Wearables

A compression spring follows Hooke's law: its force falls as it extends. In a handheld autoinjector that finishes in seconds, the drop barely matters. In an on-body system delivering a large volume of viscous biologic over minutes or hours, it does, because flow slows toward the end of the dose when resistance in the fluid path is least forgiving. A constant-force spring is a flat metal ribbon coil rather than a helical spring, and it holds near-constant load through a long extension, giving the steady flow rate an OBDS needs from the first milliliter to the last. RPK Medical engineers both types and often combines them, matching the force profile to the formulation's viscosity and the target delivery time.

On-body systems exist because some biologics cannot be delivered as a quick handheld shot: the volume is too large or the formulation too viscous. The wearable answer is to slow delivery down and lower the flow rate, which reduces the force needed and the discomfort felt. That shifts the engineering problem from peak force to force stability over time: the drive must push consistently for the whole infusion without drift, and the parts restraining it must not creep or deflect under sustained load. RPK Medical sizes the spring against viscosity, cannula geometry, and delivery window, then validates the whole drive train, spring plus the stamped parts it acts on, under the loads it will actually see.

Beyond the drive spring, the mechanism needs needle and cannula insertion and retraction components, spring seats and guides, plunger and drive interfaces, latches and triggers, and structural frames that keep everything aligned inside a low-profile housing worn against the body. Progressive-die stamping suits high-volume, simpler geometries, while multislide (fourslide) forming produces the intricate multi-bend clips, retainers, and formed springs a compact wearable depends on, with less scrap and no carrier rails. Multislide also feeds material along its grain direction, improving fatigue resistance in parts that flex or carry sustained load.

Wearables and CGMs are mechatronic, so the electrical path matters as much as the mechanical one. Contact (contactor) parts such as contact springs, battery contacts, sensor contacts, and switches carry power and signal between the sensor, the electronics, and the transmitter, and must keep working while the device is worn, bumped, flexed, and exposed to sweat and movement for days at a time. That is a harder duty than a contact actuated a few times in a handheld device. RPK Medical forms them to tight tolerances with controlled contact force and clean, burr-free edges, and selects or plates materials for stable conductivity and corrosion resistance under continuous skin-adjacent wear.

A CGM has two distinct spring-driven moments. The applicator inserts the sensor through the skin in a single fast, controlled stroke, where too weak fails to seat and too harsh hurts, so the spring and the stamped parts guiding it must deliver the same stroke every time. Then the on-body unit must keep the sensor seated and in electrical contact continuously for days, a sustained-load problem in which the retention spring cannot relax or take a set while the patient sleeps, showers, and exercises. RPK Medical engineers for both the one-shot insertion and the days-long hold.

This is the defining difference between a wearable and a handheld device. A handheld spring sits relaxed until it fires; a wearable spring may sit compressed or extended for the whole wear period, and any relaxation shows up directly as lost flow rate or a loosened sensor. RPK Medical addresses it through material choice, using stainless steels such as 316 and 17-7PH and high-performance alloys including Hastelloy and Elgiloy selected for stable elastic behavior and creep resistance, combined with stress levels set deliberately below the threshold where relaxation begins, then confirmed by load testing over the intended wear duration rather than a single actuation.