Flexible thin-film Implant with Depth Selectivity for Intraspinal Microstimulation

This paper introduces flex-ISMS, a flexible thin-film intraspinal microstimulation array that overcomes the limitations of existing microwire technologies by demonstrating high spatial selectivity, minimal tissue displacement, and the ability to evoke graded, near-natural motor responses in an acute pig model, thereby establishing a viable foundation for restoring motor function after spinal cord injury.

The Gist

The Big Picture: Fixing the "Broken Wire"

Imagine your spinal cord is like a massive, high-speed fiber-optic internet cable running down your back. It carries all the instructions from your brain to your legs and feet so you can walk, run, and dance.

When someone suffers a Spinal Cord Injury (SCI), it's like a giant storm has smashed a section of that cable. The internet is down. The brain is shouting "Walk!" but the signal gets stuck at the damage site and never reaches the legs.

Scientists have tried to fix this by placing a "booster" on the outside of the cable (called epidural stimulation). It helps, but it's like trying to tune a radio through a thick concrete wall—the signal gets fuzzy, and you can't pick out specific stations (like just moving your knee without moving your hip).

This new study introduces a smarter solution: flex-ISMS. Think of it as a microscopic, flexible "surgical spider" that doesn't just sit on the wall; it gently pokes inside the cable to reconnect the wires directly.

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The Problem with Old Tools

Before this invention, doctors used "microwires" to poke inside the spinal cord.

• The Analogy: Imagine trying to fix a delicate watch by poking it with a bundle of stiff, thick knitting needles.
• The Issues:
  1. Too Stiff: The needles are rigid, but the spinal cord is soft and squishy. If the patient moves, the needle rubs against the tissue, causing scarring (like a splinter that never heals).
  2. One Trick: Each needle only had one tip. If you wanted to stimulate a specific layer of the spinal cord, you had to pull the needle out and stick it back in at a different depth. It was clumsy and imprecise.
  3. Hand-Made: Every needle bundle was handmade, meaning no two were exactly alike.

The New Solution: The "Flexible Spider" (flex-ISMS)

The researchers created a new device called flex-ISMS. Here is how it works:

1. The Body: A Flexible Ribbon
Instead of stiff needles, this device is made of a super-thin, flexible plastic (polyimide) that is as soft as a contact lens.

• The Analogy: Imagine a silky ribbon that can bend and twist to hug the shape of the spinal cord perfectly, rather than a rigid stick that pokes it. This reduces friction and damage.

2. The Legs: 14 Tiny Arms
The device has 14 tiny "arms" (7 on each side of the spinal cord).

• The Magic: Each arm has three tiny electrodes (stimulation points) spaced out along its length.
• The Analogy: Think of it like a multi-tool screwdriver. Instead of having one screwdriver tip, you have three different-sized tips on the same handle. You can choose to turn the screw at the top, middle, or bottom of the arm without ever pulling the tool out. This allows the device to target specific layers of the spinal cord with incredible precision.

3. The Insertion: The "Needle Shield"
Because the device is so soft, it can't poke into the spinal cord on its own (it would just bend like a wet noodle).

• The Solution: The team used a special tungsten needle (like a tiny, sharp spear) to push the soft arms in.
• The Trick: Once the needle is inside, it dissolves or is pulled out, leaving the soft, flexible arm sitting perfectly inside the tissue. It's like using a rigid straw to push a soft balloon into a tight space, then removing the straw so the balloon stays put.

What Happened in the Test?

The scientists tested this on pigs (whose spines are very similar to humans). They implanted the device and started sending tiny electrical signals.

• Precision: They could make the pig move just its knee, or just its ankle, or just its hip. They could even switch from bending the knee to straightening it just by changing which of the three "tips" on the arm they used.
• Strength: The movements were strong. They could make the pig's leg kick with enough force to lift its own weight, similar to how a human walks.
• Safety: When they looked at the tissue under a microscope, the damage caused by this new "spider" was no worse than the old stiff needles, even though the new device is much more complex.

Why This Matters

This is a "proof of concept." It's like building the first prototype of a flying car and successfully driving it off a ramp. It proves the idea works.

• For the Future: If this works long-term in humans, it could allow people with spinal cord injuries to regain the ability to walk with much more natural, fluid movement than current treatments allow.
• The Catch: The study was "acute," meaning they tested it immediately after surgery and then stopped. The next step is to see if the device works safely for months or years without causing infection or scarring.

Summary in One Sentence

The researchers built a soft, multi-layered "spider" device that can gently dive into the spinal cord and turn specific "switches" to help paralyzed people move their legs with natural, precise control, all while causing less damage than the old, stiff tools.

Technical Summary

1. Problem Statement

Restoring motor function after spinal cord injury (SCI) remains a significant challenge. Current neuromodulation strategies face specific limitations:

• Epidural Stimulation (eSCS): While effective for standing and stepping, it suffers from poor spatial selectivity due to the shunting effect of cerebrospinal fluid (CSF), requiring high currents and prolonged mapping sessions.
• Conventional Intraspinal Microstimulation (ISMS): Offers higher precision but relies on manually assembled microwire arrays. These suffer from:
  • Lack of Reproducibility: Manual fabrication leads to variability.
  • Single-Site Limitation: Most microwires have only one stimulation site at the tip, limiting depth control. If the target is missed, the electrode must be extracted and re-inserted.
  • Mechanical Mismatch: Rigid microwires (e.g., 50 µm Pt-Ir) cause chronic foreign body responses (FBR) and tissue damage due to stiffness and micromotion.
  • Bulky Multisite Probes: Existing flexible multisite probes are often too thick or require fragile bonding to lead wires.

