Design Criteria for Generating Physiologically Relevant In Vitro Models in Bioreactors

Abstract

Abstract:In this paper, we discuss the basic design requirements for the development of physiologically meaningful in vitro systems comprising cells, scaffolds and bioreactors, through a bottom up approach. Very simple micro-and milli-fluidic geometries are first used to illustrate the concepts, followed by a real device case-study. At each step, the fluidic and mass transport parameters in biological tissue design are considered, starting from basic questions such as the minimum number of cells and cell density required to represent a physiological system and the conditions necessary to ensure an adequate nutrient supply to tissues. At the next level, we consider the use of three-dimensional scaffolds, which are employed both for regenerative medicine applications and for the study of cells in environments which better recapitulate the physiological milieu. Here, the driving need is the rate of oxygen supply which must be maintained at an appropriate level to ensure cell viability throughout the thickness of a scaffold. Scaffold and bioreactor design are both critical in defining the oxygen profile in a cell construct and are considered together. We also discuss the oxygen-shear stress trade-off by considering the levels of mechanical stress required for hepatocytes, which are the limiting cell type in a multi-organ model. Similar considerations are also made for glucose consumption in cell constructs. Finally, the allometric approach for generating multi-tissue systemic models using bioreactors is described.

Environmental Control in Flow Bioreactors

“…The authors conclude that the system provides several advantages thanks to the dynamic control of oxygen, achieved by programming temporal changes in the perfusion rates. However, it is known that microfluidic systems are not really representative of physiological conditions and that they present several issues related to edge effects, high shear and nutrient depletion [2]. On the other hand, the use of very large volume bioreactors can be useful for industrial purposes, but in the case of in vitro studies can lead to an unnecessary expenditure of materials.…”

“…The shear stress on the surface of a non-porous tissue construct depends linearly on the density and the viscosity of the culture medium used. In our model, the culture medium was considered an essentially aqueous media at body temperature characterized by the following constants: T = 310.15 K, ρ = 993 kg/m 3 , and η = 0.7 × 10 −3 Pa·s [2,22]. As boundary conditions, the inflow velocity v in was set to 2.12 × 10 −3 , 4.24 × 10 −3 , 6.36 × 10 −3 , and 10.6 × 10 −3 m/s (corresponding to 100, 200, 300, and 500 µL/min, respectively), on the basis of previous experiments with similar bioreactors.…”

“…Environmental parameters are also essential for the development of more physiologically-relevant in vitro models, as several studies have demonstrated the crucial role of the microenvironment on cell growth and function [1,2]. Indeed, as in the body, cells are extremely sensitive to chemical, as well as physical, stimuli, like oxygen concentration, pressure, temperature, and pH.…”

“…Finally, when the sensors are placed inside the bioreactor, we can refer to in-line measurements, or to at-line measurements when the sensors are not in the culture chamber and sampling is necessary. This distinction can be important for the correct design of culture parameter measuring systems, where the measured variables can be physical (pressure, temperature, flow, and stirrer speed), chemical (pH, pO 2 , nutrients, metabolites), or biological variables (cell metabolism, etc.). For each type of measurement, different sensing methods can be used and the best sensor for a specific application should be sterilizable and should not interfere with the medium.…”

Special Issue: Design of Bioreactor Systems for Tissue Engineering

“…This system has been tested with polyurethane and collagen scaffolds and human saphenous vein fibroblasts and endothelial cells. Design requirements for bioreactors are maturing and are normally centred on fluid mechanics and mass transfer: an example of the state-of-the-art is reported by Mattei et al [14]. This paper discusses oxygen-shear stress trade-offs and glucose consumption in cell constructs.…”