Ultraflexible photoelectrical impedance tomography-based imager for 3-axis robotic tactile sensing

Domain: biology
Published: 2026-03-14
Editorial outcome: major_revision

Abstract
Vision-based robotic triaxial tactile sensing provides superior spatial resolution and rich
multimodal data. However, employing rigid CMOS imagers suffers from limitations in mechanical
flexibility and large-area scalability. Here we present a large-area ultraflexible photoelectrical
impedance tomography (PIT)-based imager that achieves high-fidelity triaxial tactile sensing. The
5 μm-thick PIT imager integrates quantum dots/metal-oxide heterojunction layer with 16
peripheral electrodes, significantly reducing interconnects (pixel-to-interconnect ratio > 80).
The
PIT imager demonstrates a > 10⁴ photo-to-dark current ratio, resolves 1.5 mm spatiotemporal
features, and tracks up to five simultaneous UV patterns. Integrating light-scattering porous rubber
and LEDs enables triaxial force decoding via Gaussian photocurrent analysis. The system achieves
an 80 kPa dynamic range with a sensitivity of 0.04 kPa⁻¹ to normal force, 0.17 μm kPa⁻¹ shear
displacement resolution, and 96.5% topological recognition accuracy. We foresee this technology
for haptic interfaces in soft robotics, neuromorphic prosthetics, and human-machine interaction
systems.

Introduction
Tactile imaging, inspired by human touch perception, empowers robotic hands to capture spatially
distributed force and texture data, thus improving object recognition accuracy and enhancing
dexterity in manipulating fragile and irregular objects1-5. As humanoid robotics advance, achieving
large-area and high-resolution mapping quality and reliability has become crucial6-10. To meet
these demands, many tactile sensors are designed with a crossbar array structure, connecting each
sensor cell with interconnect lines1,11. Passive array like capacitive and piezoresistive-based tactile
imaging techniques feature with high sensitivity, large-area, and low-cost fabrication1,4-6,8,9,11,12.
TFT-driven active array with foundry-compatible sensing units consumes less energy and relative
higher resolution, comprising sensitivity13-15. As the resolution is related to the number of
interconnects, the increased number of interconnects leads to potential issues such as larger
crosstalk noise and greater susceptibility to interferences among sensor cells16. Moreover, single
interconnect breakage in an array architecture can lead to the failure of a row or column17,18. To
reduce number of interconnects, the adopting computational methods for pressure-sensitive
devices by calculating the distance-related resistance, capacitance, impedance and voltage changes,
have been proven effective strategies for maintaining sufficient mapping capability18-24.

As technique advances, the 3-axis tactile sensor enables a comprehensive perception of the
position, magnitude and direction of the contact force by simultaneously detecting the force
components in three orthogonal directions25-27. Thus, multiple sensing units are integrated together
to measure both normal and shear load independently. Arrays of capacitive and resistive sensors
units with response capability to both normal and tangential pressure are one of the strategies28-31.
For example, by designing liquid metal electrodes and air microchannel, a real-time 3-axis pressure
sensing array was achieved. It realized high sensitivity in sensing, and greatly eliminated crosstalk
among sensing units through compliant electrical shield layers30. Nevertheless, multiple sensing
units used for 3-axis decoupling sacrifice the resolution and make the output circuit complicated32.
To achieve high-resolution 3-axis force sensing, magnetic sensor with a sinusoidally magnetized
flexible film has been proposed, whose deformation can be detected by a Hall sensor according to
the change of magnetic flux densities under external forces33,34. Nevertheless, Hall-based tactile
sensors typically output low voltage with mV level and exhibit poor linearity with force. Also, it
needs to be weighed against environmental electromagnetic interference.

Vision-based pressure sensor, represented by Gelsight, have proven high sensitivity, high
resolution, and robustness35. The external force induces the deformation of elastomer, which
change the light propagation captured with a CMOS imager36. The compact Gelsight sensor with
a total size of 3.3 × 3.3 × 3.5 cm3 achieved a spatial resolution down to 1.2 mm on a sensing area
of 18 mm by 13 mm37. Recently, Barreiros et al realized 3-axis pressure detection using a CMOS
imager with stretchable lightguides encoding haptic stimuli into light38. By transforming the
information of light intensity and distribution to mechanical deformation, the tactile sensor (2 × 2
× 2 cm3) can correspond to a shear force over 25 N with a spatial resolution of 1.25 mm. Someya’s
group replaced rigid and bulky Si photodetectors-based CMOS imager with organic photodetectors
(OPDs) on CMOS matrix to achieve certain flexibility. The integrated tactile sensor with the
backlight, blur rubber, and flexible imager features a sensing area of 3 × 4 cm2, and can accurately
detect normal and tangential pressure up to 360 kPa and 100 kPa, respectively39.

The imager used is the determining factor for style and performance of vision-based tactile
sensor. However, rigid Si PD-based CMOS imagers could only realize limited imaging area and
its cost increases dramatically with size. Flexible OPD-based CMOS imager driven by silicon thin-
film transistor array still has a thickness of 200 μm, limiting its flexibility and conformality and
the environmental stability of OPD is still an issue. Moreover, both array-type imagers cannot
resist damage testing thus lack adaptability to harsh environmental tasks.

Here, we propose a large-area ultraflexible imager based on photoelectrical impedance
tomography (PIT) with electrodes surrounding the sensing area. The ultraflexible PIT imager
exhibits foundry compatible low manufacturing cost and damage-tolerant characteristic, which can
be used for on-skin sunscreen test and 3-axis force detection. Specially, CdSe/ZnS quantum
dots/indium-zinc-oxide (IZO) heterojunction film with 16 peripheral Al electrodes were deposited
on 5 μm-thick polyimide substrates, demonstrating high photo-to-dark current ratio, operational
stability and maintaining stable device performance when bended on a syringe needle. The PIT
imager achieves high spatiotemporal resolution with few interconnects and works well even after
unexpected puncture. Through integration PIT imager with light-scattering porous rubber and LED
arrays, we enable 3-axis force decoding with an averaged sensitivity of 0.04 kPa⁻¹ to normal force,
a shear displacement resolution of 0.17 μm kPa⁻¹, and an average discrimination accuracy of 96.5%
using neural network. We foresee this technology for next-generation haptic interfaces in soft
robotics, neuromorphic prosthetics, and immersive human-machine interaction systems.

Results
Mechanism and structure of PIT imager
Fig. 1a, b and Supplementary Video 1 compare the array-based and PIT-based imagers.
Conventional array-type imagers encounter issues from electrical crosstalk caused by parasitic
currents within electrical lines and experience a trade-off between large-area integration and
manufacturing cost40. In contrast, our PIT imager features no intersecting lines, with electrodes
positioned away from the active area, thereby potentially eliminating crosstalk issues and enabling
easy scalability. The principle and imaging mechanism of the PIT imager are described in detail in
Supplementary Notes. The schematic and the exploded view of the ultraflexible PIT imager are
shown in Supplementary Fig. 2a. and Fig. 1c. Under same interconnection numbers, PIT imager
could achieve much higher resolution. And function well even it is partially damaged. Furthermore,
foundry-compatible processing enables large-area fabrication on ultrathin polyimide (PI) substrate.
With such ultraflexible PIT imager (Fig. 1d), 3-axis tactile sensor with thin-film structure can be
assembled, consisting of a flexible light emitting diode (LED) backlight layer at the bottom, a
translucent porous rubber in the middle, and an ultraflexible PIT imager atop (Fig. 1e and
Supplementary Fig. 2b). Note that the side of the PIT imager with QDs/IZO functional layer is in
close contact with the porous rubber layer. When pressure is applied to the tactile sensor,
deformation of the porous rubber alters the light distribution. By analyzing the variations in the
peak position and width detected by the PIT imager, the 3-axis force can be reconstructed (Fig. 1f).

PIT imager manufacturing and optimization
The conformality of the e-skin is crucial for wearable devices, as it ensures optimal comfort and
high-quality signal acquisition. Moreover, the active sensing area should be homogeneous and
precisely aligned with the surrounding electrodes to ensure accurate PIT measurements. To achieve
this, foundry-compatible manufacturing processing was employed to fabricate PIT sensor on PI
substrate with good scalability and reproducibility. Specifically, PAA precursor was spin-coated
on supporting glass substrate and cured in inert atmosphere with temperatures up to 300 °C to form
ultrathin PI films. Then, multilayer IZO film was spin-coated and annealed on PI film by
combustion synthesis41,42, followed by photolithographic patterning into circular regions (1-4 cm
in diameter). Peripheral Al electrodes were subsequently deposited by thermal evaporation. Finally,
CdSe/ZnS quantum dots (QDs) were coated on IZO layer and ultraflexible PIT imager was
released from the glass substrate for usage.

Before evaluating the imaging performance of PIT imager, the photoelectrical characteristics
between two adjacent electrodes were thoroughly evaluated and optimized. IZO, the major sensing
material of the device, is basically a semiconducting material with high photosensitivity. Thus,
systematic optimization on composition, film thickness, annealing time, electrode pad’s size, and
QDs/IZO heterojunction on the photoelectric properties of PIT sensor were conducted. As the PIT
sensor is a multi-port device, the boxplots of photocurrent and response time obtained from each
port of the sensor under 5 V are presented. Supplementary Fig. 3 shows the two-terminal dynamic
photocurrent curves of PIT imager with different In-Zn ratios. Devices with an indium-zinc ratio
of 9:1 exhibited a photocurrent of ~ 6 × 10-6 A under UV light intensity of 24 mW cm-2,
approximately one order of magnitude greater than those with ratios of 7:3 and 6:4, and
approximately two order of magnitude higher than those with ratios of 5:5 and 3:7. Dark currents
for all devices were generally in the range of 10-10 to 10-9 A (Supplementary Fig. 4). Considering
the photo-to-dark current ratio (PDCR), operation stability, and the response/recovery time,
devices with an indium-zinc ratio of 6:4 provided the optimal balance performance. The number
of IZO layers and annealing time significantly influenced the device’s photoresponse
characteristics (Supplementary Fig. 5). Each additional IZO layer (approximately 30 nm per layer)
led to an order-of-magnitude increase in photocurrent due to enhanced light absorption and carrier
numbers. For PIT imager with a 60-nm thick IZO layer (Supplementary Fig. 6), photocurrent
increased exponentially with annealing durations ranging from 1 to 3 hours, correlating with
improved crystallinity. Thus, PIT imager with two-layer IZO and processed with 2 h annealing
demonstrates optimal balance between signal intensity and temporal resolution. The electrode
dimension impacts on photocurrent and temporal response were further examined through
controlled experiments with four different sizes (Supplementary Fig. 7, 8). Both photocurrent and
response time increased with electrode area due to expanded conductive IZO channel.
Consequently, electrodes sized at 0.25 × 0.25 mm² were selected as the optimal configuration.
Coating a layer of CdSe/ZnS quantum dots (QDs) on the IZO layer further enhances the
photocurrent response under UV illumination (24 mW cm-2) and extends its sensitivity to visible
light (Supplementary Fig. 9-11)43-45. Specifically, an Ohmic contact is formed between the IZO
film and Al electrodes, and the photocurrent of QDs/IZO-based PIT imager reaches 33 µA with a
PDCR of approximately 1.1×104. When UV is replaced with a visible light source, QDs/IZO-based
PIT imager exhibit a fast response and a PDCR of ~ 150, whereas the IZO-based PIT imager shows
no response (Supplementary Fig. 11). Moreover, the photocurrents of both PIT imagers exhibit
excellent linearity in response to UV light across varying intensities from 0 to 24 mW cm-2 (Fig.
2a). Electrical impedance spectroscopy (EIS) was employed to characterize the frequency-
dependent impedance between QDs/IZO and Al electrodes, which is critical to PIT imager
performance (Fig. 2b). Results indicate that the electrical characteristic between two terminals of
PIT imager swifts from resistance-type to capacitive-type with increasing light intensity. An
equivalent RC(RC)(RC) circuit model of was verified by using ZSimpWin software, showing good
consistency for the whole measured frequency under different light intensities (Supplementary Fig.
12)46.To verify the functionality and stability, cycling tests were performed under varying light
intensities and prolonged exposure (Fig. 2c, d). The devices exhibited highly repeatable
photocurrent responses under identical illumination conditions, and good stability was
demonstrated over a continuous testing period exceeding 2 h. Regarding the environmental
stability, the PIT device shows minimal performance degradation when being stored in glovebox
for 20 days, while its performance exhibits gradual deterioration during ambient storage due to the
environmental degradation of its IZO and Al thin-film components (Supplementary Fig. 13).

