Liquid flow24 May 2019
A number of disposable sensors are now available to monitor fl uid control in medical devices, and these tend to be low-cost because there are no electronics located in the sensor. IVAM CEO Dr Thomas R Dietrich talks to Kerry Taylor-Smith about the potential of these sensors for mobile and point-of-care diagnostics.
During medical treatments, the outcome of therapy and the overall well-being of a patient so often depends on the reliable and continuous administration of drugs or intravenous fluids at low or ultra-low flow rates. However, such procedures and therapies are faced with the same challenges – precisely measuring and regulating lowest flow rates of medicines and/or fluids down to the microlitre per hour.
Smart infusion pumps provide well-controlled drug delivery over an extended period of time, but are unfortunately susceptible to failure. The technology lacks the ability to directly measure the flow rate of the drug inside tubing, leading to undetected failures or oversensitive pumps giving false alarms.
Typical failures include occlusion (a blockage or closing of the blood vessel or a hollow organ); air in the line; free flow; cross flow, where multiple infusions are taking place at the same time; and infiltration or extravasation, where intravenous fluid seeps into tissue surrounding the canular inserted in the vein.
Disposable liquid flow sensors containing smart chips could be a game-changer in the biomedical field, from controlling the flow of valuable ingredients in bioreactors to monitoring infusion therapies at low and ultra-low flow rates in a clinical environment. They have been designed for single-use, high-volume biomedical applications such as drug delivery systems, and their in-built failure detection features can help counteract failures such as cross flow and extravasation.
Smart chips within the sensor can combine microelectro mechanical systems (MEMS) and CMOS (complementary metal-oxide-semiconductor, a class of integrated circuits) on a single silicon wafer, and have the potential to improve diagnostics and automation. This unique technology is safe and reliable, not to mention highly precise, and is capable of detecting the smallest change in liquid flow rate in real time at the point of interest. The integration of single-use liquid flow sensors within medical devices, such as urinary catheters or infusion sets, is simple: the sensor chip is completely isolated, encased in a plastic housing – meaning it cannot come into contact with bodily fluids or medications – that can be secured in the fluidic line using standard Luer lock fittings. The sensor is also low-cost – it has been stripped back to a bare minimum without sacrificing mechanical, electrical and fluidic connections, making it ideal for disposal after use.
The straight, open-flow channel has no moving parts (meaning nothing to break or fail), which provides excellent reliability, and its small size results in efficient and highly repeatable measurements in real time, offering unprecedented reliability and safety. It can, for example, spot occlusion in a matter of seconds by noticing a decrease in flow rate even at ultra-low flow ranges. Medical-grade wetted materials also offer excellent chemical resistance and outstanding biocompatibility as well as sterilised operation.
“A disposable liquid flow sensor provides liquid flow measurement capability from inside medical tubing, such as an infusion set or a catheter,” explains Dr Thomas R Dietrich, CEO of IVAM, an international microtechnology business network and technology marketing expert based in Dortmund, Germany. “Because of its low cost, it is suitable for disposable applications.”
Drug delivery from an infusion set, an infusion pump or other medical devices can be measured precisely and in real time. “Inside a disposable liquid flow sensor, a microchip measures the flow inside a fluidic channel,” says Dietrich. “Flow rates, typically in the range of zero to several 100ml per hour are measured with a typical accuracy of 5% of the measured value. Inert medicalgrade wetted materials are used to ensure sterile operation with no contamination of the fluid.”
The sensors’ measurement method is based on a micro-thermal principle; an incredibly tiny heating element on the chip releases an insignificantly small amount of heat into a bypassing liquid to obtain a thermal flow measurement. The shape of its heat cloud is monitored by two temperature sensors positioned symmetrically up and downstream of the heat source, which detect minute temperature differences with incredible sensitivity: these fluctuations are directly related to the flow rate inside the fluidic channel. Using this principle, liquid flow sensors can continuously and reliably measure low flow rates typical of medical applications.
Intelligent, compact and cost-effective
Single-use liquid flow sensors have the potential to revolutionise drug delivery. They are considered more effective as it is easier to oversee and regulate the flow of medication or IV fluids, and their smart chips are helping to improve diagnostics and automation. The result is better patient treatments, improved safety and support for overworked hospital staff.
