Time-of-Sight Liquid Flow Measurements in the Low Nanoliters per Minute Scale.

We describe an affordable and robust measurement technique applicable to nanoscale liquid flow. The approach can provide good precision (<1% RSD) in the 1.5-15 nL/min flow range. The motion of a conductive/nonconductive immiscible segmental interface in a capillary is followed by an admittance detector. The conductive marker segment, e.g., a salt solution, is protected on both sides from the principal flow stream by immiscible guard segments, typically a fluorocarbon (FC) liquid, of significantly greater impedance. Fluorosilylation of the capillary ensures no other liquid film between the FC segments and the wall (perfect piston). A given interface/marker can typically be used only once in interface/front tracking systems. We overcome this by putting the sensor capillary in a valved configuration where the flow direction in the sensor is reversed before the guard/marker segments escape. Several strategies are possible to interpret flow rate from the sensor output, including the rate of the interface movement. During the measurement process, a change in this rate of movement can be detected in <1 s. Small temperature variations in the 25-35 °C range did not affect sensor behavior.

Attenuation Coefficients of Tubular Conduits for Liquid Phase Absorbance Measurement: Shot Noise Limited Optimum Path Length.

We trace the history of liquid core waveguides (LCWs, also called liquid core optical fibers) and the role Teflon AF (TAF) has played in their development. We show that, in any shot noise limited situation, the optimum signal-to-noise ratio (S/N) occurs at a path length of 1/αa{ln[1 + 2(αa/αb)]}, approximately 2/αb under most conditions, αa and αb being the light attenuation coefficient due to the analyte and the background, respectively. The analysis shows that LCW length should be selected depending on the applicable αb value. An overly long LCW may exhibit a lower signal-to-noise ratio. Water-filled TAF-clad fused-silica (FS) tubes show the lowest attenuation across the wavelength range. Nevertheless, except at λ ≥ 600 nm, the observed αb values far exceed those reported for pure water: it appears that both impurities in the water and waveguide losses are involved. In examining the attenuation in various water-filled tubes, we find that the transmission of air-surrounded FS tubes is second only to TAF-clad FS tubes and is better than that of TAF tubes or externally mirrored FS tubes. Surprisingly, except for a window centered at ∼250 nm, light transmission in a water-filled poly(tetrafluoroethylene) (PTFE) tube is worse than in poly(ether ether ketone) (PEEK) tubing. Light transmission in PTFE tubes improves with increasing wall thickness.

Inline flow sensor for ventriculoperitoneal shunts: Experimental evaluation in swine.

Shunts are commonly employed to treat hydrocephalus, a severe central nervous disease caused by the buildup of cerebrospinal fluid in the brain. These shunts divert excessive cerebrospinal fluid from brain ventricles to other body cavities, thereby relieving the symptoms. However, these shunts are highly prone to failure due to obstruction from cellular debris, leading to cerebrospinal fluid accumulation in the brain and exacerbation of neurological symptoms. Therefore, there is a clinical need for a reliable, non-invasive method of monitoring shunt performance. Recently, a simple inline flow sensor was reported for monitoring ventriculoperitoneal shunting of cerebrospinal fluid in hydrocephalus treatment. The present work aimed to evaluate performance of the device in an animal model of hydrocephalus. Sensor-equipped shunt tubes were placed in anesthetized, juvenile swine. The flows reported by the sensor were compared with gravimetric flow measurements. Robust correlations (r ≈ 0.87-0.96) between the gravimetric and sensor-reported flows were obtained in 4 of the 6 experiments. The mean slope of the linear relationship of the gravimetrically determined vs. sensor flow rates was 0.98 ± 0.09 in the 6 experiments, indicating the sensor accurately reported shunt flows up to 35 ml/h. The sensor responded immediately to abrupt flow changes following cerebroventricular fluid injections. Minor hardware problems were identified and corrected. These experiments provide practical guidance for future preclinical testing of the device.

Inline Shunt Flow Monitor for Hydrocephalus.

In hydrocephalus, cerebrospinal fluid (CSF) builds up in the cranial cavity causing swelling of the head and potentially brain damage. A shunt to drain the fluid into a body cavity is now universally used, but failure is all too common. Techniques for ascertaining shunt failure are time-consuming, expertise-dependent, and often inconclusive. We report here an inline system that reliably and quantitatively measures the CSF flow rate. The system uses a single thermistor to both heat the surrounding and to sense the temperature. In the heating mode, the thermistor is subjected to a 5 s voltage pulse. In the sensing mode, it is part of a Wheatstone's bridge, the output being proportional to temperature. The signal, Vi - Vf, which is the net change ΔV in the bridge output immediately before and after the heat pulse, depends both on the flow rate and the surrounding temperature. In vitro, a single equation, flow rate = 3.75 × 10-6 × ΔV(-9.568+1.088 Vi) provided good prediction for the flow rate, with 6.3% RMS relative error. The sensor behavior is reported for flow rates between 0-52.5 mL/h at 32-39 °C, adequately covering the range of interest.