Soft electronic skin for self-deployable tape-spring hinges

Deployable structures are essential in space systems because they can be compactly stowed for launch and then expanded on orbit. Tape-spring hinges, which store elastic strain energy in the folded state and self-deploy upon release, offer a simple and lightweight deployment solution without motors or active materials. Monitoring their folding/deployment behavior is challenging due to large localized strains and complex dynamics. Optical approaches (videometry, DIC/deflectometry) face limitations with out-of-plane motion, lighting, and target visibility in space. Conventional metal strain gauges have low gauge factors (~2), while fiber Bragg grating sensors can be brittle and integration may compromise structural integrity. Strict mass and size constraints further limit sensor choices. Soft electronics, which are thin, flexible, and can tolerate large strains, present an opportunity but have been primarily explored in biomedical contexts. This work introduces a soft, multifunctional e-skin for in-situ monitoring of tape-spring hinges to address these gaps.

Optical sensing: CCD videometry has been used to monitor deployable arrays; laser Doppler vibrometry and single-camera high-speed stereo-DIC measure vibrations but are sensitive to focal plane deviations, speckle quality, and lighting. In on-orbit photogrammetry (e.g., ISS Roll-Out Solar Array), shifting lighting hindered target resolution; retroreflective targets improved data but raised adhesion concerns. Alternative strain sensing: metal gauges are robust but have low sensitivity; FBG sensors can be integrated in structures but risk defects and are brittle for highly deformable thin-walled components. Soft/2D-material strain sensors: rGO-based sensors have reported high gauge factors (e.g., 754 at 5% strain), and rGO/CNT or rGO/TPU composites can be engineered for high stretchability (55–210% ranges) and cycling durability (>1000 cycles). Hybrid rGO/CNT films with metal oxides enable added functionalities (piezoelectric, gas sensing). These advances motivate using rGO/CNT thin films for high-sensitivity, small-strain measurements (±0.5%) expected in tape-spring folds.

E-skin design and materials: A three-layer e-skin comprises (1) a flexible polyimide (PI) substrate, (2) patterned copper (Cu) traces (Dupont AR1820) forming soft circuitry, and (3) surface-mounted components and thin-film strain sensors. Serpentine Cu/PI interconnects run longitudinally along the hinge axis to accommodate folding strains; transverse PI tie-bars enhance handling and reduce weight relative to a full PI layer. The e-skin is bonded to a 6-inch (152.4 mm) steel tape-spring section (thickness 0.115 mm; flattened width 25.4 mm; transverse radius of curvature 16.9 mm) using sprayed adhesive. Sensor suite: Two MEMS accelerometers (Analog Devices ADXL335) at the two ends and one MEMS gyroscope (ST LPR503AL) at the tip, plus three longitudinal thin-film strain sensors based on reduced graphene oxide/single-walled carbon nanotube (rGO/SWNT) composites located across the hinge region. Mass: tape-spring segment 3.6 ± 0.1 g; e-skin adds 0.36 ± 0.02 g. Fabrication of soft circuitry: Cu/PI was patterned via photolithography (AZ 5214) and wet etching (Transene Copper Etchant Type 100). A backside Al shadow mask was defined to plasma-etch PI (Vision 320; O2/SF6), then removed; components were soldered with low-temperature paste (TS391LT). rGO/SWNT strain sensor fabrication: P2-SWNT (3 mg) and GO (9 mg) dispersed in DI water by probe sonication; GO reduced with HI, centrifuged, re-dispersed, further reduced with N2H2 at 90 °C for 12 h, washed via vacuum filtration, then re-dispersed in 1% SDS. The dispersion was drop-cast within PDMS molds onto electrodes; solvent evaporation yielded thin films. Typical gauge factor ~50 (calibration in Supplementary Fig. S4). Finite element analysis (FEA): Abaqus/Standard shell model of the tape spring (L=152.4 mm, t=0.115 mm, arc 86°, initial radius 16.9 mm); steel (ρ=7.85×10^3 kg/m^3, E=210 GPa, ν=0.3). E-skin regions modeled as composite patches: PI (E=2.5 GPa, ν=0.3) and Cu (E=117 GPa, ν=0.3). Ends were rotated ±45° to create 90° deflection at midspan; in-plane stress/strain extracted. Frequency analysis used a clamped boundary (12.7 mm clamp), included e-skin, linear perturbation and Lanczos solver to compute first 10 natural frequencies. Experimental setups: Quasi-static tests clamped the hinge via 3D-printed PLA block; sensors powered at 3 V and read by a 16-channel DAQ (PL3516) for 3-axis accelerometer, 2-axis gyro, and three half-bridge strain outputs. Dynamic tests mounted the hinge on a shaker (K2007E01) to soften boundary/damping for better FFT resolution. Signal processing and calibration: Gyro outputs baseline-subtracted and converted via 8.3 mV/(°/s), integrated to angle; accelerometer outputs baseline-subtracted and scaled 300 mV/g. Low-pass smoothing with a 2000-point FFT filter at 2.5 Hz used for quasi-static traces; unsmoothed windows used for vibration FFT. DIC validation performed on a bare hinge to compare strain magnitudes at sensor locations.