PIT-based UV light imaging
The PIT imager, operating on EIT principles, integrates a 16-electrode pairs to capture photo-
induced voltage variations within a central sensing cell via adjacent measurement protocols
(Supplementary Fig. 14)23,47,48. The data collection process begins by injecting a current into two
adjacent electrode pairs of the device (Fig. 2e and Supplementary Fig. 15). As the current traverses
the device, voltage measurements are simultaneously recorded from the surrounding 16 adjacent
electrode pairs. The current is then sequentially applied to the next adjacent pair, with
corresponding voltage measurements taken from the surrounding electrodes. The quad-MUX
switch matrix directs the constant current source and precision voltmeter to designated electrodes,
enabling full-frame data acquisition through 16 cyclic excitation-measurement sequences49. Image
reconstruction is achieved by using EIDORS software with finite element modeling, which reflects
internal impedance variations modulated by external light distribution and intensity (Fig. 2f)50. In
this work, the sensitivity matrix, sized [1280 × 256], incorporates the electrode positions, the
material’s boundary geometry, and the current injection and voltage measure patterns. This matrix
governs the spatial resolution of the reconstructed image, which consists of 1280 pixels. By
employing a one-step linear Gauss-Newton model and hyperparameter of 1 for mapping the light
distribution, we achieved a high-resolution grid with an effective pixel/interconnect ratio of 80.
To simulate wearable conditions on curved human body and to confirm its robustness against
bending, average voltage changes for various bending radii were measured. Upon release from the
silicon substrate, the device exhibited only a ~2.5% change in average voltage, which gradually
increased as the bending radius decreased. When the ultraflexible device was bent over a needle
with a radius of 0.3 mm, the average voltage changed slightly by 4.8% (Fig. 2g, h). The PIT
imager’s mapping capability was further validated by placing it on a pen, with results showing
only a slight deviation from those obtained on a flat surface (Fig. 2i). Spatial resolution was
subsequently assessed by identifying the position of peak light intensity within the impedance
tomography map (Fig. 2j and Supplementary Fig. 16). Following the initial block by using a 125
µm-thick PI disk with a 4 mm-diameter, the center of block was repositioned by 1.5 mm for each
step. The specific points of block were discerned through the maximal impedance point estimation
and were graphically represented, illustrating the device’s competency in distinguishing between
light exerted at 1.5-mm intervals. Furthermore, when the block adopts a movement in the shape of
the letter “M”, the resulting PIT imaging plots verify its ability for accurately locating inside the
whole sensing area (Fig. 2k and Supplementary Fig. 17).

Mapping capability under various scenarios
Initially, the capability to detect multiple points within a single cell was evaluated by utilizing PI
disks of varying sizes, numbers, and transmittance (Fig. 3a). Similar to conventional array-
structured systems, multiple points can be monitored by PIT imager with 8 or 16 surrounding
electrodes (Fig. 3b and Supplementary Fig. 18). Specifically, under UV light irradiation, 125 µm-
thick PI disks with a radius of 2 mm and 4 mm were first placed within the sensing area, the result
clearly demonstrates the region subjected to larger block exhibits a larger and more pronounced
imaging area. When PI disks of identical diameter but differing thicknesses (50 µm and 125 µm)
were introduced, the reconstructed images show similar spatial coverage but distinct intensity
levels. Subsequently, when three to five PI disks with identical thickness block the QDs/IZO area,
the disparities are visually confirmed through the impedance reconstruction, demonstrating the
device’s capability for independent high-resolution (down to 4% relative to sensing area) mapping
of simultaneous, localized block area within a single sensing region (Fig. 3b, c).

Although PIT imager can be encapsulated with flexible light-transmitting protective layer to
enhance durability, it is often insufficient in preventing unexpected mechanical damage (Fig. 3d).
Owing to its topology reconstruction mechanism, the PIT imager demonstrates robust mapping
performance even after piercing damage within the sensing area (Supplementary Fig. 19a). Fig. 3e
and Supplementary Fig. 19b show the voltage values between 256 electrode pairs for original PIT
imager and damaged PIT imagers blocked with 4 mm-diameter PI disks. When the damaged
diameter increases from 0.5 mm to 2 mm (The diameter of sensing area is 2 cm), the amplitude of
voltage fluctuations remains nearly unchanged. The corresponding imaging results (Fig. 3f) further
confirm the functionality for light distribution measurement. Such ability highlights the high
usability during practical applications and emphasizes the benefits of PIT for light imaging with
surrounding electrodes.

In addition, unlike CMOS technology, which suffers from substantial cost increases and
reduced fabrication yield as sensor area expands, the PIT imager offers superior scalability without
incurring additional manufacturing costs. As a demonstration, a large-area ultraflexible PIT imager
with a sensing diameter of 4 cm was fabricated using micro-nano fabrication techniques (Fig. 3g).
PI disks with different size are placed at the center of the sensing region to block UV light, creating
a local high-impedance region. The resulting photocurrents measured at the bottom ports indicate
a negative correlation between block area and photocurrent, with a detectable size as small as 4%
of the sensing area (Fig. 3h and Supplementary Fig. 20). When PI masks occupy 16-25% of the
sensing region, the PIT results produce mapped sizes nearly identical to the actual sizes. The
comparison among PIT, CMOS and CCD imagers are given in Supplementary Table 4,
highlighting the PIT imager’s outstanding advantages, including low-cost manufacturing,
exceptional mechanical flexibility, larger-area compatibility, fewer interconnections, and damage-
tolerance.

Furthermore, the ultraflexible PIT imagers were applied for sunscreens’ quality evaluation.
According to national standards, the sun protection factor (SPF) and protection grade of UVA (PA)
are typically determined through on-skin testing and UV-Vis spectroscopy. Seen from Fig. 3i, for
using desktop UV-Vis measurement, linear light source with different wavelengths transmits
through the sunscreen layer, then the detector receives the attenuated light, which determines the
absorption ability under different wavelengths. Five commercial sunscreens with distinct labeled
SPF and PA values, were tested. As shown in the UV-Vis absorption spectra (Fig. 3j), two
absorption peaks at UVA and UVB wavelength are observed for all sunscreens, which are
indicative of the PA and SPF performance, respectively. Dabao and Mistine’s formulas exhibit the
strongest absorption in UVA range, followed by Maxam’s one, while Wonjin and CeraVe’s
formulas exhibit the weakest UVA absorption. These results match well to their labeled values. For
wearable testing, the PIT imager directly detects incident UV light and translates it into spatially
resolved impedance images. As shown in Supplementary Fig. 21, the photocurrent between two
terminals decreases when the sensing area is centrally blocked by sunscreen coatings, and the
observed attenuation trends correlate well with the UV-Vis spectroscopy data. The imaging plots
and corresponding maximum conductivity (σ ) in Fig 3k further provide information of
max
sunscreens like UV protective ability, coating’s location, area and uniformity.

3-axis tactile sensing
The PIT imager was adapted for optical tactile sensing through integration with flexible UV
LEDs and a translucent, porous Ecoflex rubber into a thin-film structure (total thickness ~2.6 mm,
dimensions 2 × 2 cm²). This flexible configuration enables conformal attachment to curved
surfaces like a fingertip or a wooden cylinder (Fig. 4a, Supplementary Fig. 22, and Supplementary
Video 2). The illumination system consists of four UV LEDs mounted on a flexible printed circuit
(Inset of Fig. 4a and Supplementary Fig. 23). The pressure-sensitive layer was fabricated by
mixing sifted sugar particles with Ecoflex, followed by particulate dissolution post-curing, creating
a porous matrix that enhances light scattering while maintaining mechanical flexibility. Unlike
dense Ecoflex, the porous Ecoflex exhibits appearance of translucence with a higher elongation at
break and less tensile strength (Fig. 4b). Once pressure applied, the partial closure of the inner
pores of the rubber results in weaker light scattering, making the rubber more optically transparent
(Supplementary Fig. 24). As shown in Supplementary Fig. 25, when porous rubber thickness is
~1.5 mm, the light can pass through and reach the imager easily, which is not a good option for
tactile sensor. Conversely, porous rubber layer with a thickness of ~3.5 mm permits only minimal
light penetration even under substantial applied pressure. Therefore, we choose a porous rubber
with ~2.6 mm thickness, which can effectively decrease the light with no pressure and ensure thin-
film features of 3-axis tactile sensor. Seen from the displacement-pressure characteristics of porous
Ecoflex rubber (~2.6 mm thick) in Supplementary Fig. 26, the displacement increases obviously
when the pressure increases from 0 to 20 kPa, then increases slowly with increasing pressure. The
distance reaches a plateau when pressed with an 80 kPa force, further increasing the force would
not compress the rubber further. Supplementary Fig. 27 shows the porous Ecoflex layer
demonstrates remarkable mechanical robustness. After 1000 cycles of 80 kPa compression and
recovery, its internal porous architecture remains virtually unchanged.

This pressure-dependent optical modulation enables the porous rubber to serve as a tactile
interface, where mechanical stimuli alter the spatial distribution of light captured by the PIT imager.
The sensing mechanism upon normal pressure is given in Fig. 4c. Specifically, in the absence of
pressure, uniform scattering creates a small circular dark spot at the center of PIT imager. Under
normal pressure, reduced scattering leads to concentration of the UV light, creating a larger dark
spot in the center of the imager. Therefore, the photocurrent between two adjacent terminals of PIT
imager was measured with different normal pressures applied ranging from 0 to 80 kPa in the
center (Fig. 4d). The result indicates a linear relationship between photocurrent and pressure with
an averaged sensitivity of 0.04 kPa-1 within the measured range (Inset of Fig. 4d). Besides, the 3-
axis tactile sensor exhibits negligible performance degradation after 240 cycles under 80 kPa
extreme pressure (Supplementary Fig. 28), demonstrating excellent operational stability.

