Prototyping a Medical Device: Electronics, Enclosure, and What to Leave to Specialists

A clinical review exposes a cleaning problem in a handheld airflow monitor: a drop of fluid sits in the case seam, and the exposed USB connector collects lint. The instrument measures airflow through a disposable mouthpiece and needs a cleanable housing.

The founder had already scheduled an engineering review with the clinic and a regulatory consultant. The electronics team needed to improve the device package and document the design. Specialist work on regulatory strategy, verification, and certification would remain part of the development plan. The founder wanted the next prototype to answer the clinic's handling questions without implying that a clean enclosure alone made a medical product ready for use.

Follow the air from the mouthpiece to the measurement

The device used a replaceable flow tube and a Sensirion SDP31 differential-pressure sensor. An STM32L4 controller sampled the sensor and displayed a test trace. A u-blox NINA-B306 Bluetooth LE module sent session data to a development tablet. The four-layer board also included a BQ24074 charger, an 800 mAh lithium-polymer battery, and a protected USB-C service port.

The mechanical engineer defined the sensor path before the board outline. The pressure ports needed short tubing runs and a clear connection to the disposable mouthpiece adapter. He placed the SDP31 near the flow-tube sockets, separated the radio module from the sensor input, and reserved a keep-out around its antenna. The 58 by 44 millimeter board used two M2.5 mounting points and test pads for sensor supply and I2C.

The Flux agent developed the schematic and board from the measurement requirements. The mechanical engineer checked the sensor datasheet, confirmed the module's antenna keep-out, and reviewed the charger limits against the battery specification. The BOM showed current parts and their datasheets. He simulated the charging supply and checked the board rules before the founder reviewed the assembly.

Shape a housing that can be wiped down

Clinic staff asked to clean the outer case between sessions. The founder, electronics engineer, and mechanical engineer listed the surfaces that a wipe could reach: the grip, screen window, control button, USB cap, and mouthpiece connector. The first prototype used a flat lower shell with two open screw recesses. The electronics engineer redesigned it with radiused corners, a continuous outer seam, and a removable flow adapter that could be cleaned or replaced separately.

The main body used PC/ABS with a smooth, 2-millimeter wall. A silicone button boot sealed the control opening, and a recessed gasket sat below the display window. The service connector moved to the bottom under a removable cap. Two internal M2.5 bosses supported the board; the outer case had no exposed fastener heads. The mouthpiece adapter used a separate molded part so the reusable body did not contact the disposable flow path.

The electronics engineer imported the screen, battery, connector, and flow sensor models. The first X-ray view showed a battery corner 0.6 millimeters from the display cable. She rotated the cell 90 degrees and moved the board boss 2 millimeters. The screen window then cleared the board, and the battery could be replaced during prototype service without removing the pressure tubing.

The founder held the new model in the clinic's grip position. The case was 142 millimeters long and 48 millimeters wide, with a 32-millimeter grip section. The electronics engineer increased the edge radius from 1.5 to 3 millimeters and placed the button away from the user's palm. The assembly view showed the screen, flow adapter, gasket, USB cap, and board as one device.

The team chose SLA printing for the next enclosure and planned injection-molded PC/ABS for production studies. Before selecting the final cleaning material or making a clinical claim, the founder's team would document the intended cleaning agents and ask the regulatory consultant to review the verification plan. That was ordinary product development work for a medical device, not a property that could be decided from a CAD view.

Three requests from the clinic review

Design a handheld respiratory assessment prototype around the Sensirion SDP31. Keep the pressure path short, accept a replaceable flow adapter, and show a live measurement trace on the front display.

Place the STM32L4, SDP31, NINA-B306, BQ24074, and 800 mAh cell on a 58 by 44 millimeter four-layer board. Preserve the radio keep-out and add test pads for sensor supply and I2C.

Fit the board and battery in a 142 by 48 millimeter cleanable case. Keep the display cable clear, seal the button and USB cap, and show the mouthpiece adapter separate from the reusable body.

Engineering ownership and specialist review

The mechanical engineer and the electronics engineer completed the electronics and mechanical review in four hours. They checked the pressure sensor path, antenna region, battery, connector cap, board bosses, and flow adapter in a shared project. The Flux agent kept the BOM aligned as they moved the battery. The founder approved the user-facing controls and documented the open verification items for the consultant.

The revised design used the SDP31 sensor, STM32L4, NINA-B306 radio module, charger, battery, screen, button boot, gasketed window, and capped USB-C service port. The mechanical package included the PC/ABS case, flow adapter, battery supports, and board bosses. the startup exported the schematic, board files, BOM, and STEP assembly for the next prototype build.

The regulatory consultant reviewed the product's intended use, risk controls, verification plan, and certification path. The engineering team supplied the design files and test points for that work. The prototype was used to gather handling feedback and check the electronics; it was not presented as a certified device. The founder kept that distinction in the project notes and in the clinic's test instructions.

The next case arrived as an SLA print. The clinic staff could wipe the curved shell without catching the cloth on screw recesses, and the USB cap stayed closed during the handling trial. The mechanical engineer checked sensor readings against the reference setup, while the electronics engineer inspected the screen seal and adapter fit. The design review produced specific changes for the next build instead of a general request to make the product cleaner.

A medical-device prototype checklist

• Define the measurement path, sensor interface, intended user, and prototype purpose.
• Use a certified RF module and preserve its antenna keep-out in the board and case.
• Check battery, sensor, connector, and charging requirements against datasheets.
• Review seams, wipe surfaces, button boots, display seals, and disposable interfaces.
• Import component models to check battery, screen, port, and board clearance.
• Plan regulatory, verification, and certification work with qualified specialists.

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