A flexible neural implant with nanoscale heating elements blocks pain through precise temperature control, draws power wirelessly, and dissolves in the body within weeks.
(Nanowerk Spotlight) Opioids remain the most effective pharmacological option for severe pain, but they exact a devastating toll. These drugs hijack the brain’s reward pathways, creating physical dependence that traps people in cycles of addiction. Local anesthetics offer temporary relief but diffuse unpredictably through tissue and wear off within hours. Surgeons can sever pain-carrying nerves entirely, but the results are permanent and often accompanied by numbness, weakness, or paradoxical pain syndromes. For a problem as universal as pain, the available solutions have severe limitations.
Electrical nerve stimulation emerged as a more sophisticated alternative. By delivering targeted pulses to peripheral nerves, these devices can jam pain signals before they reach the brain, much like static overwhelming a radio broadcast. Patients have found relief from conditions ranging from post-amputation phantom limb pain to chronic back problems. But the hardware creates its own complications.
Most systems require permanent implantation of metal and plastic components that the body gradually encapsulates in scar tissue, triggering inflammation that can degrade device performance over months or years. Eventually, many implants must come out, requiring second surgeries with their own risks of infection and nerve damage.
The wires pose an even more immediate problem. Conventional implantable stimulators need power, and that power typically arrives through leads that penetrate the skin to connect with external batteries or controllers. Each wire tract represents a potential highway for bacteria. Patients must keep exit sites meticulously clean and limit physical activities that might dislodge connections. For acute pain management following surgery, when patients most need mobility to prevent blood clots and promote healing, these tethers prove especially counterproductive.
Bioresorbable electronics represent an attempt to escape these constraints. Engineers have developed materials that perform their electronic functions reliably, then gradually dissolve in the body’s warm, wet, chemically active environment. Temporary pacemakers built from these materials have successfully regulated heartbeats in animal studies before disappearing without a trace. Nerve regeneration stimulators have delivered therapeutic electrical pulses to injured tissue, then melted away as healing completed. But applying this concept to pain management has proven difficult.
Previous bioresorbable nerve stimulators still required transcutaneous leads, those problematic skin-piercing wires, to deliver power. Eliminating external tethers while maintaining reliable energy delivery has remained an unsolved engineering challenge.
A research collaboration spanning Northwestern University, Rice University, Washington University in St. Louis, and institutions across South Korea and the Netherlands now reports a device that sidesteps the electrical stimulation paradigm entirely. Rather than jamming nerve signals with competing electrical pulses, their implant uses heat.
Published in Advanced Functional Materials (“A Bioresorbable Neural Interface for On‐Demand Thermal Pain Block”), the work describes a soft, flexible cuff that wraps around peripheral nerves, warms them to a precise temperature that temporarily halts signal transmission, then dissolves in the body over days to weeks. Crucially, it draws power wirelessly from an external transmitter, requiring no physical connection through the skin.
Illustrations of schemes for acute pain mitigation and design features of an implantable, bioresorbable platform for thermal nerve block. a) Conceptual illustrations of (i) conventional pain management approaches and (ii) the approach presented here. b) Exploded view illustration of the layered device layout. c) Picture and diagram of the device, highlighting the serpentine traces that define the heating and sensing elements. Scale bars, 1 mm. (Image: Reproduced from DOI:10.1002/adfm.202530035, CC BY) (click on image to enlarge)
Heat blocks nerve conduction through a straightforward biophysical mechanism. Nerve signals travel as waves of electrical depolarization, driven by ion channels that open and close in rapid sequence along the nerve fiber. Elevating local temperature to between 42 °C and 45 °C disrupts this coordinated ion channel activity, effectively silencing the nerve without destroying it. When the heat source switches off and the tissue cools, normal conduction resumes. This reversibility distinguishes thermal blocking from surgical nerve cutting, which produces permanent effects.
The device integrates two functional elements into a structure thin and flexible enough to wrap around peripheral nerves averaging 1 to 2 mm in diameter. A resistive heating element generates warmth when current flows through it, the same principle that makes a toaster glow. Positioned nearby, a temperature sensor monitors local conditions continuously, enabling the system to adjust current and maintain heating within the safe therapeutic window.
Both components consist of gold traces just 120 nm thick, patterned into serpentine shapes that accommodate bending and stretching without fracturing. A 500 nm silicon dioxide layer provides electrical insulation, while bioresorbable polymers encase the entire assembly.
Two polymer options address different clinical timelines. Polyanhydride dissolves within approximately one week as water molecules break its chemical bonds. For longer applications, plasticized regenerated silk fibroin offers extended durability. This material derives from silkworm cocoons and receives glycerol treatment to improve flexibility, remaining functional for at least 14 days.
Adjusting the glycerol content changes the silk’s crystalline structure, allowing engineers to tune degradation rates for specific therapeutic windows. Both materials erode predictably from their surfaces inward, maintaining structural integrity until near the end of their operational lives rather than swelling and failing unpredictably.
Gold does not dissolve in the body, but each device contains only 113.7 µg of the metal, less than 4% of total device weight. Gold has extensive precedent in implanted medical devices, and the quantities involved here are minimal.
Temperature precision determines whether the device heals or harms. Temperatures exceeding 45 °C for extended periods destroy nerve tissue permanently. The integrated sensor enables feedback control: the system continuously reads temperature and adjusts current output to stay within safe limits. The researchers tested this capability in hydrogel phantoms that mimic body tissue at physiological temperature. Their device reached target temperatures within 30 s and held stable output at 43.5 °C ± 0.3 °C.
Animal studies validated the nerve-blocking capability. The researchers positioned heating cuffs on rat sciatic nerves, placing stimulating electrodes upstream and recording electrodes downstream to measure signal transmission. Activating the heater caused compound nerve action potentials, the aggregate electrical signals from stimulated nerve fibers, to diminish progressively.
Partial blocking appeared after 4 min of heating. Complete suppression followed at 5 min. When the heater switched off and the nerve cooled, full conduction recovered. The thermal block proved entirely reversible.
Mechanical testing confirmed durability under conditions that simulate the constant stress of body movement on implanted hardware. Resistance changes stayed within ±0.2% through 20,000 cycles of bending and twisting at physiologically relevant rates.
Wireless power delivery addresses the problem that has historically forced bioresorbable stimulators to retain skin-piercing wires. The device incorporates a receiving coil made from bioresorbable molybdenum foil. This coil harvests energy from an external radio frequency transmitter operating at approximately 13.56 MHz. Effective power transfer occurred across separation distances of 1 to 6 mm, appropriate for subcutaneous placement.
A voltage-regulating component prevents dangerous overheating regardless of fluctuations in transmitted power. Without this safeguard, device temperatures in testing climbed above 70 °C at high input levels, hot enough to cook tissue. With regulation active, temperatures held steady near 28.5 °C across the entire tested power range. Wireless operation matched wired performance, achieving the therapeutic temperature window with rapid, reproducible cycling.
The platform combines capabilities that have individually appeared in previous devices but not together: thermal rather than electrical nerve blocking, bioresorbable construction, integrated sensing for closed-loop temperature control, and wireless power delivery. Applications could span post-surgical acute pain and chronic conditions resistant to pharmaceutical treatment. Future versions might replace the gold traces with fully bioresorbable metals, achieving complete dissolution of all components. The ultimate goal is therapeutic implants that perform their function, then simply disappear.
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