In terms of oblique pressure, tangential stress causes lateral light displacement, producing
shifted dark spots with size proportional to normal force components. Then, an oblique force of 8
N was applied to the sensor with angles (θ) of 0°, 30°, 45°, and 60°, respectively (Fig. 4e). The
corresponding reconstruction results obtained from the PIT imager are shown in Fig. 4f. The
mapping results reveal two distinct opposing peaks, where the red peak corresponds to local light
intensity while the blue peak arise from EIDORS reconstruction artifacts influenced by
hyperparameters and scaling range selection. The red area progressively decreases while
displacement increases with larger tilt angles. The light intensities in the whole sensing region
were quantified by fitting the reconstructed results to a two-dimensional Gaussian function using
Equation (1):

𝐼 = 𝐴 ∗exp (−
(𝑥−𝜇𝑥)2+(𝑦−𝜇𝑦)2
)
(1)
𝑥,𝑦 2𝜎2

Here, 𝐼 represents the fitted light intensity. x and y are the coordinate index of the PIT
𝑥,𝑦
imager. µ and µ denote the position of the center of the dark spot on the PIT imager, reflecting
x y
the amplitude and direction of the tangential components of the applied pressure. σ is the standard
deviation of the 2D Gaussian function, which denotes the sharpness of the dark spot, reflecting the
value of the normal components of the applied pressure. The parameter A denotes the amplitude
and is related to the absolute light intensity. As the intensities are greatly affected by the random
distribution of the inner pores, so the absolute values of the intensities are excluded in the analysis
and are all normalized to -1 (Fig. 4g). Therefore, by fitting and quantifying the reconstructed results
from the PIT-imager, the peak position (μ) and sharpness (σ) are the two factors for 3-axis pressure
reconstruction. For simplicity, the one-dimensional profiles of the fitted results are extracted in the
center of the x direction (Fig. 4h, i), where a lateral shift and narrowed peak width of the fitted
curve are observed when oblique force is applied. The μ shifts from 68.3 to 97.1 in the y direction
and σ decreases from 21.7 to 6.85 as θ increases, corresponding to a 7.5 mm N-1 shear displacement
resolution.

To quantitatively establish the correlation between 3-axis pressure components and the optical
parameters (Δμ and σ), we present the relationships between normal pressure vs. σ in Fig. 4j and
tangential pressure vs. Δμ in Fig. 4k. The equations are formulated as follows for fitting:

𝐹 = 8.54−12.1∗𝑒−0.143𝜎 (2)
𝑁
𝐹 = −0.732+31.6∗∆𝜇 (3)
𝑇

F and F denote the normal and tangential components, respectively. Equation (2) reveals
N T
an exponential decay relationship for normal pressure detection, while Equation (3) indicates a
proportional relationship between Δμ and tangential pressure. Through these established
quantitative relationships, the 3-axis tactile sensor enables comprehensive pressure mapping by
computationally resolving both normal and tangential components from PIT imaging data.

The performance characteristics of the PIT imager-based tactile sensors are compared with
those of state-of-the-art soft tactile sensors, highlighting key advantages of employing the PIT
imager in tactile sensing (Table 1). Furthermore, as summarized in Supplementary Table 5, most
conventional tactile sensors—including those based on piezoresistive, capacitive, or thin-film
transistor (TFT) architectures—are limited to measuring forces in two dimensions. Among existing
3-axis force sensing technologies, several inherent trade-offs exist. Vision-based approaches,
such as GelSight®, often require relatively complex optical components, resulting in bulky system
configurations. Hall-effect or magnetically based sensors (e.g., PaXini®) are intrinsically
susceptible to external electromagnetic interference. Meanwhile, achieving 3-axis force
decoupling with piezoresistive arrays generally necessitates intricate layouts, which can
compromise spatial resolution and increase structural complexity. While the current performance
metrics of our PIT-based three-axis tactile sensor may not exceed those dedicated 3-axis sensors,
its performance remains adequate for many common applications, similar to commercially
available ones. Importantly, our PIT-based sensor offers several distinct advantages, notably
simplified interconnection requirements, a thin-film topology, and favorable scalability with low
associated costs—promising features for this emerging class of 3-axis tactile sensing devices.

To validate the tactile perception capability of the triaxial tactile sensor, the device was first
mounted on a robotic index finger and subjected to different geometries, including the curved
surface of a ball, the face, edge, and vertex of a cube (Fig. 5a-f). Normal press on spherical surface
produced a central red pattern with a diameter of 3.1 ± 0.2 mm (Fig. 5a). Then, a dynamic detection
scenario by sliding the robot finger fixed with the tactile sensor along different surfaces of a cube
was conducted. Firstly, the tactile sensor is pressed on the face of the cube (Fig. 5b), generating a
larger uniform dark region (diameter: ~3.8 mm). Then, the tactile sensor is slid down along the
face of the cube (Fig. 5c). During this process, a constant oblique pressure is applied, and the
direction of the tangential pressure is along the y direction. The reconstructed result presents as a
shifted dark spot with a smaller size towards the y direction, resulting from the role of the tangential
component of the pressure. Next, the tactile sensor is moved to the edge of the cube, and a rectangle
pattern (length: 5.4 ± 0.3 mm, width: 2.9 ± 0.2 mm) aligned with the contact axis is observed (Fig.
5d). Subsequently, the tactile sensor is slid along the edge to the x direction, the reconstructed
result exhibits a shift dark spot correspondingly (Fig. 5e). Finally, the tactile sensor is slid to the
end of the edge and is pressed on the vertex of the cube. The result presents as a small size
(diameter: ~ 2.3 mm) of dark spot in the center (Fig. 5f). The results demonstrate the capability of
our 3-axis tactile sensor to distinguish between various tactile perception modalities.

To validate the practical efficacy of our system, a ResNet-based tactile modality classification
framework was implemented (Fig. 5g and Supplementary Fig. 29). The network architecture
adopted an 18-layer configuration with ReLU activation and Adam optimization (learning rate =
0.0001, batch size = 16), incorporating dropout regularization (p = 0.3) to mitigate overfitting.
Training utilized 2,000 PIT imaging samples per tactile modality with 150 training epochs, with
an additional 200 test samples reserved for evaluation (Supplementary Fig. 30). The difference
between ResNet-18 and Gaussian fitting is schematically illustrated in Supplementary Fig. 31. As
shown in Fig. 5h, the primary misclassifications between F (face) and F (ball) modalities stem
N N
from their analogous pressure-induced light transmission patterns. Overall, the confusion matrix
demonstrates 96.5% average classification accuracy, confirming the system’s discriminative
capability through ResNet-enhanced PIT imaging (Supplementary Fig. 32-36).

Methods
Materials
Indium nitrate hydrate (99.9%), 2-methoxyethanol (99%), acetylacetone (99%), oxalic acid (99%),
1,7-diaminoheptane (98%), N,N-dimethylformamide (99.8%), methanol (99.5%), and isopropanol
(99.7%) were provided by Shanghai Titan Scientific Co., Ltd. Zinc nitrate (99%) was bought from
Shanghai Aladdin Biochemical Technology Co., Ltd. polyacrylic acid (PAA) solution was sold by
Changzhou Sunchem new materials co., LTD. Cd/Se/ZnS quantum dots/ethanol solution with a
concentration of 3 mg mL-1 was bought from XFNANO Materials Tech Co., Ltd. Acetone (99%)
and toluene (99%) was bought from Sinopharm Chemical Reagent Co., Ltd. AZ® nLOF 2020 was
bought from MicroChemicals GmbH and NMD-3 was bought from Tokyo Ohka Kogyo CO., LTD.
n++ Si wafer with 300 nm thick SiO was sold by Suzhou Jingxi Tech CO., LTD. Ecoflex 00-31
2
was bought from Smooth-on Inc. UV light source with adjustable intensity was acquired from a
retailer of Lemons.

Preparation of PI substrate and IZO solution
SiO /Si wafers cut with desired size was used as supporting substrate. Before use, substrates were
2
ultrasonically washed with acetone for 30 min and rinsed with isopropanol. The polyacrylic acid
(PAA) solution was diluted with N,N-dimethylformamide to a concentration of 10 wt%.
Subsequently, the diluted PAA solution was spin-coated on SiO /Si substrate with a spinning speed
2
of 600 rpm for 60 s. The cross-linking of PAA film into PI film is carried out on the hot plate in a
nitrogen glove box. The heating process performed a removing solvent bake for 20 min at T= 40 ℃,
80 ℃, 120 ℃, 140 ℃, 160 ℃ and 180 ℃ respectively, followed by a cross-linking curing for 20
min at T= 200 ℃, 250 ℃ and 300 ℃, respectively. Note that the subsequent treatment at 300 ℃
for 20 min does not compromise the integrity and mechanical strength of the PI film
(Supplementary Fig. 37).

To prepare a 0.2 M IZO precursor, 212.9 mg of indium nitrate and 119.0 mg of zinc nitrate
were dissolved in 5.0 mL of 2-methoxyethanol. 25 μL of acetylacetone and 11.25 μL of NH (aq)
3
were then added in the IZO precursor solution. The solution was stirred at 800 rpm overnight
before use.

Fabrication of ultraflexible PIT imager
First, the PI film on SiO /Si substrate was treated with oxygen plasma (PDC-36G, Hefei Kejing
2
Materials Technology Co., LTD) for 5 min. The IZO precursor solution was filtered with syringe
filters of 0.22 μm before spin-coating on the PI substrate at 3,000 rpm for 60 s and pre-annealing
at 120 °C for 60 s in a nitrogen glove box. Then, the resulting film was placed on a 300 °C hotplate
and annealed for 10 min at ambient atmosphere. To fabricate multi-layer IZO film, this process
was repeated to obtain desired film thickness. Finally, the film was annealed at 300 °C for 2 h in
ambient atmosphere.

Patterning of IZO film was achieved via a wet-etching process. First, nLOF2020 photoresist
was spin-coated and then annealed at 115 ℃ for 105 s. Then, it was exposed to UV light for 7 s
(URE-2000/35, Institute of Optics and Electronics, Chinese Academy of Sciences) and post-
annealed at 115 ℃ for 60 s. Next, the nLOF2020 film was developed in NMD-3 for 30 s followed
by deionized water washing and air-drying. After that, the exposed IZO film was dissolved in
aqueous oxalic acid (10 wt%) for 30 s and the remaining nLOF2020 was removed by acetone.
Finally, the patterned film was annealed at 300 ℃ for 10 min to remove any remains.
Subsequently, the Al electrodes with a thickness of 100 nm were deposited by thermal
evaporation. In order to achieve QDs/IZO film, QDs in toluene solution (10 mg/mL) were spin-
coated on top of the IZO layer at 2,000 rpm for 30 s. Then, the film was immersed into a 20 mM
methanol solution of 1,7-diaminoheptane, which was heated to 60 ℃ for 15 min and subsequently
soaked in pure methanol for 5 min. After rinsing with IPA and air-dried, the film was annealed at
180 ℃ for 30 min in a nitrogen glove box.

Fabrication of PIT-based 3-axis tactile sensor
The light source was designed with 4 LED arrays, with each LED evenly placed at the four corners
of the flexible printed circuit board, spaced 5 mm apart. The selected LED specifications are a
forward voltage drop of about 3.5 V and a peak wavelength of 370 nm. The 4 LEDs are connected
in parallel to the power supply of 5 V, and to ensure they operate under normal forward working
conditions, each LED is connected in series with a 200 Ω resistor. Notably, the flexible circuit of
the UV LED arrays was designed by us and subsequently fabricated by an electronics manufacturer
(Shenzhen JLC Technology Group Co., Ltd). The UV LEDs were purchased from Shenzhen YCG
Electronic Technology Co., Ltd (Model: 0603, 365-370 nm, Size: 0.8 × 1.6 mm). Then, the UV
LEDs were soldered into flexible circuits by us.

The porous rubber was fabricated by mixing sugar particles into the Ecoflex at a mass ratio of
3:1. The sugar-Ecoflex mixture was then tightly clamped by two pieces of glass with a gap of 2
mm. After curing at 50 °C for 2 h, the mixture was soaked in water to dissolve the sugar particles.
After drying, a translucent porous rubber was formed with an average pore diameter of 100 μm.

The light source, porous rubber, and PIT layer were laminated together. To enhance interlayer
adhesion and avoid wrinkle and buckle effect (Supplementary Fig. 38), a 10-μm thick patterned
double-side TPU tapes (without covering the sensing and LED area) was employed among PIT
imager, porous rubber and LED layers. After assembling, they need to be integrated into a single
system that can be electrically controlled simultaneously. Note here that the 3-axis tactile sensor
was mounted on to robotics’ finger with help of double-side tape.

