Pith. sign in

REVIEW 4 major objections 3 minor 4 references

Miniaturized liquid metal composite circuits with energy harvesting coils for battery-free bioelectronics and optogenetics

T0 review · 4 major / 3 minor · reviewed 2026-08-10 · deepseek-v4-flash

Challenge this review Re-run · record.json Download PDF Read on arXiv ↗ Review JSON

Pith's one-line read The paper reports that laser-patterned liquid-metal composite coils with 50 µm trace spacing harvest 178 mW/cm² through near-field inductive coupling, enough to operate battery-free optogenetic implants.

desk verdict The fabrication story is genuinely useful and the integration is new; the headline power figure needs a proper measurement chain before it can be quoted. read the letter →

arxiv 2501.11016 v1 pith:FRSQYOQR submitted 2025-01-19 physics.med-ph physics.bio-ph

Denis Rocha , Pedro Lopes , Paulo Peixoto , Anibal de Almeida , Mahmoud Tavakoli This is my paper · ORCID
classification physics.med-phphysics.bio-ph
keywords liquidmetalelectronicsbattery-freebioelectronicswirelesspowerharvestingoptogeneticslaserpatterningnear-fieldinductivecouplingsoftstretchablecircuitsSMDintegration
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The reading

This paper tries to establish that soft, stretchable circuits made from a biphasic liquid-metal/silver ink can be miniaturized enough to harvest wireless power for battery-free bioelectronics, and specifically for optogenetic implants. It reports coils with $50\ \mu\mathrm{m}$ trace spacing that capture $178\ \mathrm{mW/cm^2}$ through 13.56 MHz near-field coupling, and a soft coil that delivered more than 140 mW, about 25% less than an equivalent copper coil. The fabrication route is fast and cleanroom-free: laser patterning of the ink plus vapor-assisted soldering of surface-mount chips. If the claims hold, optogenetic experiments on freely moving mice could be done with soft, untethered implants rather than rigid head-mounted or tethered devices.

What carries the argument

The load-bearing object is the biphasic liquid-metal composite coil: a planar spiral of EGaIn-Ag ink that is laser-ablated on a sacrificial PVA layer, transferred to a stretchable SIS substrate, and tuned to resonate at 13.56 MHz by parallel capacitors. Two mechanisms carry the argument: the nondestructive laser ablation that achieves $50\ \mu\mathrm{m}$ spacing without smearing short circuits, and the vapor-assisted soldering that lets SMD packages self-adhere and self-encapsulate by a polymer-gel transition of the substrate. The optimization loop is powered by the planar-spiral inductance expressions from reference [67] together with a bulk DC conductivity value for the ink, evaluated exhaustively over the parameter space.

What would settle it

Measure the inductance, resistance, and Q-factor of a fabricated soft coil (for example ID3) at 13.56 MHz with a vector network analyzer and compare them with the values the search algorithm predicted from the planar-spiral inductance model. If the measured Q deviates by more than a few tens of percent, or if re-running the search with measured parameters selects a visibly different trace width or turn count, the asserted optimality and the $178\ \mathrm{mW/cm^2}$ figure lose their stated basis.

Watch

Extended reading notes

Core claim

The central claim is that miniaturized energy-harvesting coils can be made entirely from soft liquid-metal composite materials without sacrificing enough performance to matter. By patterning a biphasic EGaIn-Ag ink with a 1064 nm pulsed fiber laser and attaching SMD components through a toluene-vapor polymer-gel transition, the authors produced working coils with $150\ \mu\mathrm{m}$ trace width and $50\ \mu\mathrm{m}$ trace spacing. A search algorithm over trace width, spacing, and turn count, based on planar-spiral inductance expressions, selected geometries with Q above 25, resistance below $3\ \Omega$, inductance above $0.7\ \mu\mathrm{H}$, and inner diameter above $2\ \mathrm{mm}$. The best soft coil harvested more than $140\ \mathrm{mW}$ at its peak, roughly 25% below its copper counterpart, and cage-level wireless transfer was demonstrated in a $35 \times 25\ \mathrm{cm}$ enclosure.

