Pressure sensors in medical fluid-handling devices — infusion pumps, dialysis machines, ventilators, anesthesia delivery systems, and patient monitoring systems — must respond accurately to the pressure of the fluid they monitor while being protected from direct fluid contact that would cause corrosion, contamination, or biologically problematic material transfer to the patient fluid. The adhesive encapsulant around the pressure sensor element is the barrier that separates the electronic sensing die from the process fluid, and it must satisfy three simultaneous requirements that are often in tension: it must transmit the pressure signal accurately (compliance requirement), protect the sensor from chemical and moisture attack (barrier requirement), and not transfer harmful extractables to the patient fluid (biocompatibility requirement). Selecting and applying medical-grade epoxy for pressure sensor encapsulation in fluid-handling medical devices requires understanding how these requirements interact and how the encapsulant design addresses each.

The Pressure Transmission Requirement

Pressure sensors in medical applications measure gauge pressure of fluid — the pressure relative to atmospheric reference — by deflecting a thin diaphragm that generates a strain signal proportional to the applied pressure. The sensing element is isolated from the process fluid either by the diaphragm itself (if the fluid is compatible with silicon or stainless steel) or by a secondary diaphragm and oil-filled pressure transfer cavity.

Where adhesive encapsulant is in contact with the primary diaphragm or the secondary diaphragm on the fluid side, the encapsulant modulus affects the pressure measurement. A very rigid encapsulant constrains the diaphragm deflection, reducing the sensor’s sensitivity. A very soft encapsulant provides inadequate mechanical support and may introduce pressure measurement error from compliance of the encapsulant itself.

Medical epoxy encapsulants for pressure sensor applications are formulated with modulus values in a specific range — typically 1 to 5 GPa — that provides adequate mechanical support without significantly constraining diaphragm deflection. Some sensor designs use the encapsulant only for peripheral support and leave the diaphragm area open to fluid or gas; in these configurations, the encapsulant modulus does not directly affect sensitivity but must still provide adequate structural retention of the sensor package.

Thermal expansion of the encapsulant affects the zero offset of the pressure measurement through temperature. As temperature changes, the encapsulant expands or contracts, applying a parasitic stress to the diaphragm that the sensor reads as pressure even in the absence of applied fluid pressure. Low-CTE encapsulant formulations minimize this thermal zero shift. The CTE of the encapsulant should be matched as closely as possible to the CTE of the sensor package substrate (typically stainless steel or ceramic) to minimize differential thermal stress.

Barrier Properties Against Fluid Ingress

The encapsulant’s barrier function protects the sensor electronic components from moisture and process fluid ingress. Moisture ingress causes corrosion of bond wire, die metallization, and lead frame materials; electrolytic leakage between high-impedance circuit nodes; and swelling of the epoxy package that applies stress to the die and causes mechanical sensitivity drift.

Water vapor transmission through the cured epoxy is the key barrier property parameter. Standard medical-grade epoxy has water vapor transmission rates of approximately 0.1 to 1 g/(m² · day) at 25°C and 90 percent relative humidity for a 1 mm film — adequate for most fluid-handling applications at ambient temperature. At higher temperatures or in continuous immersion, transmission rates increase and the moisture barrier function degrades over time.

Chemical barrier against the specific fluids in the device depends on the formulation chemistry. For medical saline and aqueous biological fluids, standard aromatic amine-cured or anhydride-cured epoxy provides adequate resistance. For devices handling lipid-containing fluids (parenteral nutrition solutions, lipid emulsions) or organic solvents (in pharmaceutical compounding equipment), the encapsulant must be specifically evaluated for swelling and adhesion loss in those chemical environments.

Fully void-free encapsulation — achieved through vacuum potting or centrifuge potting — ensures that the moisture barrier is continuous without air pockets or voids that provide pathways for fluid to reach the sensor elements. Voids in the encapsulant are both mechanical weak points (initiation sites for cracking under pressure cycling) and moisture ingress pathways.

For encapsulant barrier characterization data for specific fluid environments in your device, Email Us — Incure can provide immersion test data and moisture permeability measurements for relevant formulations.

Biocompatibility Requirements for Fluid-Contact Encapsulants

When the encapsulant is in contact with process fluid that subsequently contacts the patient — the fluid path of a dialysis machine, the infusion fluid in a pump, or the respiratory gas in a ventilator — the encapsulant is a fluid-contact material and requires the same extractables and leachables assessment as any other fluid-pathway material.

ISO 10993-12 extraction testing on the encapsulant at conditions simulating the device fluid contact — aqueous extraction at body temperature or at the device’s process temperature, for the duration of patient contact — generates the extractables data. ISO 10993-17 toxicological risk assessment confirms that the identified extractables are below patient safety thresholds.

For pressure sensor encapsulants that contact aqueous process fluids at physiological conditions, the extractables of primary concern are: amine hardener residuals (potentially cytotoxic at elevated concentrations), plasticizers and diluents (extracted rapidly into aqueous media), and catalysts and accelerators (variable toxicity profiles requiring compound-specific assessment). Medical-grade encapsulant formulations designed for fluid-contact applications are characterized for low aqueous extractable content.

Pressure Sensor Encapsulation Process

Sensor encapsulation in device manufacturing uses a controlled dispense process: the sensor package is placed in the encapsulant mold or housing, the medical-grade epoxy is dispensed to fill the housing to the specified height, and the assembly is cured under a defined schedule.

Bubble-free fill is essential for consistent pressure response and barrier integrity. Vacuum degassing the mixed epoxy before dispensing removes dissolved gas that would form bubbles during cure. For viscosity ranges compatible with vacuum potting, the assembly can be potted under vacuum — simultaneously degassing and filling under reduced pressure to prevent entrapment.

Cure at the temperature specified for the formulation develops the full mechanical and chemical properties. For low-temperature-sensitive sensor assemblies, formulations with cure cycles at 80°C to 100°C allow encapsulation without thermal damage to adhesive bonds or solder joints elsewhere in the sensor package.

Post-cure pressure cycling — cycling the encapsulated sensor through the rated pressure range repeatedly before calibration — stabilizes any residual cure-induced stress in the encapsulant and reveals any mechanical instabilities in the package before calibration. A stabilized sensor maintains its calibration through the device’s service life more reliably than an uncycled sensor.

Contact Our Team to discuss medical-grade epoxy selection, modulus optimization, void-free potting processes, and biocompatibility documentation for pressure sensor encapsulation in fluid-handling medical device applications.

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