Signal acquisition and imaging reconstruction
Optoelectronic properties among terminals of PIT were acquired by a Keithley 2400 source meter.
PDCR is defined as (I −I )/I , where I is the photocurrent and I is the dark current. The rise and
p d d p d
decay time (t and t ), defined as the time interval when photocurrent changes from 10% to 90%
r d
and vise visa.

To implement the measurement module in PIT system, the switching and voltage measurement
were accomplished by Keysight 34970A Data Acquisition/Data Logger Switch Unit. Keysight
34970A had 2 multiplexers with 16-channel. One multiplexer was connected to the output of 2400
SMU and served as the current injection and another one served for voltage measurement. The
switching and measurement were controlled by laptop through general-purpose interface bus
(GPIB) communication with Keysight 34970A. To reduce measurement error, an average of five
consecutive measurements was used for one voltage data and the system’s timing parameters (500
ms sampling interval) and excitation current (1 mA) were optimized for test (Supplementary Fig.
39). For 16-electrode PIT, 256 voltage data in total were obtained in sequence for one imaging
frame. The acquired data were then imported into MATLAB for EIT reconstruction based on
EIDORS.

3-axis force measurement and reconstruction
The setup for calibrating the response of the tactile sensor under external force included a
motorized test stand (ESM 303) to apply the external load, a force gauge (model M5-10, Mark-10)
to measure the amplitude of the force, and the above PIT measurement system. The oblique force
was achieved by putting the sensor on bases with different tilt angles. The contact area is same to
the size of tactile sensor.

