# A Review of Passive Constant Flow Regulators for Microfluidic Applications
## 5 Conclusions
**Nomenclature**  
**Symbol** **Name** **Units**  
H  Height  m  
L  Length  m  
\(p\)  Pressure  Pa  
\(P_{\rm in}\), \(P_{\rm out}\)  Inlet/Outlet pressure  Pa  
\(\Delta p\)  Pressure gradient  Pa  
\(\Delta P_{\rm reg\,min}\), \(\Delta P_{\rm reg\,max}\)  Minimum/Maximum pressure gradient to achieve  
\(\Delta P_{\rm reg\,min}\), \(\Delta P_{\rm reg\,max}\)  the nominal flow rate  
\(\Delta P_{\rm off}\)  Shut-off pressure gradient  Pa  
\(Q\), \(Q_{0}\)  Flow rate, Nominal flow rate  m\({}^{3}\).s\({}^{-1}\)  
\(R_{h}\)  Hydrodynamic resistance  Pa\(\cdot\)s\(\cdot\)m\({}^{-3}\)  
T  Temperature  K  
v  Velocity  m\(\cdot\)s\({}^{-1}\)  
W  Width  m  
\(\eta\)  Dynamic viscosity  Pa\(\cdot\)s  
\(\lambda\)  Relaxation time of a polymer solution  s  
_Re_  Reynolds number -  
_Kn_  Knudsen number -  
**Abbreviation** **Name**  
BioMEMS  Biomedical microelectromechanical system  
CSF  Cerebrospinal fluid  
CTQ  Critical-to-quality  
EOF  Electroosmotic flow  
FEM  Finite element method  
FSI  Fluid-structure interaction  
HF  Hydrofluoric acid  
ICP  Intracranial Pressure  
KOH  Potassium hydroxide  
LOC  Lab-on-a-chip  
MEMS  Microelectromechanical system  
NPH  Normal-pressure hydrocephalus  
PDMS  Poly(dimethylsiloxane)  
PE  Polyethylene  
PET  Polyethylene terephthalate  
PEEK  Polyether ether ketone  
PVC  Polyvinyl chloride  
Si  Silicon  
SOI  Silicon on insulator  
Ti  Titanium  
\(\mu\)TAS  Micro total analysis system  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Author and Year** & **Name/Description** & **Construction** & **Working Fluid** & \begin{tabular}{c} **Plateau Pressure** \\ **(mbar)** \\ \end{tabular}  & \begin{tabular}{c} **Flow Rate** \\ **(mL/min)** \\ \end{tabular}  & \begin{tabular}{c} **SD (\%)** \\ \end{tabular}  \\ \hline \hline \multicolumn{7}{c}{**Min**} & **Max** & \\ \hline \hline \multicolumn{7}{c}{**MECHANICAL PASSIVE CONSTAT FLOW REGULATORS**} \\ \hline \hline \multicolumn{7}{c}{Five-layer-stacked PDMS device—membrane (W200, L300, H130) in um} \\ \hline \multicolumn{7}{c}{Five-layer-stacked PDMS device—membrane (W200, L400, H130) in um} \\ \hline \multicolumn{7}{c}{} \\ \hline \multirow{2}{*}{[PERSON] 2016 [44]} & \multirow{2}{*}{Passive flow regulator} & \multirow{2}{*}{Five-layer-stacked PDMS device—membrane (W150, L300, H70) in um} \\ \cline{3-7}  & & & & & \\ \cline{3-7}  & & & & & \\ \hline \multirow{2}{*}{[PERSON] 2017 [45]} & \multirow{2}{*}{Massive Microfluid autoregulatory valve (channel height 145 um)} & \multirow{2}{*}{Water} & \multirow{2}{*}{200} & \multirow{2}{*}{700} & \multirow{2}{*}{1.36} & \multirow{2}{*}{3.4\({}^{*}\)} \\  & & & & & & \\ \hline \multirow{2}{*}{Zhang 2017 [45]} & \multirow{2}{*}{Massive Microfluid autoregulatory valve (channel height 90 um)} & \multirow{2}{*}{Server-layer-based structure (polymer films and PDMS membranes)} & \multirow{2}{*}{Water} & \multirow{2}{*}{200} & \multirow{2}{*}{700} & \multirow{2}{*}{0.29} & \multirow{2}{*}{4.2\({}^{*}\)} \\  & & & & & & \\ \hline \multirow{2}{*}{Zhang 2019 [46]} & \multirow{2}{*}{Massive flow regulator} & \multirow{2}{*}{Power-layer structure with thin PDMS} & \multirow{2}{*}{Water} & \multirow{2}{*}{700} & \multirow{2}{*}{1300} & \multirow{2}{*}{25.2} & \multirow{2}{*}{5\({}^{*}\)} \\  & & & & & & \\ \cline{3-7}  & & & & & & & \\ \hline \multirow{2}{*}{[PERSON] 2014 [55]} & \multirow{2}{*}{ \begin{tabular}{c} CSF shunt/Infagra OSV \\ II (Integra LifeScience)— normal flow \\ \end{tabular}  } & \multirow{2}{*}{Moving elastomeric membrane + notched pin} & \multirow{2}{*}{Water (test)/CSF} & \multirow{2}{*}{9} & \multirow{2}{*}{36} & \multirow{2}{*}{0.3} & \multirow{2}{*}{15.0} \\ \cline{2-7}  & & & & & & \\ \hline \multirow{2}{*}{[PERSON] 2018 [56]} & \multirow{2}{*}{ \begin{tabular}{c} CSF shunt/Infagra NPH \\ (Integra LifeScience)— low flow \\ \end{tabular}