The on-body injector industry is the only mechanical engineering domain that directly couples a spring to a variable load without a transmission.
- Every car has a gearbox between its engine and wheels.
- Every clock had a fusee between its mainspring and escapement.
- Every hydraulic press has an intensifier between its pump and ram.
- The LBOI industry directly couples a spring to a plunger against a variable load — an architecture that was adequate for 1–2 mL autoinjectors but is fundamentally wrong for 10 mL high-viscosity delivery.
- The solution isn't a better spring; it's a transmission.
- The force gap is 5–10 N at end of stroke — well within the range addressable by proven mechanical transmission principles combined with viscosity reduction that the formulation likely already provides.
If you prioritize speed and certainty, pursue the compound cam solution — it's 8–14 months to production-ready with >99% confidence. If you're building a multi-product LBOI platform and can accept regulatory risk, pursue pulsatile delivery in parallel — it's transformative if validated.
Compound Solution: Cam + Shear-Thinning + Body-Heat Warming
Three modest interventions stack to provide >99% completion with no single element needing heroic performance — blocked only by a $5–15K rheometry study that sizes the cam.
Pulsatile Delivery via Cam with Dwell Sections
Exploit tissue viscoelastic relaxation during 12-second pauses to keep backpressure 30–50% lower throughout injection — blocked by absence of regulatory precedent for pulsatile SC biologics.
- If this were my project, I'd start Monday morning by shipping a 50 mL sample of the formulation to a capillary rheometry lab with a PO for $10K.
- That single data point — the Cross model fit at needle-relevant shear rates — determines whether we're solving a 5 N gap or a 10 N gap.
- Every downstream decision gets easier with that number.
- While waiting for rheometry (2–4 weeks), I'd have a mechanism designer start the cam profile synthesis assuming worst-case 40 cP.
- The cam design is a well-posed math problem: input the spring's measured force curve and the target output curve (Hagen-Poiseuille at actual viscosity + Doughty's tissue backpressure data), compute the cam profile, run a Monte Carlo tolerance stackup, and generate the mold geometry file.
- When the rheometry comes back, we update the target curve and the cam gets simpler — wider tolerances, lower lateral loads, maybe no barrel diameter change needed.
- In parallel, I'd spend $5–10K on a benchtop vibrator prototype and test insertion force in porcine skin.
- This is a cheap screening gate for the gauge shift concept.
- If the vibrator shows >40% force reduction, we have a potential game-changer worth a $200–400K clinical study.
- If not, we've lost $10K and gained certainty.
- The pulsatile delivery concept is the one that keeps me up at night — in a good way.
- The physics is compelling, the MPD analogy is quantified, and if it works, it's a platform technology that transforms every LBOI product in the pipeline.
- I'd design the ex vivo porcine tissue study to run concurrent with the cam development.
- At $10–25K, it's cheap insurance against the possibility that we're optimizing the wrong variable.
- What I would NOT do is pursue the elastomeric diaphragm force transformer (concept-9) or the mechanical flow governor (concept-10) at this stage.
- The diaphragm's ±20–25% unit-to-unit variation is a dealbreaker compared to the cam's ±3–5%.
- The governor only regulates velocity, not pressure, and doesn't address the fundamental force deficit.
- Both are intellectually interesting but practically inferior to the cam.
- Biggest risk I'd manage actively: the BD patent.
- Before spending more than $30K on cam tooling, I'd have an IP attorney review US10,159,800 claims against our specific LBOI implementation.
- If it's blocking, a four-bar linkage achieves the same force transformation in a different IP space — it's a 4–6 week design pivot, not a project killer.