Spinal cord injury produces two parallel injury processes: the primary lesion (axonal disruption preventing motor command transmission) and secondary muscle denervation atrophy (progressive sarcomere loss, neuromuscular junction degradation, and type I-to-II fiber conversion that compounds the primary disability). Current functional electrical stimulation (FES) approaches address the second process incompletely: single-channel surface stimulation produces rapid fatigue, recruits superficial fibers non-selectively, and cannot achieve the deep muscle penetration required for sustained tonic maintenance. We describe a full-body haptic exosuit system delivering continuous sub-perceptual electrical micro-stimulation across all major muscle groups via a 256-electrode garment worn under or as clothing. The stimulation uses high-frequency interference (HFI) patterns — constructive wavefronts from multiple electrode pairs creating deep-tissue recruitment without the surface discomfort of conventional FES. The suit operates in two modes: (1) passive maintenance — tonic micro-stimulation at minimum contractile threshold to preserve sarcomere integrity and neuromuscular junction viability; (2) active rehabilitation — patterned stimulation sequences encoding voluntary motor programs from the Arc headband, enabling progressive motor re-education. Three garment configurations are specified: Config A, a 1-piece graphene-neoprene waterproof wetsuit for overnight and continuous wear (IPX7); Config B, a 2-piece silver-nylon therapy garment for daytime active rehabilitation; and Config C, a tracksuit inline electrode lining — a replaceable conductive panel any tailor can install in 20 minutes, invisible inside a standard tracksuit with the stimulator unit docking at the lower back. All configurations share a single stimulator unit (165 × 75 × 18 mm, 320 g, USB-C PD 45W charge). R&D cost amortized at $0.33 per unit over 15 million global SCI patients against a $5M cap. Complete system retail: $1,270–2,490 depending on configuration. Released under the GPL-3.0 as open prior art.
The spinal cord injury rehabilitation field correctly focuses on the primary injury -- the lesion. But while rehabilitation teams address the cord, the muscles below the lesion are quietly dying.
Skeletal muscle requires two things to survive: neural trophic support (axonal contact at the neuromuscular junction releasing BDNF, CNTF, and IGF-1 that maintain sarcomere protein turnover) and mechanical loading (tension-sensitive signaling via titin, integrin complexes, and mTORC1 activation that triggers muscle protein synthesis). SCI eliminates both simultaneously.
The result is predictable: within 6 weeks of complete injury, muscle cross-sectional area begins declining at 3-6% per week in the acute phase. Over years, this produces the characteristic lower limb muscle morphology of long-duration SCI: atrophic, largely replaced by intramuscular fat, with neuromuscular junctions that have partially retracted. Type I slow-twitch fibers (fatigue-resistant, oxidative) convert to type II fast-twitch (glycolytic, fatigue-prone), further limiting any rehabilitation potential.
For a patient injured decades ago, this process has run to near-completion in the muscles below the lesion. The muscles are not gone -- but they are substantially compromised. Any restoration of function (through the Arc headband, through exoskeleton, through future cord repair) will land on a muscle substrate that has been neglected for decades.
Conventional FES uses surface electrodes (2-4 per muscle group) delivering biphasic pulses at 20-50 Hz. Limitations:
The high-frequency interference (HFI) approach addresses limitations 1, 3, and 4 simultaneously. The principle: two high-frequency carrier signals (e.g., 4 kHz and 4.1 kHz) are delivered through electrode pairs positioned on opposite sides of the target muscle. Individually, each carrier is above the frequency range of neural activation (action potentials cannot follow >1 kHz continuously). But where the two wavefronts intersect inside the tissue, they produce a beat frequency (4,100 - 4,000 = 100 Hz) -- which is within the neural activation range and which selectively activates the motor units at the interference focus.
Result: stimulation occurs deep in the tissue at the geometric intersection of the two beams, not at the skin surface. Surface current density is sub-threshold; the interference node is at threshold. This is the same principle used in interferential current therapy (IFC) for pain management, now applied with precision electrode placement for deep motor unit recruitment.
Extensions:
Three garment form factors are specified, sharing identical electrode coverage maps and stimulator protocol. Selection depends on use context: overnight maintenance, active therapy, or daily public wear. All three connect to the same stimulator unit.