The reconstructed images were stored in MATLAB for all postprocessing. All of the image
elements were normalized with the minimum value set to -1and then fitted to a 2D Gaussian
function by the trust-region method. The 1D profile was extracted from the 2D fitted result and
fitted to a Gaussian function. The SD (σ) of the Gaussian function was used to reconstruct the
normal pressure, and the center positions shift to their original values (μ ) were used to reconstruct
y
the tangential pressures.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Specific concerns include:\n1、The abstract states: “Optical-based tactile sensing inherently provides superior accuracy, structural simplicity, and\nelectromagnetic interference resistance.” While resistance to electromagnetic interference is indeed a notable advantage of\noptical tactile sensing, the claims regarding accuracy and structural simplicity are not sufficiently rigorous.\n2、The principle and structure of the PIT imager are not described in adequate detail, and the imaging mechanism remains\nunclear. Although Figures 1a and 1b are included, there is no corresponding explanation in the text.\n3、The authors mention that the PIT imager exhibits low crosstalk and is easily scalable. Crosstalk in spatial force sensing\nis critical for tactile perception, so the extent of crosstalk in three-axis force measurement should be explicitly addressed.\n4、The mechanical limits such as ultimate strength, fatigue strength, and environmental adaptability of the flexible three-\ndimensional tactile sensor based on the PIT imager are not evaluated.\n5、Throughout the manuscript, the fabrication of the PIT imager does not appear straightforward. How does this sensor\noutperform existing alternatives? Table 3 does not convincingly demonstrate the advantages of PIT. A quantitative\ncomparison with state-of-the-art flexible electronic devices is recommended.\n6、The authors claim that the PIT imager remains functional after puncture, but this is not experimentally verified.\n7、The supporting video is of low quality and inconsistent with the images presented in the manuscript, raising doubts about\nits authenticity.", "reviewer_2": "The authors present a highly innovative ultraflexible photoelectrical impedance tomography (PIT) imager that achieves an\nimpressive 1.5 mm spatiotemporal resolution and is capable of simultaneously detecting up to five UV patterns. Notably, this\nremarkable performance is achieved through an elegantly simple fabrication process and structural design. A particularly\nstriking feature is the device’s robust functionality under extreme mechanical deformation, distinctly differentiating it from\nconventional tactile imaging systems. The demonstrated extension to 3-axis tactile sensing with competitive detection\ncapabilities further underscores its technological significance. This work establishes a novel imaging framework that\nsubstantially advances the field of tactile sensing.\nWhile recommending minor revision, I propose the following constructive suggestions to enhance the manuscript:\n#Comment 1. Can the authors discuss potential semiconductor alternatives to the IZO layer? A brief commentary on material\nselection criteria and compatibility with the PIT mechanism would strengthen the methodology section.\n#Comment 2. The authors should discuss whether surface wrinkling induced by conformal attachment to curved substrates\nimpacts sensor response characteristics.\n#Comment 3. Clarify the distinction between the PIT approach and conventional array-based imagers. A concise conceptual\ncomparison—highlighting the elimination of discrete pixels, reduction in interconnect complexity, and differences in data\nacquisition schemes—would help readers appreciate how the PIT architecture departs from traditional active or passive\nmatrix systems.\n#Comment 4. Provide a detailed cross-sectional schematic of the device architecture. An annotated illustration showing\neach functional layer (polyimide substrate, electrode, IZO, QD layer, encapsulation, etc.) would clarify the physical\nconfiguration and assist readers in understanding the mechanical flexibility and optical pathways.\n#Comment 5. Figure 2: Improve figure readability by enlarging annotations to ensure legibility in print format. Consider\nincreasing font sizes or using bolder typography for critical labels.\n#Comment 6. Figure 2k: Enhance visual contrast by adding a distinct border delineation to the circular image area,\npreventing blending with the background.\n#Comment 7. Formatting revisions:\n– Line 158: Correct spacing in scientific notation to “6 × 10⁻⁶ A.”\n– Line 501: Implement paragraph indentation (2-character space) for the opening sentence.\n– Include missing citation for reference [52] in the appropriate context.\n– Standardize reference formatting according to journal guidelines.\n– Page 3, line 88: Standardize dimensional notation by revising “3 cm by 4 cm” to “3 × 4 cm” for consistency with SI unit\nconventions.", "reviewer_3": "The manuscript titled “Ultraflexible Photoelectrical Impedance Tomography-based Imager for 3-Axis Robotic Tactile\nSensing”. presents a significant advancement in tactile sensing technology by introducing a Photoelectrical Impedance\nTomography (PIT) imager for high-fidelity 3-axis force decoding. The proposed PIT architecture reduces the number of\noutput connections compared to conventional array imagers, a topological advantage that successfully mitigates the cross-\ntalk effect inherent in traditional sensing arrays while leveraging excellent photoelectric characteristics to achieve high\nsensitivity. However, after careful review, I believe that several parts of the manuscript require major revisions to address\ncertain scientific and technical points before it can be considered for publication.\n1. The PI substrate is flexible but not stretchable. Therefore, even if a porous rubber layer is placed beneath the PI substrate,\nthe PI substrate will not stretch when pressure is applied; instead, it will tend to wrinkle and buckle (or crumple).\nConsequently, there is a serious question as to whether the device can accurately recognize the diverse pressure\nenvironments related to the Z-axis, which the authors claim is part of their 3-axis sensing capability. Furthermore, since\n80kPa is a considerably high pressure, it is questionable whether accurate measurements can be achieved given the\npotential for significant device deformation at this force level.\n2. In the proposed 3-axis tactile sensing application based on the PIT imager, the stacked structure from top to bottom is:\nImager, Porous Rubber, UV LED. The mechanism for 3-axis pressure recognition involves the UV LED emitting light, which\nis transmitted through the porous rubber to the imager; pressure application is intended to modulate the light scattering\nwithin the porous rubber, which is then sensed by the imager. However, given that the imager's substrate is polyimide, which\ninherently does not transmit UV light, the premise that the UV light from the bottom LED layer can pass through the PI\nsubstrate to reach the photosensitive QD/IZO layer is physically unfounded. This necessitates a major clarification or\ncorrection of the device structure and its underlying sensing mechanism.\n3. Te pressure sensitivity will likely vary across different pressure ranges. The authors are required to present the sensitivity\nvalues for these segmented ranges and clearly specify the pressure range when stating the pressure sensitivity in the\nmanuscript. Furthermore, while Figure 4d shows results for a wide pressure range, the authors are recommended to show\nwhat happens when pressure higher than 80kPa is applied.\n4. Since the sensor is fabricated based on EIT (Electrical Impedance Tomography), where measurements rely on the\nrelative change from a value taken before each measurement, initial calibration is extremely important, and a detailed\nexplanation of this procedure is required.\n5. The Abstract mentions a PDCR > 104, but later the manuscript notes that the PDCR is 150 under visible light (though still\nhigh when compared to IZO-only devices). Therefore, to avoid misrepresentation and ensure accuracy, the authors are\nadvised to specify the exact illumination conditions (e.g., UV light) when referencing the PDCR > 104 in the Abstract.\n6. The authors refer to a Flexible UV LED. It is necessary to clarify whether this component was fabricated in-lab or\npurchased as a ready-made product. If it was fabricated in-lab, a detailed fabrication process with illustration is required. If it\nwas a purchased product, the product name (manufacturer and model) must be clearly specified.\n7. Figure 4f clearly shows two peaks, but the subsequent analyses in 4g/4e (and the processed profile in 4f) treat only a\nsingle peak. The manuscript does not explain how one peak was isolated from the 4f data (e.g., polarity masking, absolute-\nvalue transform, ROI selection). Without a transparent description of this preprocessing, the single-peak analysis is not\nreproducible, and its credibility is weakened.\n8. The manuscript introduces two tools for analyzing 3-axis data—Gaussian fitting and ResNet-18—but does not specify\nwhen each is used or under what conditions. While ResNet-18 is applied to classify ball/face/edge, it remains unclear\nwhether a Gaussian-based analysis can achieve the same distinctions, and how the two tools are intended to complement\neach other. Please clarify the use criteria, workflow, and any quantitative comparison between the two pipelines.\n9. The comparison with CMOS presented in the paper seems premature, as PIT is trading off the core performance metric of\nhigh resolution for secondary benefits like cost and flexibility. Until PIT's resolution is improved to a truly competitive level for\na tactile sensor, a direct comparison with CMOS seems to highlight PIT's drawbacks more than its advantages.\n10. The paper's claim of 96.5% accuracy in recognizing \"complex topographies\" seems overstated. The test subjects used—\na ball, a flat face, an edge, and a vertex, along with their associated tangential or normal forces are merely simple geometric\nshapes. It is questionable to categorize these basic forms as \"complex topographies.\n11. Polyimide is generally known for its high thermal stability. However, the authors describe a process where the PI film is\nformed from PAA solution, which includes an annealing step at 300°C. It is necessary to confirm whether the PI film\nundergoes degradation or undesirable deformation when annealed at 300°C for 20 minutes, especially after a prior\nannealing step at 250°C for 20 minutes. The authors should provide evidence or relevant characterization data to ensure the\nultra-thin PI substrate maintains its required mechanical and structural integrity throughout the entire high-temperature\nfabrication process.\n12. The authors mention using a lift-off process for patterning the IZO film. Generally, the conventional lift-off process\ninvolves patterning the photoresist, depositing the material via a solution process or deposition over the resist pattern, and\nthen dissolving the photoresist to lift off the unwanted material. However, in this study, the IZO film was dissolved using\noxalic acid after photoresist patterning. Since the material was selectively etched (dissolved) from the exposed areas rather\nthan physically lifted off the substrate, the terminology \"lift-off process\" is inappropriate for describing the IZO film patterning\nprocedure. This step should be accurately described as an etching process (specifically, wet etching).\n13. The authors used thermal evaporation to deposit the Al electrodes onto the PI (Polyimide) substrate. It must be clarified\nwhether there were any adhesion issues between the Al film and the PI substrate. If an additional adhesion layer was used\n(e.g., Chromium or Titanium), the authors should specify this. Furthermore, to fully validate the robust mechanical properties\nclaimed for this ultraflexible device, the manuscript requires evidence of durability, specifically through repeated cyclic\nbending tests of the PIT imager."}, "author_response": "1) The abstract states: “Optical-based tactile sensing inherently provides superior accuracy, structural\nsimplicity, and electromagnetic interference resistance.” While resistance to electromagnetic interference\nis indeed a notable advantage of optical tactile sensing, the claims regarding accuracy and structural\nsimplicity are not sufficiently rigorous.\nReply: Thanks for reviewer’s comments. We revised the sentence in the abstract as follows:\n“Optical-based tactile sensing inherently provides superior spatial resolution, rich multimodal data, and\nelectromagnetic interference resistance.”\n2) The principle and structure of the PIT imager are not described in adequate detail, and the imaging\nmechanism remains unclear. Although Figures 1a and 1b are included, there is no corresponding\nexplanation in the text.\nReply: Thank you for the comments. Actually, the imaging mechanism of PIT imager was given in Fig.\n2e, f and Supplementary Fig. 15. As our paper proposed a novel PIT imager and demonstrated its\npotential application in 3-axis tactile sensor. The corresponding text of Fig. 1 mainly points out the\ninnovations and advantages of PIT imager; thus, we did not explain the principle and structure in detail.\nAs the following part (corresponding to Fig. 2) focuses on the characterization of PIT imager. We\ndescribed the mechanism, structure, fabrication procedures, equipment and the processing of PIT imager\nin detail.\nHowever, to make the PIT imager of Fig. 1 easy to understand, we added detailed descriptions in\nSupplementary Notes. Please check them in Supplementary Information.\n3) The authors mention that the PIT imager exhibits low crosstalk and is easily scalable. Crosstalk in\nspatial force sensing is critical for tactile perception, so the extent of crosstalk in three-axis force\nmeasurement should be explicitly addressed.\nReply: Good point! We fully agree that minimizing the crosstalk in spatial force sensing is critical and\nmeaningful. Our PIT imaging inherently relies on the device structure to achieve low crosstalk. However,\nwhen applied to 3-axis tactile sensing, a porous Ecoflex layer with a thickness of approximately 2.6 mm\nmust be inserted between the PIT imager and the LED arrays. Under multi-point force loading, this\nporous layer inevitably introduces pronounced interference. Therefore, more advanced force-optical\nreconstruction algorithms—potentially including AI-based approaches—are required to further suppress\ncrosstalk, which will be the focus of our future work. A discussion regarding spatial force crosstalk has\nbeen provided in the Discussion Section.\n4) The mechanical limits such as ultimate strength, fatigue strength, and environmental adaptability of\nthe flexible three-dimensional tactile sensor based on the PIT imager are not evaluated.\nReply: Thank you for valuable comments. The 3-axis tactile sensor consists of a flexible PIT imager, a\nporous rubber and flexible LED light source. Thus, the compressive properties of the tactile sensor are\nrelated to the porous rubber. As shown in following Fig. R1, when porous rubber thickness is ~1.5 mm,\nthe light can pass through and reach the imager easily, which is not a good option for tactile sensors.\nConversely, porous layer with a thickness of ~3.5 mm permits only minimal light penetration even under\nsubstantial applied pressure. Therefore, we choose a ~2.6 mm-thick porous rubber which can effectively\ndecrease the light with no pressure and ensure thin-film features of 3-axis tactile sensor.