2. Methodology

The authors developed flex-ISMS, a thin-film, polyimide-based intraspinal microstimulation array designed for depth selectivity and mechanical compliance.

• Device Design:

  • Substrate: Flexible polyimide (PI) film (40 µm wide, 8 µm thick).
  • Architecture: 14 flexible arms (7 per side of the spinal cord) spaced 6 mm apart rostrocaudally.
  • Electrodes: Each arm contains 3 stimulation sites (75 × 15 µm) spaced 500 µm apart along the dorsoventral axis, enabling depth control. Total of 42 sites per large array.
  • Materials: Electrodes are sputtered with Iridium Oxide (SIROF) and coated with PEDOT/PSS to enhance charge injection capacity.
  • Strain Relief: Serpentine regions integrated into the arms to accommodate spinal cord movement.

• Insertion Strategy:

  • Due to the thinness of the polyimide arms, a custom 125 µm tungsten insertion aid (tapered to a 20 µm tip) was developed.
  • Procedure: The tungsten rod is threaded through a reinforced hole at the tip of the arm and secured with Polyethylene Glycol (PEG). The assembly is inserted into the spinal cord; the PEG dissolves in ~10–15 seconds, releasing the flexible arm while the tungsten aid is retracted.

• Experimental Model:

  • Subjects: Acute in vivo implantation in the lumbosacral enlargement of three female domestic pigs (approx. 56 kg).
  • Configuration: Tested both a "small" array (3 arms/side) and a "large" array (7 arms/side).
  • Data Collection: Kinematic analysis (range of motion), isometric joint force measurements, and electromyography (EMG) were recorded at stimulation amplitudes of 25–300 µA.
  • Histology: Post-mortem H&E staining to assess acute tissue damage compared to conventional microwires.

3. Key Contributions

1. Novel Device Architecture: First demonstration of a high-channel-count (42 sites), depth-selective, thin-film ISMS array that conforms to the spinal cord surface.
2. Insertion Aid Innovation: A reliable method using dissolvable PEG and tungsten aids to implant ultra-thin (8 µm) flexible arms without buckling.
3. Depth Selectivity: Proven ability to selectively activate different motor networks by stimulating at different depths (dorsal vs. ventral) on the same arm.
4. Biocompatibility Proof: Demonstration that the acute tissue damage caused by the 125 µm insertion aid is comparable to that of standard 50 µm microwires, despite the higher electrode density.

4. Key Results

• Functional Motor Output:

  • The device successfully evoked graded, unilateral movements in the hip, knee, and ankle joints.
  • Range of Motion (ROM): Achieved knee extension of 40° and isometric forces up to 30 N, approaching levels seen during natural locomotion in pigs.
  • Selectivity: Electrodes separated by 500 µm produced distinct movement patterns (e.g., switching from flexion to extension) and force magnitudes.
  • Recruitment: Force and ROM increased linearly with stimulation amplitude, indicating natural motor unit recruitment.

• Electrochemical Performance:

  • Impedance: 10 kΩ at 1 kHz.
  • Charge Injection Limit (CIL): In vitro CIL was 2.2 mC/cm². In vivo measurements suggested a conservative safe limit of ~0.8 mC/cm² (due to tissue voltage drops), corresponding to a safe current amplitude of ~50 µA for chronic use.
  • Performance: Despite small geometric surface areas (1125 µm²), individual electrodes evoked strong movements, outperforming previous reports requiring larger electrode groups.

• Tissue Response:

  • Acute Damage: Histological analysis showed acute insertion damage widths of ~100 µm (axial) and ~200 µm (coronal).
  • Comparison: The damage profile was indistinguishable from that caused by conventional 50 µm microwires, despite the use of a larger 125 µm tungsten aid. The flexible arms remained in situ during sectioning, confirming minimal subdural volume displacement (40 × 8 µm per arm).

5. Significance and Future Outlook

• Clinical Translation: The flex-ISMS platform addresses the critical trade-off between selectivity and mechanical compatibility. By enabling depth-specific stimulation, it reduces the need for high currents and complex mapping required by eSCS.
• Chronic Viability: The extreme flexibility (400–10,000x more compliant than Pt-Ir wires) suggests a significant reduction in chronic foreign body response and glial scarring compared to rigid implants.
• Future Work:
  • Long-term Studies: Chronic implantation in SCI models to verify long-term stability and safety.
  • Design Refinements: Moving to larger wafer fabrication for monolithic arrays, implementing multilayer metallization to increase surface area for safer charge injection, and developing robot-assisted insertion for higher precision.
  • Integration: Potential coupling with epidural recording implants to create closed-loop systems that translate motor intent into specific intraspinal stimulation patterns.

Conclusion: The study establishes the acute feasibility of flex-ISMS as a superior alternative to microwire arrays, offering high spatial selectivity, naturalistic motor recovery, and improved mechanical compatibility with neural tissue.