A neck-to-ankle wetsuit-style garment for overnight therapeutic wear, shower compatibility, and extended continuous use between attendant care visits. The electrode function is moved from discrete adhesive hydrogel pads into the fabric itself: a 3 mm closed-cell neoprene base with a 0.3 mm graphene nanoplatelet-loaded PDMS layer laminated to the body-contact surface. The graphene layer is laser-patterned into 256 isolated conductive zones separated by 2 mm non-conductive gaps (isolation resistance >1 MΩ between adjacent zones).
The original daytime therapy configuration: long-sleeve top + full-leg tights in 80% polyester / 20% spandex substrate with silver-coated nylon conductive traces woven in. Lighter per piece, easier independent donning, lower cost. Designed for waking hours when caregiving availability permits assisted donning.
A replaceable conductive lining layer that installs inside any standard tracksuit. The tracksuit itself is unmodified in appearance — the electrode system is entirely internal. Designed for patients who need daily wear without visible medical device aesthetics.
The stimulator unit is a single module shared across all three garment configurations. Its form factor is designed to disappear: 165 mm × 75 mm × 18 mm, 320 g — smaller than a wallet. It docks at the lower back via a magnetic recessed port in all three suit configurations, sitting flush against the lumbar in a purpose-sewn sleeve. Under a tracksuit jacket: invisible. On a clinical garment: no worse than a phone at the belt.
The lower back dock is intentional — the device sits over a region of low motion, low pressure, and reliable reach-access for the patient. In a tracksuit, the back pocket is the cleanliest location: no lateral compression when seated, no interference with wheelchair armrests, no clip slippage that would shift the garment. A rear-dock unit worn under a tracksuit jacket is indistinguishable from a lumbar support brace.
256 electrodes cover:
The stimulator unit sits at the lumbar — the closest rigid-body proxy to the body's center of mass available without an implant. This is the optimal location for fall detection: the pelvis angular velocity and linear acceleration at fall onset are the highest-signal, lowest-noise indicators in the kinematic chain. The ICM-42688-P IMU at 1 kHz poll rate gives 1 ms resolution on the event that precedes ground contact by 300–500 ms.
Two independent fall signatures are used in combination to minimize false positives:
On confirmed fall detection:
The secondary IMU functions are equally important for therapeutic use:
IMU BOM contribution: $3–6. No additional MCU load beyond a DMA-driven SPI peripheral reading at 1 kHz and an interrupt line for threshold events. This is not optional functionality — at $4 average cost, every stimulator unit ships with it. The alternative is a patient on the floor for hours because nobody built fall detection into a device that was already wearing the right component.
Goal: minimum contractile threshold stimulation sufficient to maintain sarcomere integrity and NMJ viability without producing observable movement or patient discomfort.
Parameters:
Evidence basis: animal denervation studies demonstrate that as little as 8-12 Hz tonic electrical stimulation at sub-threshold levels is sufficient to prevent the type I → type II fiber conversion and maintain NMJ morphology. In humans, even low-dose passive stimulation in acute SCI (within 6 weeks) attenuates the atrophy trajectory significantly. The maintenance protocol targets the minimum effective dose for maximum long-term wearability.
Triggered by Arc headband motor intent decoding or by scheduled rehabilitation session:
For complete cervical SCI patients, autonomic dysreflexia (AD) -- sudden dangerous hypertension triggered by stimuli below the lesion -- is a life-threatening complication. The suit can serve as an early detection and intervention platform:
The suit and headband form a closed sensorimotor loop:
[Arc: OPM motor intent decode] → [Intent signal: "stand"]
↓
[Suit stimulator: activate stand program]
↓
[Quadriceps + gluteus stimulation → joint torque → stance]
↓
[Exoskeleton provides structural support]
↓
[OPM: detect motor cortex execution signal confirmation]
↓
[Classifier: label as successful → update model]
The critical design principle: the suit provides the peripheral execution that the cord cannot. The headband provides the central intent that the patient generates. The cord lesion is bridged not by repairing it but by routing around it: cortex → air-gap (radio) → periphery.
For patients with intact sensation below the lesion (incomplete SCI), proprioceptive feedback from the suit-driven movement reaches the cortex through preserved posterior column pathways, closing the sensorimotor loop neurologically and accelerating motor learning.