\nFig. R1\nFrom the distance-pressure characteristics of porous Ecoflex rubber (~2.6 mm thick) in Fig. R2, the\ndisplacement increases obviously when the pressure increases from 0 to 20 kPa, then increases slowly\nwith increasing pressure. The distance reaches a plateau when pressed with an 80 kPa force, further\nincreasing the force would not compress the rubber further. Thus, the ultimate strength is 80 kPa.\nFig. R2\nIn this 3-axis tactile sensor, the fatigue strength of the device is mainly determined by the porous rubber\nlayer. As shown in Fig. R3, the porous Ecoflex layer demonstrates remarkable robustness. After 1000\ncycles of extreme compression (under 80 kPa) and recovery testing, its internal porous architecture\nremains virtually unchanged. After careful assembly, as shown in Supplementary Fig. 29, the 3-axis\ntactile sensor exhibits negligible performance degradation after 240 cycles under 80 kPa extreme\npressure, demonstrating excellent operational stability.\nFig. R3\nThe environmental adaptability of 3-axis tactile sensors is mainly dependent on the PIT imager. For the\nultraflexible PIT imager, we optimized the preparation process of the PI substrate to ensure its\nmechanical strength. The flexible PIT imager exhibits negligible performance degradation under various\nbending conditions, as well as during long-term cycling tests (Fig. 2c, d, h). Besides, the device shows\nlittle performance degradation when being stored in glovebox for 20 days (Supplementary Fig. 13).\nHowever, as we know, the IZO layer is a n-type semiconductor, the majority charge carriers are electrons,\nwhich are sensitive to O and H O in the atmosphere. In addition, the conductivity of the Al electrode\n2 2\nwould decrease due to its slow oxidation in the ambient environment. Hence, the performance of the PIT\nimager gradually degrades in the air. We anticipate the issue could be solved by adequately\nencapsulating the PIT imager.\nIn revised manuscript, we added Fig. R1-R3 as Supplementary Fig. 26-28, respectively. And added the\nrelated discussions in Page 10.\n5) Throughout the manuscript, the fabrication of the PIT imager does not appear straightforward. How\ndoes this sensor outperform existing alternatives? Table 3 does not convincingly demonstrate the\nadvantages of PIT. A quantitative comparison with state-of-the-art flexible electronic devices is\nrecommended.\nReply: We appreciate reviewer’s valuable suggestions. The detailed fabrication process of the PIT\nimager was provided in the Methods Section. As you can see from Figure 1 c, it has very simple device\nstructure with ultraflexible PI substrate, QDs/IZO sensitive layer, and surrounding Al electrodes. More\nimportant, the fabrication is compatible with semiconductor micro-nano fabrication technology, ensures\ngood consistency and high throughput.\nRegarding the PIT imager and 3-axis tactile sensor performance, we provided Supplementary Table 4\nand Supplementary Table 5 to compare with other devices quantitatively. Our PIT imager combines\nultraflexibility with a high pixel/interconnect ratio, scalable and cost-effective large-area fabrication, and\ninherent damage tolerance. Furthermore, the ultraflexible PIT-based 3-axis tactile sensor exhibits\nperformance that rivals or surpasses its state-of-the-art counterparts.\n6) The authors claim that the PIT imager remains functional after puncture, but this is not experimentally\nverified.\nReply: Thank you for pointing out this, we also recognized the importance, Thus, the experimental data\nof the PIT imager after puncturing was shown in original Fig. 3d-f. When the damaged diameter\nincreases from 0.5 mm to 2 mm (The diameter of sensing area is 2 cm), the amplitude of voltage\nfluctuations remains nearly unchanged. The corresponding imaging results (Fig. 3f) further confirm the\nfunctionality for light distribution measurement.\n7) The supporting video is of low quality and inconsistent with the images presented in the manuscript,\nraising doubts about its authenticity.\nReply: We sincerely apologize for this oversight. After carefully reviewing the supplementary video and\nthe manuscript, we found that two of the mapping images in Supplementary Video 1 do not correspond\nto the figures in the manuscript (Fig. 2f and 3h). Specifically, Fig. 2f is only a schematic illustration of\nthe reconstruction process; accordingly, the corresponding image in Supplementary Video 1 was\nrandomly chosen from the set of mapping results. For Fig. 3h, the 4% mapping image in Supplementary\nVideo 1 does not match, due to variations in algorithmic parameters that lead to differences among the\ngenerated images. We have changed Supplementary Video 1 with updated images.\nReviewer 2\nThe authors present a highly innovative ultraflexible photoelectrical impedance tomography (PIT)\nimager that achieves an impressive 1.5 mm spatiotemporal resolution and is capable of simultaneously\ndetecting up to five UV patterns. Notably, this remarkable performance is achieved through an elegantly\nsimple fabrication process and structural design. A particularly striking feature is the device’s robust\nfunctionality under extreme mechanical deformation, distinctly differentiating it from conventional\ntactile imaging systems. The demonstrated extension to 3-axis tactile sensing with competitive detection\ncapabilities further underscores its technological significance. This work establishes a novel imaging\nframework that substantially advances the field of tactile sensing. While recommending minor revision,\nI propose the following constructive suggestions to enhance the manuscript.\nReply: We really appreciate the reviewer’s positive comments and constructive suggestions to the\nmanuscript.\n1) Can the authors discuss potential semiconductor alternatives to the IZO layer? A brief commentary on\nmaterial selection criteria and compatibility with the PIT mechanism would strengthen the methodology\nsection.\nReply: We appreciate the inspiring comments. Based on our investigation, the criteria of light-sensitive\nmaterials for PIT imager include adequate conductivity, easy fabrication and scalability, high light\nsensitivity, and operation stability. Thus, we choose CdSe/ZnS QDs and IZO as semiconductors, but we\nbelieve other semiconductors, like In O , perovskite, other QDs, and even organic semiconductors might\n2 3\nbe potential alternatives to the IZO layer.\nWe have added such descriptions in Discussion Section.\n2) The authors should discuss whether surface wrinkling induced by conformal attachment to curved\nsubstrates impacts sensor response characteristics.\nReply: Thanks for reviewer’s questions. Surface wrinkling in the ultraflexible PIT imager can, under\ncertain conditions, perturb the sensor’s response. In the 3-axis tactile sensor, wrinkles may shift the\npressure-induced dark spot from its true center in the QDs/IZO functional area, thereby reducing\nsensitivity and accuracy. Despite this, the PIT imager itself remains mechanically robust, exhibiting\nnegligible performance degradation under various bending states (Fig. 2g, h).\nTo suppress wrinkle-induced effects, the PIT layer was carefully laminated onto the porous Ecoflex\nrubber substrate to minimize surface deformation (Supplementary Fig. 34). Moreover, this issue can be\neliminated by introducing a thin patterned TPU interlayer between the PIT and porous Ecoflex layers,\nwhich effectively mitigates mechanical mismatch and prevents wrinkle formation.\n3) Clarify the distinction between the PIT approach and conventional array-based images. A concise\nconceptual comparison—highlighting the elimination of discrete pixels, reduction in interconnect\ncomplexity, and differences in data acquisition schemes—would help readers appreciate how the PIT\narchitecture departs from traditional active or passive matrix systems.\nReply: We appreciate reviewer’s suggestions. Similar to the comment 2 from the Reviewer 1, we made a\ncomparison between the PIT approach and conventional array-based images from several aspects in\nDiscussion Section, Supplementary Notes, and added more content in the manuscript in Page 3.\n4) Provide a detailed cross-sectional schematic of the device architecture. An annotated illustration\nshowing each functional layer (polyimide substrate, electrode, IZO, QD layer, encapsulation, etc.) would\nclarify the physical configuration and assist readers in understanding the mechanical flexibility and\noptical pathways.\nReply: We appreciate reviewer’s suggestions. The schematic of the ultraflexible PIT imager and\ncross-sectional schematic illustration of 3-axis tactile sensors are provided as follows and added in the\nmanuscript as Supplementary Fig. 2.\nFig. R4\n5) Figure 2: Improve figure readability by enlarging annotations to ensure legibility in print format.\nConsider increasing font sizes or using bolder typography for critical labels.\nReply: We thank the reviewer for the suggestion. The annotations in Fig. 2 have been enlarged and\nadjusted to ensure optimal legibility in print formats, with improved font weight for critical labels.\n6) Figure 2k: Enhance visual contrast by adding a distinct border delineation to the circular image area,\npreventing blending with the background.\nReply: Thanks for reviewer’s suggestions. We improved the quality of Fig. 2k according to your advice.\n7) Formatting revisions:\n– Line 158: Correct spacing in scientific notation to “6 × 10-⁶ A.”\n– Line 501: Implement paragraph indentation (2-character space) for the opening sentence.\n– Include missing citation for reference [52] in the appropriate context.\n– Standardize reference formatting according to journal guidelines.\n– Page 3, line 88: Standardize dimensional notation by revising “3 cm by 4 cm” to “3 × 4 cm2” for\nconsistency with SI unit conventions.\nReply: We appreciate reviewer’s careful inspections and comments. All above-mentioned problems have\nbeen addressed, with the corresponding changes highlighted in the manuscript.\nReviewer 3\nThe manuscript titled “Ultraflexible Photoelectrical Impedance Tomography-based Imager for 3-Axis\nRobotic Tactile Sensing”. presents a significant advancement in tactile sensing technology by\nintroducing a Photoelectrical Impedance Tomography (PIT) imager for high-fidelity 3-axis force\ndecoding. The proposed PIT architecture reduces the number of output connections compared to\nconventional array imagers, a topological advantage that successfully mitigates the cross-talk effect\ninherent in traditional sensing arrays while leveraging excellent photoelectric characteristics to achieve\nhigh sensitivity. However, after careful review, I believe that several parts of the manuscript require\nmajor revisions to address certain scientific and technical points before it can be considered for\npublication.\nReply: We sincerely appreciate the reviewer’s positive assessment of our work as a significant\nadvancement in tactile sensing technology. We have carefully revised the manuscript to incorporate your\nsuggestions.\n1) The PI substrate is flexible but not stretchable. Therefore, even if a porous rubber layer is placed\nbeneath the PI substrate, the PI substrate will not stretch when pressure is applied; instead, it will tend to\nwrinkle and buckle (or crumple). Consequently, there is a serious question as to whether the device can\naccurately recognize the diverse pressure environments related to the Z-axis, which the authors claim is\npart of their 3-axis sensing capability. Furthermore, since 80 kPa is considerably high pressure, it is\nquestionable whether accurate measurements can be achieved given the potential for significant device\ndeformation at this force level.\nReply: Good points! PI substrate is generally considered as flexible but not stretchable. In fact, it\ntypically exhibits a small amount of stretchability. In our work, we fabricated ultrathin PI film with a\nthickness of ~5 um, which exhibits ~5% stretchability and good conformality to curve surface. Also, we\nadopted 10 μm thick patterned double-side TPU tapes (without covering the sensing and LED area)\namong PIT, porous rubber and LED layers to improve the adhesion. Seen from the figures below, there\nis almost no wrinkle and buckle effect by high oblique pressure with edge and even vortex contact.\nFig. R5\nIn the revised manuscript, we added discussions in Page 17 and added Fig. R5 as Supplementary Fig. 34.\n2) In the proposed 3-axis tactile sensing application based on the PIT imager, the stacked structure from\ntop to bottom is: Imager, Porous Rubber, UV LED. The mechanism for 3-axis pressure recognition\ninvolves the UV LED emitting light, which is transmitted through the porous rubber to the imager;\npressure application is intended to modulate the light scattering within the porous rubber, which is then\nsensed by the imager. However, given that the imager’s substrate is polyimide, which\ninherently does not transmit UV light, the premise that the UV light from the bottom LED layer can pass through the PI\nsubstrate to reach the photosensitive QD/IZO layer is physically unfounded. This necessitates a major\nclarification or correction of the device structure and its underlying sensing mechanism.\nReply: Thank you for pointing out this, we are sorry for oversighting the mistake in Fig. 1e. Actually, we\nreversed the PIT imager and covered the porous rubber layer during experiment, thus the UV light can\ndirectly reach imager. In revised manuscript, we changed the deep blue color of QDs/IZO layer into a\nblur one to avoid misinterpretation.\n3) The pressure sensitivity will likely vary across different pressure ranges. The authors are required to\npresent the sensitivity values for these segmented ranges and clearly specify the pressure range when\nstating the pressure sensitivity in the manuscript. Furthermore, while Figure 4d shows results for a wide\npressure range, the authors are recommended to show what happens when pressure higher than 80 kPa is\napplied.\nReply: Thank you for the good suggestions. we had responded to Reviewer 1, The 3-axis tactile sensor\nconsists of a flexible PIT imager, a porous rubber and flexible LED light source. Thus, the compressive\nproperties of the tactile sensor are replied to the porous rubber. As shown in following Fig. R1, when the\nporous rubber layer is ~1.5 mm, incident light can easily penetrate the material and reach the imager,\nwhich is undesirable for stable tactile sensing. In contrast, a ~3.5 mm thick porous layer allows only\nminimal light transmission even under substantial applied pressure, thereby diminishing sensor\nresponsiveness. To balance these requirements, a porous rubber layer with a thickness of approximately\n2.6 mm was selected, as it effectively attenuates light in the absence of pressure while maintaining\nthin-film features of 3-axis tactile sensor.\nFig. R2 presents the distance-pressure characteristics of the porous Ecoflex layer (~2.6 mm), the\ndisplacement increases obviously when the pressure increases from 0 to 20 kPa, then increases slowly\nwith increasing pressure. The distance reaches a plateau when pressed with an 80 kPa force, further\nincreasing the force would not compress the rubber further.\nBesides, as shown in the inset of Figure 4d, our PIT device exhibits good linearity within the measured\nrange of 0-80 kPa, thus the sensitivity values for these segmented ranges are almost same.\nIn revised manuscript, we added Fig. R1 and Fig. R2 as Supplementary Fig. 26 and Supplementary Fig.\n27, respectively. And we specify the pressure range when stating the pressure sensitivity.\n4) Since the sensor is fabricated based on EIT (Electrical Impedance Tomography), where measurements\nrely on the relative change from a value taken before each measurement, initial calibration is extremely\nimportant, and a detailed explanation of this procedure is required.