Load-bearing premise

The optimization and the claimed power figures assume that a standard planar-spiral inductor formula and a bulk DC conductivity value accurately predict the Q-factor of rough, laser-ablated liquid-metal traces at 13.56 MHz, and the paper supplies no measured-versus-predicted check for L, R, or Q.

Editorial extensions

If this is right

  • Battery-free optogenetic implants can be prototyped and iterated in minutes with a laser engraver and solvent vapor, with no cleanroom steps.
  • The harvested power density of $178\ \mathrm{mW/cm^2}$ is enough to run NFC chips, LEDs, and capacitors, so future devices could add sensing or telemetry without a battery.
  • Because the soft coil loses only about 25% of the harvested power of a copper coil, soft-matter circuits can plausibly substitute for rigid coils in wearable and implantable power links.
  • Cage-level wireless powering enables multi-agent optogenetic studies where several untethered animals are stimulated on-the-fly in the same enclosure.
  • The proposed figure-of-merit of miniaturization gives designers a single number for comparing coil designs under implant size constraints.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • The headline power figures come from coils in close contact with the transmitter; at separations beyond roughly 1.5 cm the induced peak-to-peak voltage falls below 1.5 V, so the practical operating range for a true implant is much smaller than the cage-level demonstration suggests.
  • The optimized geometry is only as trustworthy as the planar-spiral inductance model and the bulk DC conductivity used for the rough, laser-ablated traces; a measurement-calibrated search could select a different geometry.
  • If vapor-assisted soldering generalizes, it could become a standard low-cost route for integrating SMD chips into other stretchable circuits, not just energy-harvesting coils.
Share X Bluesky LinkedIn Reddit HN

Signed reviews

No signed human review yet.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Referee Report

4 major / 3 minor

Summary. The manuscript reports a fabrication and integration route for stretchable, battery-free bioelectronic circuits based on biphasic liquid-metal composite inks. A MOPA fiber laser is used to pattern 50 µm-spaced micro coils, and a toluene-vapor process is used to solder SMD components into the soft substrate. A design-search algorithm based on published planar-spiral inductance formulas is claimed to optimize coil geometry for 13.56 MHz resonant inductive harvesting, and experimental comparisons are presented between copper and soft coils, including distance-dependent voltage measurements, tissue-insertion tests, and a cage-level wireless powering demonstration. The abstract's central quantitative claim is 178 mW/cm² harvested power from micro coils, and the Results report a peak of more than 140 mW for the soft coil ID3, about 25% below its copper counterpart. The broader claim is that these soft circuits lay the foundation for wireless optogenetics in freely moving animals.

Significance. If the quantitative claims are verified, the work would be a meaningful step toward soft, stretchable, battery-free implants, because it combines high-resolution patterning of liquid-metal composites with SMD integration in a few minutes and without cleanroom lithography. The paper contains genuine measured hardware results and does not reduce the power figures to a fitted model, so there is no core circularity; the design search uses standard inductance formulas, but the headline power numbers are experimental. The main value is the demonstrated integration of liquid-metal coils with NFC chips, capacitors, and LEDs, which could be useful to the soft bioelectronics and optogenetics communities. However, the significance is currently limited by the absence of an auditable measurement chain for the headline power figure, missing measured inductance/resistance/quality-factor validation, and several missing numerical values in the performance comparison.

