Why an ultrasonically welded flow sensor was critical to a ventilator design

Didier Perret, medical business development manager, Branson Welding and Assembly at Emerson explains the role ultrasonically welded flow sensors played in Hamilton Medical’s ventilators.

Hamilton Medical produces intelligent ventilation solutions for intensive-care units and critical-care transports. To meet the exploding demand for ventilators during the COVID-19 pandemic, Hamilton Medical, with the support of a local team from General Motors, established a new ventilator production site in Reno, Nevada, USA.

In four months the new Hamilton Medical site moved from bare floor to full production, delivering the first of thousands of HAMILTON-T1 ventilators purchased under contract by the U.S. Department of Health and Human Services in September 2020.   

Since 1983, the proximal flow sensor has been the centrepiece of all Hamilton Medical ventilators. It provides valuable data directly from the patient’s airway opening. The sensor must be able to detect even tiny pressure changes that signal a patient’s attempts to breathe naturally, adjusting to the patient’s own inhalation process to comfortably support it with the required volume of oxygenated air. 

Successfully mass-producing highly engineered and extremely precise flow sensors from disposable components was not simple; it posed significant design, assembly and cost challenges for Hamilton Medical. Fortunately, the company had already invested in years of design and development efforts in Switzerland, collaborating with Emerson and local machine builders to create and validate automated systems for assembling all-plastic flow sensors using Branson ultrasonic welding technology. Ultrasonic welding combines high-frequency vibration and compressive force to create frictional heating and targeted melting at the interface of mating thermoplastic parts, enabling them to join permanently. The process is well suited to flow-sensor assembly, since it virtually eliminates risk of heat, mechanical stress, or contamination to the sensor’s fragile membrane.

The fully automated flow-sensor assembly system was built by Switzerland-based Imperia Systems under the supervision of Hamilton, with Emerson providing medical-production-ready Branson 2000Xc ultrasonic welders, weld tooling, and technical and integration support to the fast-track assembly project. Following delivery, the system underwent successful IQ/OQ/PQ (installation, operational and performance qualification) in time for the September 2020 production launch. 

During operation, the assembly system robot picks and positions the three sensor components atop the lower weld tool. First comes the lower half, into which is inserted the flap containing the membrane. After the robot validates proper ring-membrane position, the upper half is placed on top. Then, the actuator of the Branson 2000Xc welder descends, compressing the upper weld tooling onto the sensor components. Compressive force actuates the ultrasonic weld, which is completed in less than one second. 

As the ultrasonic weld is made, the digital controls in the Branson 2000Xc welder monitor all weld parameters in real time and save a complete weld data record for each part. Medical-ready Branson welders are equipped with data security features including hierarchical password protection and high-grade data encryption. They support compliance with leading global manufacturing standards including ISO 13485, as well as medical device production and part traceability requirements in the European Union’s Medical Device Regulation and the U.S. Food and Drug Administration’s (FDA) 21 CFR Part 11 regulations.

How ventilators work

Mechanical ventilators like the HAMILTON-T1 provide respiratory support to the lungs of patients who are unable to breathe sufficiently by themselves due to injury or respiratory disease. Following intubation — the insertion of an endotracheal tube into a patient’s trachea — ventilators offer multiple modes for safely providing the lungs with sufficient oxygen and removing carbon dioxide. Modes are selected based on the patient’s remaining respiratory capacity and range from fully automated to supportive (e.g., triggered by the patient’s own efforts to breathe).

And especially with the help of intelligent digital controls, automating the human respiratory process involves great precision and complexity. Ventilators may deliver a high-oxygen mixture at a carefully managed pressure, breath volume, respiration rate, humidity level and temperature. Hamilton Medical ventilators follow the principle of intermittent positive pressure ventilation. Gas with a defined oxygen concentration moves under positive pressure to the lungs, passing through bronchial tubes into tiny individual air sacs (alveoli). In the alveoli, the gas exchange takes place: Oxygen is exchanged with carbon dioxide at the red blood cells. During expiration, the ventilator’s exhalation valve opens to relieve the pressure, and the gas enriched with carbon dioxide can escape through the alveoli from the lungs and out of the body.  

A circuit of two tubes, attached to the ventilator and the patient’s airway, constitutes the breathing circuit. The two tubes meet at a Y-piece, with the inhalation tube and valve on one branch, the exhalation branch and valve on the other. At the Y-piece the breathing gas passes through the proximal flow sensor, moving to and from the patient airway (see Figure 2).

This process itself is relatively simple — alternating cycles of higher pressure for inhalation and pressure release for exhalation. The complexity stems from the precision needed to replicate fine adjustments that a patient’s brain would otherwise make to regulate the breathing process. This calls for accurate real-time control of myriad factors — mixture, flow rate, pressure and even the timing of each breath.