\nReply: Good point! The initial calibration is important, thus we set different test conditions for our PIT\nimager, as shown in Supplementary Fig. 35. Both the channel delay and excitation current have effect on\nthe data accuracy. Also, we need to check the obtained 256 voltage data to see whether it exhibits\nreasonable curve (shown in Supplementary Fig. 19b). Besides, in revised manuscript, the details of\nEIDORS processing are provided in Supplementary Note 2.\n5) The Abstract mentions a PDCR > 104, but later the manuscript notes that the PDCR is 150 under\nvisible light (though still high when compared to IZO-only devices). Therefore, to avoid\nmisrepresentation and ensure accuracy, the authors are advised to specify the exact illumination\nconditions (e.g., UV light) when referencing the PDCR > 104 in the Abstract.\nReply: Thank you. We revised it in the Abstract.\n6) The authors refer to a Flexible UV LED. It is necessary to clarify whether this component was\nfabricated in-lab or purchased as a ready-made product. If it was fabricated in-lab, a detailed fabrication\nprocess with illustration is required. If it was a purchased product, the product name (manufacturer and\nmodel) must be clearly specified.\nReply: We appreciate your rigorous comments. Regarding the flexible UV LED arrays, we first designed\nthe circuit with predetermined dimensions by AutoCAD, then, we sent the files to a company for\nmanufacturing. In the revised manuscript, we added the manufacturer information in the Methods\nSection.\n7) Figure 4f clearly shows two peaks, but the subsequent analyses in 4g/4e (and the processed profile in\n4f) treat only a single peak. The manuscript does not explain how one peak was isolated from the 4f data\n(e.g., polarity masking, absolute-value transform, ROI selection). Without a transparent description of\nthis preprocessing, the single-peak analysis is not reproducible, and its credibility is weakened.\nReply: Thank you for pointing out the concerns! In imaging patterns of Fig. 4f, they exhibit two\nopposite peaks, the red one corresponds to light intensity in this area, the blue one actually depends on\nthe reconstruction process of EIDORS, which is related to the hyperparameter of inv_model and the set\nscale ranges. Their contrast can be easily tuned by changing these parameters, which has no effect on the\nactual results. Also, the blue area is opposite to useful peaks, during the processing, we can simply cut\nthis part. Besides, for 1D gauss fitting, we need to choose a line pass through the central dark red area,\nthus the blue area would not affect the calculated results.\nIn revised manuscript, we have added these descriptions in Page 12.\n8) The manuscript introduces two tools for analyzing 3-axis data—Gaussian fitting and ResNet-18—but\ndoes not specify when each is used or under what conditions. While ResNet-18 is applied to classify\nball/face/edge, it remains unclear whether a Gaussian-based analysis can achieve the same distinctions,\nand how the two tools are intended to complement each other. Please clarify the use criteria, workflow,\nand any quantitative comparison between the two pipelines.\nReply: Thank you for the good comments. The Gaussian fitting and ResNet-18 play different roles for\nanalyzing 3-axis data. As mentioned in the manuscript and shown in Fig. R6, Gaussian fitting is used to\nquantify the reconstructed results by fitting the PIT data matrix along the X and Y axes. In this way,\nwe can actually obtain the applied force and angles through the peak position (μ) and sharpness (σ). On the\nother hand, the ResNet-18 is used for the classification of different scenarios directly by using PIT\nimaging patterns.\nTherefore, Gaussian methods can precisely calculate pressure magnitude and angles, while ResNet-18\ncannot achieve this.\nOn the other hands, both approaches can perform scene classification, but Gaussian methods require an\nadditional step: converting PIT patterns into 1D data followed by precise pressure and angle calculations\nfor differentiation. In contrast, ResNet-18 directly leverages AI image recognition principles to classify\nthrough PIT patterns without intermediate steps.\nWe added a schematic illustration of the difference between Gaussian fitting and ResNet-18 as following\nFig. R6 and added it as Supplementary Fig. 32.\nFig. R6\n9) The comparison with CMOS presented in the paper seems premature, as PIT is trading off the core\nperformance metric of high resolution for secondary benefits like cost and flexibility. Until PIT’s\nresolution is improved to a truly competitive level for a tactile sensor, a direct comparison with CMOS\nseems to highlight PIT’s drawbacks more than its advantages.\nReply: Thank you for critical comments. We understand your suggestion, but CMOS imagers have been\ndeveloped for over 50 years and are widely used. In these years, their key performance parameters have\nbeen continuously improved, establishing CMOS technology as the current mainstream in optical\nimaging. In this way, we cannot simply neglect it.\nIn contrast, PIT, as a different optical imaging approach, employs a unique imaging mechanism that\noffers several advantages that are hard to achieve for CMOS, including low cost, interconnect-free\ndesign, mechanical flexibility, and scalability. In Discussion Section, we acknowledge that PIT currently\nlags behind mature CMOS technology in spatial resolution. Nevertheless, with future enhancements,\nsuch as increasing the number of peripheral electrodes, optimizing material systems, and refining\nreconstruction algorithms, PIT has significant potential to advance the field of optical sensing.\n10) The paper’s claim of 96.5% accuracy in recognizing “complex topographies” seems overstated. The\ntest subjects used a ball, a flat face, an edge, and a vertex, along with their associated tangential or\nnormal forces are merely simple geometric shapes. It is questionable to categorize these basic forms as\n“complex topographies”.\nReply: We appreciate your good comments. In the revised manuscript, we have changed the expression\nusing simple topologies, please see it in Abstract.\n11) Polyimide is generally known for its high thermal stability. However, the authors describe a process\nwhere the PI film is formed from PAA solution, which includes an annealing step at 300 °C. It is\nnecessary to confirm whether the PI film undergoes degradation or undesirable deformation when\nannealed at 300 °C for 20 minutes, especially after a prior annealing step at 250 °C for 20 minutes. The\nauthors should provide evidence or relevant characterization data to ensure the ultra-thin PI substrate\nmaintains its required mechanical and structural integrity throughout the entire high-temperature\nfabrication process.\nReply: Thanks for reviewer’s comments. We supplemented tensile tests as shown in Fig. R7,\ndemonstrating that the treatment at 300 ℃ for 20 min does not impair the integrity and mechanical\nstrength of the PI film. We added Fig. R7 as Supplementary Fig. 33 and added comments in Page 16.\nFig. R7\n12) The authors mention using a lift-off process for patterning the IZO film. Generally, the conventional\nlift-off process involves patterning the photoresist, depositing the material via a solution process or\ndeposition over the resist pattern, and then dissolving the photoresist to lift off the unwanted material.\nHowever, in this study, the IZO film was dissolved using oxalic acid after photoresist patterning. Since\nthe material was selectively etched (dissolved) from the exposed areas rather than physically lifted off\nthe substrate, the terminology “lift-off process” is inappropriate for describing the IZO film patterning\nprocedure. This step should be accurately described as an etching process (specifically, wet etching).\nReply: Thank you very much for pointing out this mistake. We revised the sentences in the manuscript\nas follows: “Patterning of IZO film was achieved via a wet-etching process.”\n13) The authors used thermal evaporation to deposit the Al electrodes onto the PI (Polyimide) substrate.\nIt must be clarified whether there were any adhesion issues between the Al film and the PI substrate. If\nan additional adhesion layer was used (e.g., Chromium or Titanium), the authors should specify this.\nFurthermore, to fully validate the robust mechanical properties claimed for this ultraflexible device, the\nmanuscript requires evidence of durability, specifically through repeated cyclic bending tests of the PIT\nimager.\nReply: Thank you. We have ~10 years’ experience in dealing with Al with oxide devices. For our case,\nwe do not observe any delamination issue during test. Also, Al is very important as the work function\nmatches well with oxide used. If using interfacial layer e.g. chromium or titanium, it could help adhesion,\nbut the devices may not work well.\nRegarding the mechanical durability of PIT imager, we have demonstrated them in in Fig. 2g-i. The\ndevice exhibits minimal performance variation after release and under extreme bending, with voltage\nfluctuations remaining within 5%. Even when mounted on a pen, the PIT imager maintains accurate\nmapping, showing only slight deviations relative to measurements on a flat surface."}, {"round": 2, "reviewer_comments": {"reviewer_1": "Although the authors have added several experiments to verify the ultimate strength, fatigue resistance, and environmental adaptability of the flexible three-dimensional tactile sensor, the fundamental issues of the work remain unresolved:\n1、From Table 5, no clear performance advantage of this work over existing state-of-the-art studies can be identified, and the authors do not provide a convincing explanation or discussion in this regard.\n2、Although the PIT imager is claimed to be ultraflexible, the overall flexibility of the device is in practice constrained by the Ecoflex layer and the supporting substrate. Based on the physical appearance shown in Supplementary Fig. 26, the achieved flexibility does not meet the expected level.\n3、The reported 96.5% represents only a classification accuracy. The accuracy of the sensor's three-axis force measurements relative to standard reference sensors is not explained. In addition, although the authors mention potential strategies to mitigate crosstalk in the discussion section, the magnitude of the crosstalk itself is not quantitatively evaluated. I therefore question whether the device can deliver practical and reliable performance in real applications.\n4、The overall presentation of the work appears overly idealized, and the application demonstrations are relatively limited.\nAt present, numerous single-point and multi-point flexible 3-axial force sensors have been reported, many of which, to my\nknowledge, demonstrate superior performance and more convincing supporting videos (e.g., DOI: 10.1126/sciadv.adv212).\n5、Other issues: The image quality in the main text is insufficient. Moreover, the responses details should be as detailed as possible, and the results directly related to each comment should be clearly presented immediately below the corresponding\nresponse.", "reviewer_2": "The authors have conducted a thorough and well-balanced review of the relevant literature and have clearly positioned their work within the current state of the field. The motivation, technical approach, and claimed contributions are presented in a coherent and convincing manner.\nIn particular, the manuscript demonstrates a solid understanding of prior optical tactile sensing technologies and adequately addresses existing limitations through the proposed approach.\nOverall, the manuscript is well prepared, technically sound, and clearly written. I do not have major concerns regarding the content or presentation. Therefore, I consider this manuscript acceptable in its current form.", "reviewer_3": "The authors have addressed several of my previous concerns; however, some critical technical details regarding the device architecture and the specifications of the core components remain insufficiently explained. To ensure the reproducibility of the study and the accuracy of the physical model, the following points must be clarified and addressed in the manuscript.\n1. While the authors provided an explanation of the device structure, there is a persistent ambiguity regarding the actual experimental setup. According to Figure 1e, the layer sequence is illustrated as: PIT imager / Porous rubber / Flexible LEDs. However, the authors’ previous response implies that the experimental verification involved two separate units, suggesting that the PIT sensor was flipped (inverted) and then the flexible LED layer was placed on top. The authors must explicitly clarify whether the \"PIT-based Imager\" is a single integrated system as depicted in Fig. 1e, or a stacked configuration of two independent devices used for testing.\n2. The UV LED is the key component of the Photoelectrical Impedance Tomography mechanism. While the authors mentioned the flexible circuit manufacturer (JLC), they have yet to provide specific details regarding the UV LEDs themselves. The authors must specify whether the UV LEDs were fabricated in-lab or purchased as commercial ready-made products. A comprehensive description of the fabrication process or manufacturer’s name and specific product/model number must be added to the Methods section."}, "author_response": "Reviewer 1\n1. Although the authors have added several experiments to verify the ultimate strength,\nfatigue resistance, and environmental adaptability of the flexible three-dimensional\ntactile sensor, the fundamental issues of the work remain unresolved:\nReply: We sincerely appreciate your acknowledgement on the verification of\nmechanical and stability of tactile sensor. Regarding your other concerns, we provide\nour responses below one by one.\n2. From Table 5, no clear performance advantage of this work over existing state-of-\nthe-art studies can be identified, and the authors do not provide a convincing\nexplanation or discussion in this regard.\nReply: Thank you for this insightful comment. Supplementary Table 5 summarizes the\nperformance metrics of state-of-the-art both 2D and 3D tactile sensors, positioning of\nour PIT-based 3-axis tactile sensor within the landscape of existing technologies.\nThe Table includes both 2D and 3D sensing technologies, which serve\nfundamentally different capabilities. In this context, the primary advantage of our\nsensor is its inherent ability to resolve 3D force vectors, a functionality that most 2D\nsensors (including many piezoresistive, capacitive, and TFT-driven devices) cannot\nprovide.\nRegarding a comparison with other 3D tactile sensing methodologies, as shown in\nTable R1, the field is broadly dominated by three approaches: (i) vision-based sensors\n(e.g., GelSight®), (ii) Hall/magnetic-effect-based sensors (e.g., PaXini®), and (iii)\npiezoresistive array-based sensors (e.g. Nature Electronics 2025; doi:10.1038/s41928-\n025-01515-x). As noted in the main text, each of these established approaches presents\ncertain trade-offs. Vision-based systems often require complex optical components,\nresulting in larger form factors. Hall-effect-based designs can be susceptible to\nelectromagnetic interference. Piezoresistive arrays for 3D force decoupling typically\ninvolve intricate structures and can face resolution limitations.\nWe acknowledge that the absolute performance parameters of our current PIT-\nbased prototype may not surpass those of some mature, commercialized 3D sensors like\nGelSight or PaXini. However, its performance remains suitable for common\napplications. More importantly, our technology introduces a distinct set of advantages\ncentered on simplicity and scalability: it features significantly simplified\ninterconnections, a thin-film topology, and a low projected cost for scaling production.