major comments (4)
  1. [Results, Performance comparison of copper versus soft coils] The headline power figures are not auditable because the measurement chain is undefined. The text reports that ID1 and ID2 harvest 'more than 180 mW' and ID3 'more than 140 mW at its peak,' but it does not state the load resistance, whether the power was measured as DC power after the half-bridge rectifier or inferred from AC Vpp, the transmitter power or geometry, the coil-to-coil distance, or the number of trials and error bars. Moreover, the abstract's 178 mW/cm2 value does not appear in the Results section, so its derivation from ID3's peak power is not shown. These omissions are load-bearing because the paper's central quantitative claim is the energy-harvesting capability, and a reader cannot tell whether the quoted power is actually available to drive an LED or NFC chip.
  2. [Results, Performance comparison of copper versus soft coils] The claim that the soft coil's resistance is '9 times' that of its copper counterpart is unverifiable because the measured resistance values are missing from the supplied text. The relevant sentence reads 'was resistance of its copper counterpart, ID2, was,' which lacks the numerical data for ID2 and ID3. Without these values, the 9x resistance comparison and the attributed 25% power penalty cannot be checked.
  3. [Results, Coil design optimization and tuning] The 'exhaustive search algorithm' is presented as optimizing the coil geometry, but the manuscript provides no measured-versus-predicted comparison for L, R, or Q. The optimization relies on Mohan et al.'s planar-spiral inductance expressions (ref. 67) with a bulk DC conductivity for the biphasic ink, and no check is reported for skin effect, trace roughness, or laser-ablation damage at 13.56 MHz. The constraints Q>25, R<3 Ω, L>0.7 µH, and inner diameter >2 mm are stated without justification. The optimality claim is therefore not established, even though the fabricated devices may still perform as reported.
  4. [Discussion, Towards wireless optogenetics for neuroscience research] The manuscript states that the miniaturized implant 'can be used for wireless optogenetic stimulation' and claims to be the first demonstration of miniaturized liquid-metal coils being wirelessly powered in a cage, but no optogenetic experiment is reported, and the cage-level result is only a voltage/power-harvest map with no quantitative power values or demonstration of an LED driven solely by the cage field. The optogenetics-readiness framing therefore outruns the evidence presented in the paper.
minor comments (3)
  1. [Results, Performance comparison of copper versus soft coils] The FOMminiaturization metric is described in words, but the actual formula is not displayed; the text reads 'FOMminiaturization: Being L the inductance (µH) and Vpp the peak-to-peak voltage (V)...' without providing an equation. Since this metric is used to compare coil designs, the formula should be given explicitly as a numbered equation.
  2. [Supplementary materials] The optimization relies on 'supplementary formulas 3 and 4,' but these formulas are not included in the supplied main text, making it impossible to reproduce the search algorithm. The supplementary material should be made available and cross-referenced clearly.
  3. [Throughout] Several passages are truncated or contain corrupted wording, for example 'ID1 is a race race race race trace race spacing' in the Performance comparison section and 'fully soft optogenetic implants for neuromodulati' in the Discussion. These appear to be missing words or OCR artifacts, but as supplied the manuscript is not complete, and the authors should provide a clean, proofread version.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: headline harvest powers are measured hardware results, the coil optimization uses standard published inductance formulas, and the authors' self-citations supply materials/process inputs rather than the claimed result.

full rationale

The paper's central claims are (1) a fabrication route for miniaturized liquid-metal circuits and (2) measured energy-harvesting performance. The coil geometry is selected by an exhaustive search using Mohan et al.'s published planar-spiral inductance expressions and stated ink conductivities; these are external, parameter-free inputs, not quantities fitted to the measured power. The reported values ('more than 140 mW at its peak' for ID3; '178 mW/cm2' in the abstract) are measurements on fabricated coils in the described setups, not outputs of the optimization model. The imposed constraints (Q>25, R<3 Ω, L>0.7 µH, inner diameter >2 mm) are explicit design specifications, not hidden fits. Self-citations to refs. 19, 20, and 45 supply the biphasic ink and vapor-soldering chemistry from prior work; those are reproducible materials/process results with independent content and are not invoked to forbid alternatives or to derive the measured harvesting figures. The FOMminiaturization metric is explicitly author-defined as a comparator ('The FOMminiaturization formula was created so that it values...'), and the visible text does not use it to generate a physical prediction; it is a design-ranking tool. The genuine weaknesses—unreported load resistance and rectifier losses for the 140 mW figure, and the unvalidated use of Mohan formulas plus bulk DC conductivity for laser-ablated rough traces—are correctness/auditability concerns, not circularity.

