63
Two-dimensional (2D) and three-dimensional (3D) in vitro models may be used
to study vascular aging. 2D models are relatively cheap and simple; however, they
may not recapitulate important aspects of vascular biology [37] and in vivo predictivity [12] because cells are not exposed to normal mechanical signals, including
fluid shear stress, tension, and compression. These limitations have led to increased
interest in 3D models which provide more predictive data for in vivo tests [38].
Three-dimensional models may replicate both anatomical macro- and microstructures, including appropriate cell types, ECM, and suitable physiological cues [102].
Microfluidic devices offer the possibility of culturing living cells, in continuously perfused micrometer chambers, in order to model physiological functions of
tissues and organs [12]. Advantages of microfluidic devices comprise (1) the possibility of incorporating physical forces, including fluid shear stress, cyclic strain,
and mechanical compression, (2) the possibility of making a 3D microenvironment
by using hydrogels as scaffolds, (3) the possibility of mimicking relevant tissues by
incorporating human cells (cell lines, primary cells, stem cells differentiated to specific lineages as well as the differentiation process itself), and (4) controlling the
distribution of chemical variables.
Cells cultured under flow conditions have different biological properties. Shear
stress, which is the tangential force to the cells surface, is known to induce substantial morphological and biochemical changes in vascular cells through mechanotransduction [109]. The necessary flow rates, to reach typical shear stress forces,
vary from vessel to vessel. In healthy conditions, the rate is estimated to be between
1 and 6 dyn/cm
2
in the venous system and between 10 and 70 dyn/cm
2
in the arterial
system [69]. There are a variety of techniques used to achieve this, ranging from
pneumatic and syringe pumps to electro-kinetics to control shear stress in microfluidic devices. Constant flow, typical of many capillaries, can be applied through
gravity flow, where a height difference between inlet and outlet reservoirs is used to
provide steady differential pressure [92]. However, if flow rates are extremely high,
other fluid systems such as using liquid level sensing and computer-controlled
valves to transmit the proper amount of media may be valuable in maintaining a
certain pressure and flow rate [133]. Pulsatile flow can be achieved using an elastomeric microfluidic cell shearing chamber, interfaced with computer-controlled
movement of piezoelectric pins [110]. Additional approaches to generate high shear
stress in the fluidics include the incorporation of solenoid pinch valves into gravity
flow to turn vessel flow on and off electronically [114], external peristaltic pumps
[25], and pneumatic micropumps [108].
Microfluidic systems with progeria cells have been instrumental in the study of
pathological aging [97, 111]. In one case, a microfluidic system was developed with
a top fluidic channel, a middle thin polydimethylsiloxane (PDMS) membrane, and
a bottom vacuum channel (Fig. 3.2a). SMCs derived from human-induced pluripotent stem cells obtained from HGPS patients (HGPS-iPSC-SMCs) were cultured on
top of the membrane which was deformed by applying different amount of pressures on the bottom channel. This aged model combined biomechanical strain and
flow and thus allowed for a better understanding of the inflammatory response of
aged SMCs to strain, characterized by an increase in levels of inflammation markers
3 Physiological and Pathological Vascular Aging
Two-dimensional (2D) and three-dimensional (3D) in vitro models may be used
to study vascular aging. 2D models are relatively cheap and simple; however, they
may not recapitulate important aspects of vascular biology [37] and in vivo predictivity [12] because cells are not exposed to normal mechanical signals, including
fluid shear stress, tension, and compression. These limitations have led to increased
interest in 3D models which provide more predictive data for in vivo tests [38].
Three-dimensional models may replicate both anatomical macro- and microstructures, including appropriate cell types, ECM, and suitable physiological cues [102].
Microfluidic devices offer the possibility of culturing living cells, in continuously perfused micrometer chambers, in order to model physiological functions of
tissues and organs [12]. Advantages of microfluidic devices comprise (1) the possibility of incorporating physical forces, including fluid shear stress, cyclic strain,
and mechanical compression, (2) the possibility of making a 3D microenvironment
by using hydrogels as scaffolds, (3) the possibility of mimicking relevant tissues by
incorporating human cells (cell lines, primary cells, stem cells differentiated to specific lineages as well as the differentiation process itself), and (4) controlling the
distribution of chemical variables.
Cells cultured under flow conditions have different biological properties. Shear
stress, which is the tangential force to the cells surface, is known to induce substantial morphological and biochemical changes in vascular cells through mechanotransduction [109]. The necessary flow rates, to reach typical shear stress forces,
vary from vessel to vessel. In healthy conditions, the rate is estimated to be between
1 and 6 dyn/cm
2
in the venous system and between 10 and 70 dyn/cm
2
in the arterial
system [69]. There are a variety of techniques used to achieve this, ranging from
pneumatic and syringe pumps to electro-kinetics to control shear stress in microfluidic devices. Constant flow, typical of many capillaries, can be applied through
gravity flow, where a height difference between inlet and outlet reservoirs is used to
provide steady differential pressure [92]. However, if flow rates are extremely high,
other fluid systems such as using liquid level sensing and computer-controlled
valves to transmit the proper amount of media may be valuable in maintaining a
certain pressure and flow rate [133]. Pulsatile flow can be achieved using an elastomeric microfluidic cell shearing chamber, interfaced with computer-controlled
movement of piezoelectric pins [110]. Additional approaches to generate high shear
stress in the fluidics include the incorporation of solenoid pinch valves into gravity
flow to turn vessel flow on and off electronically [114], external peristaltic pumps
[25], and pneumatic micropumps [108].
Microfluidic systems with progeria cells have been instrumental in the study of
pathological aging [97, 111]. In one case, a microfluidic system was developed with
a top fluidic channel, a middle thin polydimethylsiloxane (PDMS) membrane, and
a bottom vacuum channel (Fig. 3.2a). SMCs derived from human-induced pluripotent stem cells obtained from HGPS patients (HGPS-iPSC-SMCs) were cultured on
top of the membrane which was deformed by applying different amount of pressures on the bottom channel. This aged model combined biomechanical strain and
flow and thus allowed for a better understanding of the inflammatory response of
aged SMCs to strain, characterized by an increase in levels of inflammation markers
3 Physiological and Pathological Vascular Aging