ from mm Hg(Reprinted with permission from Refs. [5,12], Copyright 2017, Elsevier).

Figure 2

Construction of hypoxic microenvironment using microfluidic platform. (A) Microfluidic device capable of generating oxygen gradients using spatially confined chemical reactions (Reprinted with permission from Ref. [84], Copyright 2011, Royal Society of Chemistry); (B) Construction of hypoxic microenvironment with sodium sulfite low-oxygen layer [85]; (C) Constructing hybrid hypoxic microenvironments using polydimethylsiloxane polycarbonate (PDMS-PC) (Reprinted with permission from Ref. [67], Copyright 2014, Royal Society of Chemistry); (D) Construction of hypoxic microenvironment by hypoxia gas (Reprinted with permission from Ref. [86], Copyright 2021, American Chemical Society).