“Treatments will become more effective as they become easier to monitor and control,” says Dietrich. “Patients’ safety is improved by the automatic detection of failure modes like clogging, free flow, air bubbles, or leaks in the tubing connection.” This serves to increase the accuracy of drug delivery and automatic diagnostic equipment.
Such single-use liquid flow sensors can detect a drop in a primary infusion flow due to cross-flow errors from a secondary line where multiple infusions are taking place simultaneously, and includes bubble detection to identify air inside the infusion tubing.
Furthermore, the sensors are so sensitive that they can even detect a heartbeat, a direct indication of an intact connection of the infusion cannula to the vein of the patient. The absence of a pulse could indicate an interrupted connection caused by a kinked tube, ruptured tubing or a dislodged cannula, which could lead to infiltration or extravasation (leaking of intravenous fluid into the tissue surrounding the vein). Such incidents can occur when damage is caused to the vein during catheter insertion and can cause irritation if the fluid is a non-vesicant agent, or damage to nerves, tendons and joints if the fluid is a vesicant agent such as a chemotherapy drug.
These intelligent, compact and cost-effective liquid flow sensors have the potential to change drug delivery from the ground up, offering safer, more mobile and reliable care in hospitals and peoples’ homes.
“Disposable liquid flow sensors make the use of point-of-care diagnostic tools affordable for everyday use,” says Dietrich. “Even patients and family members without any medical education can use automatic equipment for diagnostics and drug delivery.”
The use of these devices at home is made possible as the sensors are cheap and easy to use. “This will give elderly and chronically ill people the opportunity to stay longer at home and avoid long clinical stays or movement to nursing homes,” explains Dietrich.
It is known that being mobile can reduce the length of a patient’s hospital stay while simultaneously improving their quality of life. However, ambulatory infusion pumps can be problematic in terms of ensuring the required flow rate is accurate – during a typical day it can be difficult for a patient to follow the handling instructions that ensure the specified performance, resulting in real-life flow rates deviating drastically from the intended values.
The use of a single-use liquid flow sensor could help overcome this by allowing the continuous delivery of highly concentrated medication over several days. This could be especially useful for those receiving chemotherapy where the constant administration of chemotherapy drugs over time has a better pharmacodynamic influence on effectiveness and toxicity compared with traditional bolus injections every day. There is even the potential for these single-use liquid flow sensors to be incorporated into wearable drug delivery devices, further enhancing a patient’s quality of life while they undergo treatment.
Another issue is the clogging of the restrictor of the pump – usually a thin capillary in the pressure drop element – or at the injection site. It can take hours before an occlusion is noticed by the medical team or the patient themselves, but the use of a disposable liquid flow sensor within the infusion set can spot such a failure much more quickly – even within seconds.
Such disposable liquid flow sensors can also prove useful in treating babies and young children who require much smaller doses of drugs due to their low body weight. Once integrated into an infusion set, the single-use liquid flow sensor allows for the accurate monitoring of administered flow rates – critical if excreted fluids are also being monitored – as well as overcoming other common failures related to current infusion technology quickly and reliably.
But these sensors aren’t just for administering drugs and fluid to the body, they can also measure and record things coming out of it. They can also help to diagnose acute kidney injury (AKI) by automatically measuring and recording urine output, a task currently carried out manually at regular intervals by nurses. Using disposable liquid flow sensors in this manner could allow for a timelier diagnosis and potentially prevent the onset of AKI, while also eradicating human error and freeing up medical staff’s time.
Integrating disposable liquid flow sensors into infusion tube sets has the potential to progress infusion therapies and allow for controlled drug delivery on a much broader scale. Measurements and vital signs could be obtained and stored automatically and electronically, eliminating human error and saving valuable time, which medical staff can dedicate to actually treating patients.
They could detect currently unnoticed failures and maybe even prevent them, increasing patient safety and well-being. Introducing such a ground-breaking technology could also present massive savings for the healthcare system, while decreasing workloads and stress for hospital staff.