Assumptions & free parameters 6 free parameters · 5 assumptions · 1 invented entities

The central claims rest on material properties and fabrication processes from the authors' prior work (biphasic ink, conductivity, vapor soldering), on standard inductor formulas used without validation for the printed traces, and on measurements whose detailed supporting tables are not visible. The design search adds hand-chosen constraints and a purpose-built FOM metric. These are not all circular, but they are assumptions or unvalidated inputs the reader must accept from earlier papers.

free parameters (6)
  • Coil geometry (trace width, spacing, turns, outer dimensions) = ID1/ID2 near-optimal designs, exact values only in supplementary table 2
    Selected by the exhaustive search under imposed constraints (Q>25, R<3 Ω, L>0.7 µH, inner diameter>2 mm); no sensitivity analysis or measured-versus-predicted validation is shown.
  • EGaIn:Ag ratio in the TPU ink = 1:1 for the primary ink; 2:1 and 3:1 also tested
    Chosen empirically for viscosity and conductivity; conductivity values are cited from the authors' prior work rather than measured here.
  • Laser patterning parameters = Power 100%, speed 1000 mm/s, 50 kHz, 40 ns Q-pulse, 10 loops, 0.01 mm hatching
    Hand-tuned to achieve 50 µm ablation isolation without damaging the substrate; no process window data are provided.
  • Toluene vapor soldering time = 45 minutes
    Selected to induce the polymer-gel transition and chip pad adhesion; the optimization trials are in supplementary tables 3 to 5, which are not visible in the provided text.
  • Tuning capacitor value = Determined experimentally via VNA S11 and impedance curves, exact values not reported
    Chosen to place the LC resonance at 13.56 MHz; this is standard impedance matching rather than a theoretical fit, but the final values are absent from the main text.
  • FOMminiaturization weights = L and Vpp favored over area, fill ratio, and R; exact exponents not given
    New metric introduced ad hoc to rank coil designs for implantable use; it has no independent validation or comparison to alternative metrics.
assumptions (5)
  • domain assumption Planar spiral inductance can be computed from Mohan et al. formulas for the printed soft coils.
    Invoked in 'Coil design optimization and tuning'; the paper provides no measured-versus-predicted validation or correction for rough, soft, laser-ablated traces at 13.56 MHz.
  • domain assumption The biphasic ink's bulk DC conductivity, taken from prior work, applies to laser-patterned traces without additional frequency-dependent loss or skin-effect modeling.
    Conductivity values (up to 1.08e6 S/m for AgEGaIn-TPU) are cited from refs. 20 and 51; the paper does not measure conductivity of the patterned traces or account for RF losses.
  • domain assumption Solvent-vapor soldering creates reliable, low-resistance chip-pad contacts.
    The mechanism is established in the authors' own ref. 19, but the present paper does not report contact resistance, adhesion strength, or assembly yield.
  • domain assumption Magnetic resonant coupling theory from refs. 16 and 17 applies to these coils at 13.56 MHz with ex-vivo tissue present.
    Tissue effects are shown qualitatively in fig. 4G, but no quantitative model of dielectric loss or detuning is provided.
  • domain assumption Ex-vivo mouse tissue is a valid proxy for in-vivo tissue in the power-transfer measurements.
    A piece of C57/BL6 mouse tissue was inserted between the coils, but no comparison to live tissue or to an implanted scenario is made.
invented entities (1)
  • FOMminiaturization metric
    purpose: A normalized figure of merit for comparing coil designs of different sizes in implant-constrained applications.
    Defined in 'Performance comparison of copper versus soft coils' using L, Vpp, area, fill ratio, and R. The exact weighting is unspecified and no external benchmark is used, so it cannot independently validate the claimed miniaturization advantage.

