Drop sensors from a drone over a forest. They need to survive impact, collect data, and transmit it. But they also need to stack flat for transport — you can't carry hundreds of three-dimensional objects in a small UAV payload.
The solution: sensors that are flat during storage and three-dimensional after deployment, with the transformation requiring no motors, no batteries, and no active control. A passive elastic hinge — made by laminating commercial sheet materials to rigid circuit boards — folds the sensor into its operational shape the moment it's released. Gravity does the work. The geometry of the laminate determines the fold angle.
The fold angle is programmed by a single oven-heating step during fabrication. Heat sets the elastic strain in the laminate; when released, the stored strain produces a predictable fold. Different laminate geometries produce fold angles from 10 to 100 degrees with 4-degree standard deviation. A geometric model links the laminate parameters to the resulting angle, making the design predictive rather than empirical.
Field tests demonstrate the complete pipeline: drone releases flat sensors, sensors fold during free fall, land intact, collect environmental data, and transmit via LoRa over distances exceeding 10 km. The trajectory during descent is modeled using atmospheric wind profiles, enabling controlled distribution over target areas.
The structural insight: what seems like a constraint — the sensor must be flat for storage — is actually a feature. The flat-to-3D transformation creates a deployment mechanism that requires no power, no moving parts, and no communication with the drone. The constraint of stackability produces the solution of self-deployment. The geometry that makes them packable is the same geometry that makes them deployable.