Disposable liquid flow sensors open up new possibilities for medical devices

Smart infusion pumps have become an ubiquitous sight in many clinical settings. They deliver medication to a patient´s body in a controlled, precise, and automated manner and offer tremendous benefits to both patients and clinical personnel. However, these benefits are not without risks, as statistics from medical device reports show. Between 2003 and 2009, over 56,000 medical device reports associated with infusion pumps have been issued – including 710 related deaths. While one callback of over 200,000 infusion pumps may have generated particular public interest, more pertinent is that a total of 87 callbacks were issued by the FDA during this period. All parties agree that changes must be made to the overall systemic design of drug delivery. Improvements also have to be introduced to both the software and hardware design of the pumps. Disposable liquid flow sensors offer a hardware solution which drastically enhance safety and reliability and completely change the design of infusion pumps altogether. The use of a microchip to measure the current drug flow inside the infusion tubing enables failure modes to be detected with unprecedented reliability and will inspire a whole new generation of medical devices.

Common Failure Modes When Operating Infusion Pumps
While a multitude of possible failure modes can arise when operating an infusion pump (e.g. interface handling problems, software errors, battery issues, etc.), we focus on failures occurring in the infusion tubing that have a severely adverse effect on the patient, and show how a disposable liquid flow sensor can detect such failures. This analysis includes the failure modes Free Flow, Clogging, Bubbles, Open Line and Cross Flow in Multi-Infusion Settings.

Modern infusion pumps use sophisticated technological approximation without actual flow measurement to combat most of these failure modes. However, the reliability of these features is insufficient and results in a high number of false alarms, which in turn lowers the staff´s attention to individual alarms. Genuine alarms may therefore go unnoticed. Increasing the reliability of such alarms is crucial to ensuring safer infusion therapies. According to a patient safety expert, clinicians override some 90% of alarms triggered. The following failure modes are the most common in infusion tubing.

Free Flow occurs when the drug’s passage to the patient is unhindered, thereby causing an overdose (along with the related adverse effects). Although mechanical measures are implemented – mostly involving a free flow clamp designed to prevent such a failure mode – dangerous over-infusions from free flow still occur due to incorrect handling or malfunctioning mechanics in the pump. By integrating a liquid flow sensor, the drug flow rate is measured and an alarm is triggered if a defined upper threshold is exceeded.

Clogging refers to the blockage of the infusion line, thereby interrupting the delivery of the drug. This primarily occurs in kinked tubes. The clogging alarm is triggered if the liquid flow sensor detects a drug flow rate below a certain threshold.

Bubbles in the infusion tubing above a certain size can lead to a venous embolism and put the patient in mortal danger. False alarms triggered by bubble are very common in modern infusion pumps. Disposable liquid flow sensors feature bubble detection and can reliably detect possible dangers.

Open Line is a consequence of repeated mechanical stress or improper handling of the infusion set, and involves the tearing of the tubing or the disconnection of fluidic adapters. This results in an open line in the infusion. In addition to the drug delivery being interrupted, the blood can freely flow backwards through the tubing and cause blood loss in the patient. Disposable flow sensor technology functions bi-directionally, meaning that a blood drain can be detected as a negative flow.

Cross Flow in Multi-Infusion Settings occurs where “piggyback" infusions result in cross flow from one infusion line into the other. This can adversely affect the actual delivery rate of the drug and significantly alter medication results. By placing a bi-directional liquid flow sensor in the right section of the medical tubing, flow rate changes can be detected and looped back to the multi-infusion setup, thereby enabling the errors to be compensated or an alarm to be triggered.

Flow sensor technology that is successfully integrated into medical devices needs to meet three crucial requirements: performance, size and cost. The advanced technology used in Sensirion’s liquid flow sensors has powerful advantages in all three disciplines.

Illustration 1: Thermal measurement principle.Sensirion’s Liquid Flow Sensor Technology
The basis of this measurement technology (see illustration 1) is a small heater that introduces a negligible amount of heat to the drug solution. Two thermal sensors positioned upstream and downstream monitor this “thermal cloud" whose shape is directly related to the flow in the tubing. By using this principle, the liquid flow sensor has an excellent performance, especially in the low flow rates used in drug delivery. Since the sensor measures flow rate from outside the tubing, there are no contamination issues or obstructions inside the medical tubing.

The sensing element is an integrated part of a microchip, which – using Sensirion’s CMOSens technology – features all the signal processing, linearization, calibration and communication elements on a footprint of just 7.4 mm2. In other words, this is approximately the size of a pinhead. This combination of a sensing element with CMOS logic is based on semiconductor technology, which is also used in the production of computer chips.

The highly efficient production and economy of scale achievable in the semiconductor technology reduce the cost of a liquid flow sensor to a level where it is viable for use in a disposable infusion set. There are numerous ways of integrating the microchip into the tubing and – on a macroscopic level – into medical devices as well as hospital infrastructure as a whole. It is therefore extremely versatile in terms of application, from actual infusion pumps with a sensor inside the tubing to homecare devices for mobile therapies.

Integration of the Sensor
A plastic disc-shaped housing contains the microchip and Luer locks for connecting the sensor to the infusion set. Therefore the sensor is directly integrated into the infusion tube. There are also solutions with much stronger integration imaginable and a few examples are discussed below. Two criteria must be met to ensure that integration is successful: firstly, the microchip must be put in contact with the flow path. Secondly, there needs to be an electrical connection to the power supply and communication must be established. These requirements can be met in many different ways. One example features the microchip molded into an infusion set, which is then loaded into an infusion pump similar to models in use today. The power supply and communication are established wirelessly through near field communication once the pump hatch is closed. Additional functions such as a digital code for identifying drug types in the infusion could further increase safety.

Illustration 2: Mobile system for drug delivery.In an example of mobile drug delivery, the sensor is used in an infusion set, which is connected to a device worn on the arm of the patient (see illustration 2). This device houses a power supply, display, LEDs and/or acoustic alarms. Equipped with a valve and the requisite intelligence, the device can replace an infusion pump altogether in the case of gravity- or pressure-driven infusion.

The sensor chip could even be integrated directly into butterfly needles and measure the flow from a position as close to the patient’s body as possible. Other wearable devices like smart watches are conceivable in terms of communication and power supply.

We can conceive of devices that always consist of one part housing the complex electronics, e.g. an infusion pump or a wearable device, and another disposable part containing the microchip alone. In accordance with this concept, it is possible to envision a new generation of medical devices that yields increased safety, reliability and/or mobility.

Influence of a Disposable Liquid Flow Sensor on Today’s Infusion Pumps
In addition to improving a patient’s welfare, there is a strong economic motivation behind the use of disposable liquid flow sensors. Reducing the number of infusion pump incidents saves on costs for hospitals – each preventable adverse drug event (ADE) has been reported to cost $8,750. Therefore, many hospitals invest heavily in training and safety systems. The use of disposable liquid flow sensors in infusion pumps leads to much simpler designs and significantly increased safety and reliability. The failure modes discussed above can be detected much more assuredly, which will reduce false alarms and improve the awareness of clinical staff as to genuine failures.

Regarding the improvement of existing systems, the use of a disposable flow chip in infusions offers a sound technological basis for all-new concepts. Wearable devices with a valve and a sensor in a closed-loop, gravity-driven flow from an infusion bag offer a mobile solution for infusion therapies.

The use of smart, small and cheap liquid flow sensors will change our perception of drug delivery technology and provide safer, more reliable and mobile solutions for hospital and homecare therapies.

• Drug Delivery