how reviews work

0 comments
Cite this review

Pith. "Pith review of Miniaturized liquid metal composite circuits with energy harvesting coils for battery-free bioelectronics and optogenetics." pith.science (2026). https://pith.science/paper/FRSQYOQR

@misc{pith2026250111016,
  author       = {Pith},
  title        = {Pith review of: Miniaturized liquid metal composite circuits with energy harvesting coils for battery-free bioelectronics and optogenetics},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/FRSQYOQR}},
  note         = {Machine review of arXiv:2501.11016}
}
read the original abstract

Over the past years, rapid progress has been made on soft-matter electronics for wearable and implantable devices, for bioelectronics and optogenetics. Liquid Metal (LM) based electronics were especially popular, due to their long-term durability, when subject to repetitive strain cycles. However, one major limitation has been the need for tethering bioelectronics circuits to external power, or the use of rigid bulky batteries. This has motivated a growing interest in wireless energy transfer, which demands circuit miniaturization. However, miniaturization of LM circuits is challenging due to low LM-substrate adhesion, LM smearing, and challenges on microchip-interfacing. In this article, we address these challenges by high-resolution laser-assisted micropatterning of biphasic LM composites and vapor-assisted LM microchip soldering. Through development of a search algorithm for optimization of the biphasic ink coil performance, we designed and implemented micro coils with trace spacing of 50 {\mu}m that can harvest a significant amount of energy (178 mW/cm2) through near field inductive coupling. We show miniaturized soft-matter circuits with integrated SMD chips such as NFC chips, capacitors, and LEDs that are implemented in a few minutes through laser patterning, and vaporassisted soldering. In the context of optogenetics, where lightweight, miniaturized systems are needed to provide optical stimulation, soft coils stand out in terms of their improved conformability and flexibility. Thus, this article explores the applications of soft coils in wearable and implantable devices, with a specific focus on their use in optogenetics.

Discussion (0). Continue with ORCID to comment.

Reference graph

Works this paper leans on

4 extracted references · 4 canonical work pages

  1. [13]

    et al.Self-powered deep brain stimulation via a flexible PIMNT energy harvester

    Hwang, G.-T. et al.Self-powered deep brain stimulation via a flexible PIMNT energy harvester. Energy Environ Sci 8, 26772684 (2015). 14. Yao, G. et al. Effective weight control via an implanted self-powered vagus nerve stimulation device. Nat Commun 9, 5349 (2018). 15. Park, S. Il et al. Soft, stretchable, fully implantable miniaturized optoelectronic sys...

    work page 2015

  2. [31]

    S., Tresco, P

    Polikov, V. S., Tresco, P. A. & Reichert, W. M. Response of brain tissue to chronically implanted neural electrodes. J Neurosci Methods 148, 118 (2005).32. Yang, Y. et al. Wireless multilateral devices for optogenetic studies of individual and social behaviors. Nat Neurosci 24, 10351045 (2021). 33. Seo, D. et al. Wireless Recording in the Peripheral Nervo...

    work page 2005

  3. [48]

    Liu, S., Yuen, M. C. & Kramer-Bottiglio, R. Reconfigurable electronic devices enabled by laser-sintered liquid metal nanoparticles. Flexible and Printed Electronics 4, 015004 (2019). 49. Pan, C. et al. Visually Imperceptible Liquid-Metal Circuits for Transparent, Stretchable Electronics with Direct Laser Writing. Advanced Materials 30, 1706937 (2018). 50....

    work page 2019

  4. [65]

    Silver, H. W. & League, A. R. R. The ARRL Handbook for Radio Communications 2021. (ARRL, 2020). 66. DeMaw, D. First Steps in Radio. (American Radio Relay League, 1986). 67. Mohan, S. S., Hershenson, M. del M., Boyd, S. P. & Lee, T. H. Simple accurate expressions for planar spiral inductances. IEEE J Solid-State Circuits 34, 14191424 (1999).68. Lu, L. et a...

    work page 1999

Pith tools

Reviewed August 10, 2026 · model on record in the stance chip above.