How did a design and development consultancy develop the world's first personal capnometer?

A small battery powered personal capnometer which can be used to measure the amount of carbon dioxide in exhaled breath has been developed.

The device, N-Tidal, aims to measure the changes in carbon dioxide concentration as a patient breathes in and out in order to assess the health of a patient's lungs.

N-Tidal works by measuring changes in the infra red optical absorption at the carbon dioxide absorption wavelength. Therefore, the device requires custom plastics, electronics, and infra red optics.

It is thought that the device is advantageous to current hospital practice where capnometers are typically a large bedside machine connected to a patient’s face mask, or a sensor located in life support equipment. Whereas N-Tidal is suitable for use as a personal respiratory monitor as the device claims to be accurate, low cost and low power.

There were multiple plastic design challenges involved with the development of the N-Tidal device including the ability to avoid cross contamination between patients. This is because the device requires the patient to breathe in and out of the device for a minute whilst their normal breath profile is measured, and therefore in between patients the breath tube must be replaceable.

Commenting on the challenges faced, Wideblue mechanical engineer Callum Stewart explained: “The breath tube assembly had a number of design constraints and requirements. It needed to be sealed so that no breath from one patient could be inhaled by another patient, and needed to be solid so that electronics connections could reliably be made to the breath tube.

“It also needed to be relatively soft so that a patient could put the mouthpiece in the mouth.

“We chose a medical grade acrylonitrile butadiene styrene (ABS) body to hold the electronics and infra red components and a compliant medical grade polypropylene for the mouthpiece. The two materials provided a good seal, a solid fit and are comfortable to use.”

A further consideration was designing a breath path through the device with enough clearance to insert the breath tube, and also with enough alignment accuracy to carry out a precision infra red measurement.

Colin Phimister, another Wideblue mechanical engineer, worked on this problem and commented: “The brief from the client was to design and manufacture a stable infra red optical platform with enough clearance to get the replaceable breath tube through.

“We designed a robust infra red optical platform suitable for injection moulding. The material needed to accept specialist infra red optical coatings but also be stiff enough to avoid warping in normal hand held use.

“We selected a glass filled polycarbonate material. Optical coatings have good adhesion and durability on polycarbonate but the material was not sufficiently stiff without the addition of the glass filling.”

Russell Overend, Wideblue's managing director, adds: “Wideblue and our customer Cambridge Respiratory Innovations Limited (CRiL) chose Kansas based Pivot International to manufacture the product as they were able to manufacture the injection mould tooling, mould the parts to the required tolerances and finally assemble the plastic parts, infra red optical parts, electronics parts, program the electronics and test the assemblies. This made it easy for us to make the transition from design and development into production.

“The device is to be used by patients with respiratory related diseases such as asthma and chronic obstructive pulmonary disease.

“Early clinical results show that the data from the device can be used to predict attacks or exacerbations in advance by measuring changes in carbon dioxide profiles. Doctors can then decide if a change in medication is required to prevent an attack.

“The device is currently undergoing clinical/user trials and has already produced superb clinical results. Subject to successful completion of these trials and regulatory approvals we expect the units to go into commercial production in 2020."

The medical device was produced by design and development consultancy Wideblue for client CRiL.