Total R&D budget cap: $5,000,000. Global SCI population requiring this device: 15,000,000. Amortized research cost per device set: $0.33. This $0.33 is included in every configuration price below. It is not a rounding error. It is the correct answer to the question of what it costs each person for the knowledge to exist.
| Component | Config A (1-Piece) | Config B (2-Piece) | Config C (Tracksuit Lining) |
|---|---|---|---|
| Conductive garment material | $180–260 (graphene-neoprene, 2 rolls) | $120–200 (silver-nylon Lycra, roll-to-roll) | $60–90 (silver-nylon panel material) |
| Electrode zone patterning | $40–60 (CO₂ laser trace isolation) | $160–280 (256× carbon-silicone hydrogel pads) | $140–240 (256× carbon-silicone hydrogel pads) |
| Inductive charging coils | $55–80 (8× litz coils, embedded in seam) | $55–80 (8× litz coils, sewn-in) | $40–65 (6× litz coils, lining-embedded) |
| Garment construction | $60–90 (wetsuit factory, blind-stitch seam) | $35–55 (standard sportswear factory) | $20–35 (sewn panel + snap installation) |
| Connectors + hardware | $30–50 (IPX7 magnetic 32-pin) | $35–55 (waist interlock + hip connector) | $18–28 (12 press-stud pairs + back port) |
| R&D amortization | $0.33 | $0.33 | $0.33 |
| Garment BOM total | $365–540 | $405–670 | $278–458 |
| Component | Cost at scale |
|---|---|
| STM32H7 MCU + 8-channel HV DAC array | $55–80 |
| 256-channel multiplexer ICs (4× MAX14900E class) | $60–90 |
| 100V compliance boost converter | $25–40 |
| 5,000 mAh LiPo cell + BMS | $40–65 |
| BLE 5.2 + 2.4 GHz radio module | $12–18 |
| ICM-42688-P 6-axis IMU (accel + gyro) | $3–6 |
| USB-C PD charging IC + connector | $8–14 |
| Enclosure (aluminium CNC + gasket) | $35–55 |
| Magnetic dock connector (32-pin) | $18–30 |
| PCB fabrication + assembly | $45–70 |
| Stimulator BOM total | $301–468 |
| Configuration | Garment BOM | + Stimulator BOM | = Total BOM | Retail (2.2×) | Replacement lining |
|---|---|---|---|---|---|
| A — 1-Piece Full (IPX7) | $365–540 | $298–462 | $663–1,002 | $1,460–2,200 | New garment ~$800–1,200 |
| B — 2-Piece Therapy | $405–670 | $298–462 | $703–1,132 | $1,550–2,490 | Pad set replacement ~$120–200 |
| C — Tracksuit Inline | $278–458 | $298–462 | $576–920 | $1,270–2,020 | Lining only: $60–90 + tailor labour |
For comparison: the Bioness StimRouter (single-channel peripheral nerve stimulator, implanted) lists at $8,500–$12,000 per channel. The Ottobock C-Brace (passive exoskeleton, no stimulation) retails at $30,000–$50,000. This suit delivers active 256-channel HFI stimulation across the full body for under $2,500 at retail in the highest-specification configuration.
At first-article prototype scale (pre-volume manufacturing):
The path from prototype to volume is well-trodden: wetsuit manufacturing is a mature global industry (Indonesia, Thailand, China), silver-nylon compression garment manufacturing is commodity sportswear, and the stimulator electronics have direct analogues in consumer medical devices at volume. The engineering work to reach prototype is the R&D cost. The $0.33 already paid for it.
Meta-analysis of FES in chronic SCI (>1 year post-injury):
These outcomes are from conventional FES. HFI-based stimulation has been shown in acute studies to achieve equivalent or superior muscle force production at significantly lower surface current density -- projecting to better compliance and longer achievable treatment duration.
Based on FES literature extrapolated to full-body continuous maintenance dose:
For patients injured decades ago: outcomes attenuated but positive. Any improvement in muscle bulk directly reduces pressure sore risk (the leading cause of hospitalization and death in chronic SCI), reduces caregiver burden for daily hygiene, and improves the substrate available for Arc-driven functional movement.
The secondary injury of SCI -- muscle denervation atrophy -- is largely preventable with technology that already exists. The haptic exosuit converts that technology from clinical FES equipment requiring technician operation to a wearable garment worn under clothing, providing continuous maintenance with zero daily burden on patient or caregiver.
At manufacturing scale, the annual cost of the suit is equivalent to three days of professional attendant care. The clinical case for deployment is unambiguous. The barrier, again, is not physics.
Released under the GPL-3.0. Build it.
Funding: This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors. This is a design specification based on published evidence and does not represent new experimental work.
Conflict of Interest: None declared.
Data Availability: This paper presents no original data. All performance parameters are extrapolated from published literature cited in the references. All design specifications are provided in full within the text.
License: GPL-3.0 Prior art date: 2026-02-22