• Mechanical accommodation: FEA showed induced longitudinal/transverse strains in e-skin under equal-/opposite-sense bending remain below the elastic limits of Cu (~1%) and PI (~3%); inclusion of e-skin slightly increases local strains in patterned regions by ~0.24% compared to a bare hinge, without changing overall distribution or maxima.
• Multimodal sensing: The e-skin captured quasi-static hinge kinematics via the gyroscope and measured local strains via rGO/SWNT sensors (gauge factor ~50). Equal-sense folding produced primarily rotation about the transverse axis with minimal twist; opposite-sense folding showed measurable rotations about both axes and twisting.
• Strain magnitudes: During equal-sense folding, longitudinal tensile strains at the folded region were <0.3%; during opposite-sense folding, compressive strains were <0.4%. Twisting induced much smaller strains consistent with minor curvature change. DIC on a bare hinge measured ~0.33% tensile strain at sensor locations vs ~0.3% by sensors (~10% difference), attributed to setup differences.
• Sliding detection: Strain sensor triad tracked the sliding of the folded region along the hinge. Shifts in which sensor read ~0.3% vs ~0.2% identified the folded zone approaching or moving away from specific sensor locations; analysis inferred sliding near SS3 reaching SS2 but not passing beyond SS3.
• Deployment dynamics: Upon release from ~90° equal-sense fold, accelerometer z-axis registered vibration with amplitudes up to about ±4 g, decaying within ~0.5 s. FFT of z-axis acceleration revealed prominent peaks at ~90 Hz and ~179 Hz. FEA predicted the first twisting and first bending modes at 86.0 Hz and 185.9 Hz, respectively; measured–simulated discrepancies were <5%, likely due to boundary-condition differences (PLA clamp compliance) and the shaker-based setup.

The study demonstrates that a thin, flexible e-skin can be conformally integrated onto tape-spring hinges to provide robust, multimodal sensing without interfering with large local deformations. Serpentine Cu/PI interconnects and compliant rGO/SWNT thin-film sensors enable strain accommodation well within elastic limits, overcoming limitations of rigid sensors and optical methods in highly deformable, thin-walled space structures. Gyroscope and accelerometer data quantify quasi-static folding and rapid deployment dynamics; high-sensitivity strain sensors quantify small longitudinal strains and detect folded-region sliding, addressing a known uncertainty in tape-spring mechanics. The measured vibration modes agree closely with FEA, validating the approach while highlighting the sensitivity to boundary conditions. Overall, the findings support e-skin as a practical, lightweight path for in-situ monitoring of foldable space structures and provide a framework for combining soft electronics with structural modeling for design and diagnostics.

A soft, lightweight structural e-skin integrating serpentine soft circuitry, rGO/SWNT high-gauge-factor strain sensors, and MEMS inertial sensors was designed, fabricated, and demonstrated on a 6-inch tape-spring hinge. The system captured quasi-static folding/rotation, measured localized strains (<0.3% tensile, <0.4% compressive), detected folded-region sliding, and identified post-deployment vibration modes (~90 Hz and ~179 Hz) consistent with FEA. This establishes a viable route for multifunctional, conformal sensing on highly deformable deployable structures. Future work includes: assessing and mitigating space environmental effects via encapsulation (e.g., PI, SiO2, SiNx) and thermal compensation; advancing shape reconstruction using dense strain-sensor arrays augmented by inertial sensing; embedding e-skin concepts into composite deployables; scaling manufacturing via printing and modular sensor-node arrays with thin-film power and wireless links; and system-level optimization under strict mass, flexibility, and frequency-shift constraints.

• Durability testing was limited; no extended cyclic folding/deployment campaigns were performed (application typically requires single successful deployment). Sensors showed stable operation on the order of ~100 cycles; broader lifetime in space remains unverified.
• Accelerometers and components were not placed on the centerline, which may introduce small asymmetries in dynamic measurements.
• Sliding experiments captured clear plateaus during static holds, but fast sliding transients were under-sampled.
• Frequency comparisons to FEA are affected by imperfect replication of boundary conditions (PLA clamp compliance, shaker mounting).
• Added mass (~0.36 g) and local stiffness from the e-skin may slightly perturb dynamics; optimization for minimal impact is needed.
• Environmental robustness (thermal-vacuum, atomic oxygen, UV, radiation) and encapsulation trade-offs (added thickness/weight vs flexibility) were not experimentally evaluated.