\nThese characteristics represent a promising alternative pathway for 3D tactile sensing,\nparticularly for applications where form factor, system complexity, and cost are critical\nconstraints.\nIn direct response to your comment, we have revised Supplementary Table 5 to\nexplicitly include columns for “size” and “cost of scaling”. We also expanded the\ndiscussion in the manuscript for interpreting the Supplementary Table 5 (Page 13):\n“Furthermore, as summarized in Supplementary Table 5, most conventional tactile\nsensors—including those based on piezoresistive, capacitive, or thin-film transistor\n(TFT) architectures—are limited to measuring forces in two dimensions. Among\nexisting 3-axis force sensing technologies, several inherent trade-offs exist. Vision-\nbased approaches, often require relatively complex optical components, resulting\nin bulky system configurations. Hall-effect or magnetically based sensors are\nintrinsically susceptible to external electromagnetic interference. Meanwhile,\nachieving 3-axis force decoupling with piezoresistive arrays generally necessitates\nintricate layouts, which can compromise spatial resolution and increase structural\ncomplexity. While the current performance metrics of our PIT-based three-axis\ntactile sensor may not exceed those dedicated 3-axis sensors, its performance\nremains adequate for many common applications, similar to commercially\navailable ones. Importantly, our PIT-based sensor offers several distinct advantages,\nnotably simplified interconnection requirements, a thin-film topology, and\nfavorable scalability with low associated costs—promising features for this\nemerging class of 3-axis tactile sensing devices.”\nTable R1. Performance metric of existing 3-axis tactile imaging.\nSensitivity (S) or\nresolution (R) or Resolution or\nDetection Dynamic Cost of\nMechanism Root mean Sensing area Year\nlimit range scaling\nsquare error (Sizes)\n(RMSE)\nPiezoresistive 0.7 mm\n0.008 N-1 0.1 N 0-0.6 N Low 2024\narray (10×10× ~1 mm3)\nPiezoresistive 20 mm\n0.0625 kΩ N-1 0.05 N 0-8 N Low 2025\narray (20×20× ~0.5 mm3)\n3 mm\nHall sensor 0.01-0.27 kPa−1 0.5 N 0-8 N Low 2021\n(8×8×5.6 mm3)\n0.01 kPa−1 6 mm\nHall sensor n/a 0-28 N Low 2024\nRMSE = 0.04 N (21×21× ~4 mm3)\n0-516 1 cm×1 cm2\nMagnetoelastic 0.02 kPa-1 n/a Medium 2024\nkPa (n/a)\nOptical, CMOS RMSE ~ 0.03 2.54 mm\n0.5 N 0-5 N High 2022\n(Tac3D) mm (40×22×24 mm3)\nOptical, CMOS 0.4 mm\nR = 0.03 N 0.03 N 0.03-2 N High 2022\n(Insight) (π×400×70 mm3)\nOptical, CMOS 1.25 mm\n0.07 N-1 > 1 N 0-25.2 N High 2022\n(Lightguide) (110×75× ~17 mm3)\nOptical, TFT- RMSE = 6.7 kPa 0-360 5 mm\n1 kPa High 2023\nOPD array (or 0.67 N) kPa (30×40× ~1.5 mm3)\n0.5 mm\nOptical, CMOS RMSE = 0.18 N 0.5 N 0-2.5 N High 2024\n(π×400×45 mm3)\n0.04 kPa-1 1.5 mm\nOptical, PIT <1 N 0-8 N Low This work\n(or 0.4 N-1) (20×20× ~2 mm3)\n3. Although the PIT imager is claimed to be ultraflexible, the overall flexibility of the\ndevice is in practice constrained by the Ecoflex layer and the supporting substrate.\nBased on the physical appearance shown in Supplementary Fig. 26, the achieved\nflexibility does not meet the expected level.\nReply: Thanks for your comments. Unlike 2D sensors, 3D tactile sensors inherently\ninvolve greater structural complexity, which makes it challenging to achieve the same\nlevel of mechanical flexibility. We provide photos of three types of 3D tactile sensors\nand compare their sizes. As we can see, the 3D tactile sensors (commercially Gelsight\nmini and Paxini MC-M2020-Elite) generally possess more complex structure than\nconventional 2D counterparts due to the requirement for decoupling of 3D forces. By\ncontrast, our PIT-based sensor offers much simple and flexible structure with minimal\nvolume, as well as easy scaling production.\nFig. R1. Images of three different types of 3D tactile sensors.\n4. The reported 96.5% represents only a classification accuracy. The accuracy of the\nsensor’s three-axis force measurements relative to standard reference sensors is not\nexplained. In addition, although the authors mention potential strategies to mitigate\ncrosstalk in the discussion section, the magnitude of the crosstalk itself is not\nquantitatively evaluated. I therefore question whether the device can deliver practical\nand reliable performance in real applications.\nReply: Good points! We evaluated two types of commercial sensors for comparison. As\nshown in Fig. R2, 3, a 2D piezoresistive sensor (Model: M1616, Luoyang Guantuo\nElectronic Technology Co., Ltd) was tested. It can detect normal forces at multiple\nspatial locations (Fig. R2), but it failed to sense shear forces as it was designed as a 2D\nsensor (Fig. R3).\nIn addition, Fig. R4 shows a commercial Hall-effect-based 3D sensor (Model:\nMC-M2020-Elite, Paxini Perception Technology (Shenzhen) Co., Ltd). The sensor\ncould decouple 3-axis forces easily. However, it cannot detect multiple points 3-axis\nforce. In contrast, it integrates two force signals into one total force (Fig. R4). Moreover,\nas this sensor reply on array structure, it can only distinguish the force information of\namplitude and direction, it fails to classify different tactile scenarios (Fig. R5). By\ncontrast, our PIT-based 3-axis tactile sensors rely on imaging data, providing the\ncapability in classifying contacts with different tactile modalities.\nRegarding the crosstalk, it is important to frame this issue within the current state\nof the art. In practical robotic manipulation—a key target application—the exact\ndecoupling of every infinitesimal contact point is not always the primary control\nobjective. An advanced 3D tactile sensor (2 cm × 2 cm in size) described in recent\nliterature (Nature Electronics 2025; doi:10.1038/s41928-025-01515-x), which\nintegrates an origami-inspired single-degree-of-freedom transmission mechanism, a\nsoft pneumatic actuator, and an embedded 3-axis force-sensing skin, remains essentially\na single-point 3D force sensor. In terms of the Hall-based 3D sensor (Paxini), as our\ntests show, it cannot detect multiple points 3-axis force (Fig. R4). Even for Gelsight,\nthe entire contact area is often treated as a single “contact point”. By integrating or\ntaking a weighted average, the total force and torque acting on this contact point are\ncalculated for stable grip and manipulation. Regarding our PIT-based tactile sensor, as\nshown in Fig. R5, the mutual interference between two forces on deformation of PIT\nimager is obvious, especially with larger force and closer distance.\nFig. R2. A commercial piezoresistive sensor.\nFig. R3. Detection under different surfaces and force state.\nFig. R4. A commercial Paxini sensor with 3-axis force detection capacity.\nFig. R5. Detection under different surfaces and force state by Paxini sensor.\nFig. R6. Mutual interference between two forces under different distances.\nWe supplemented Fig. R2-6 as Supplementary Fig. 33-37 with above discussion\n(Page 40-44).\n“Supplementary Fig. 33. (a) A photo of a commercial piezoresistive sensor (Model:\nM1616, Luoyang Guantuo Electronic Technology Co., Ltd.). (b-d) The sensor can\ndetect normal forces at multiple spatial locations.”\n“Supplementary Fig. 34. Detection under different surfaces and force states. It is\nincapable of sensing shear forces and therefore does not possess 3-axis tactile\nsensing capability”\n“Supplementary Fig. 35. A commercial Hall-effect-based 3D tactile sensor\n(Paxini®, Model: MC-M2020-Elite, Shenzhen Paxini Perception Technology Co.,\nLtd). The sensor could decouple 3-axis forces easily. However, it cannot detect\nmultiple points 3-axis force. In contrast, it integrates two force signals into one\ntotal force.”\n“Supplementary Fig. 36. Detection under different surfaces and force states by\nPaxini® 3D tactile sensor. It can only distinguish the force information of amplitude\nand direction, but it fails to classify different tactile scenarios.”\n“Supplementary Fig. 37. Mutual interference between two forces under different\ndistances. The mutual interference between two forces on deformation of PIT\nimager is obvious, especially with larger force and closer distance. In practical\nrobotic manipulation—a key target application—the exact decoupling of every\ninfinitesimal contact point is not always the primary control objective. An\nadvanced 3D tactile sensor, which integrates an origami-inspired single-degree-of-\nfreedom transmission mechanism, a soft pneumatic actuator, and an embedded 3-\naxis force-sensing skin, remains essentially a single-point 3D force sensor.50 In\nterms of the Hall-based 3D sensor (Paxini®), it cannot detect multiple points 3-\naxis strong forces (Supplementary Fig. 35). Even for Gelsight® derived Tac3D®,\nthe entire contact area is often treated as a single contact point. By integrating or\ntaking a weighted average, the total force and torque acting on this contact point\nare calculated for stable grip and manipulation.”\n5. The overall presentation of the work appears overly idealized, and the application\ndemonstrations are relatively limited. At present, numerous single-point and multi-\npoint flexible 3-axial force sensors have been reported, many of which, to my\nknowledge, demonstrate superior performance and more convincing supporting videos\n(e.g., DOI: 10.1126/sciadv.adv2124).\nReply: Thank you for raising this important point. As mentioned above, most reported\ntactile sensors are actually 2D force sensors. Only vision-based (e.g., GelSight®),\nHall/magnetic-based (e.g., PaXini®), and some of piezoresistive array-based sensors\nare classified to be 3D sensors.\nWe have carefully examined the work (DOI: 10.1126/sciadv.adv2124) highlighted\nby the reviewer. This work indeed represents a significant achievement in achieving\nspatial super-resolution for pressure mapping. However, as the authors state, it is\nactually a 2D tactile sensor, is distinct from the 3D force vector field. The comparison,\ntherefore, underscores a difference in sensing paradigm (2D vs. 3D) rather than a direct\nperformance shortcoming within the same category.\nBesides, the central innovation of our work is the ultraflexbile large-area imagers\nbased on a novel PIT mechanism and our primary goal was to establish and validate the\nfundamental working principle and basic capability of the PIT mechanism for 3D tactile\nimaging. Consequently, the entire supporting infrastructure—from data readout circuits\nand PIT image reconstruction algorithms to the software pipeline that transforms\nreconstructed conductivity distributions into calibrated 3D force vectors—had to be\ndeveloped from the ground up. Achieving real-time, closed-loop robotic manipulation\nvideos requires substantial further engineering optimization of this entire pipeline,\nwhich was beyond the scope of this initial proof-of-concept manuscript.\nWe acknowledge the current limitation in real-time application videos. However,\nthe feasibility of real-time data acquisition and processing for similar electrical\ntomography techniques is well-established. Recent work on EIT-based tactile sensors\n(e.g., Sci. Adv. 10, eadr1099 (2024); Sci. Adv. 11, eadv3359 (2025)) has demonstrated\nreal-time human-machine interaction, confirming that the path toward real-time\noperation for our PIT-based system is viable.\nTherefore, we further modified the Discussion Section as follows (Page 15):\n“…While this study establishes the foundational principle and characterization of\nthe PIT-based 3-axis tactile imager, real-time, closed-loop robotic manipulation\nrepresents a critical next step and the proven real-time capabilities of analogous\nelectrical impedance tomography systems provide a clear engineering pathway…”\n6. Other issues: The image quality in the main text is insufficient. Moreover, the\nresponses details should be as detailed as possible, and the results directly related to\neach comment should be clearly presented immediately below the corresponding\nresponse.\nReply: We really appreciate your suggestions. The high-resolution images have been\nuploaded separately. As requested, we have added detailed explanations below in the\ncorresponding response.\nReviewer 2\nThe authors have conducted a thorough and well-balanced review of the relevant\nliterature and have clearly positioned their work within the current state of the field.\nThe motivation, technical approach, and claimed contributions are presented in a\ncoherent and convincing manner.\nIn particular, the manuscript demonstrates a solid understanding of prior optical tactile\nsensing technologies and adequately addresses existing limitations through the\nproposed approach.\nOverall, the manuscript is well prepared, technically sound, and clearly written. I do\nnot have major concerns regarding the content or presentation. Therefore, I consider\nthis manuscript acceptable in its current form.\nReply: We really appreciate your recognition of our revised manuscript.\nReviewer 3\n1. The authors have addressed several of my previous concerns; however, some critical\ntechnical details regarding the device architecture and the specifications of the core\ncomponents remain insufficiently explained. To ensure the reproducibility of the study\nand the accuracy of the physical model, the following points must be clarified and\naddressed in the manuscript.\nReply: We sincerely appreciate your positive assessment and recognition of our revised\nmanuscript. We further revised the manuscript according to your suggestions.\n2. While the authors provided an explanation of the device structure, there is a persistent\nambiguity regarding the actual experimental setup. According to Figure 1e, the layer\nsequence is illustrated as: PIT imager/porous rubber/flexible LEDs. However, the\nauthors’ previous response implies that the experimental verification involved two\nseparate units, suggesting that the PIT sensor was flipped (inverted) and then the\nflexible LED layer was placed on top. The authors must explicitly clarify whether the\n\"PIT-based Imager\" is a single integrated system as depicted in Fig. 1e, or a stacked\nconfiguration of two independent devices used for testing.\nReply: Thank you for the comments. We are sorry for the previous misleading reply.\nFig. 1e shows the schematic of the PIT-based 3-axis tactile sensors. The 3-axis sensor\nincludes an ultraflexible PIT imager, a porous rubber layer and flexible LED layer. After\nassembling the three layers together, they need to be integrated into a single system that\ncan be electrically controlled simultaneously.\nTo explicitly clarify it, we made revision in the Method part (Page 17):\n“After assembling, they need to be integrated into a single system that can be\nelectrically controlled simultaneously.”\n3. The UV LED is the key component of the Photoelectrical Impedance Tomography\nmechanism. While the authors mentioned the flexible circuit manufacturer (JLC), they\nhave yet to provide specific details regarding the UV LEDs themselves. The authors\nmust specify whether the UV LEDs were fabricated in-lab or purchased as commercial\nready-made products. A comprehensive description of the fabrication process or\nmanufacturer’s name and specific product/model number must be added to the Methods\nsection.\nReply: We appreciate your rigorous comments. The UV LEDs are commercially\navailable, purchasing from Shenzhen YCG Electronic Technology Co., Ltd (Model:\n0603, 365-370 nm, Size: 0.8 × 1.6 mm). Then, the UV LEDs are soldered into flexible\ncircuits (Shenzhen JLC Technology Group Co., Ltd) by us.\nWe added the comments in the Methods Section as follows (Page 17):\n“The UV LEDs were purchased from Shenzhen YCG Electronic Technology Co.,\nLtd (Model: 0603, 365-370 nm, Size: 0.8 × 1.6 mm). Then, the UV LEDs were\nsoldered into flexible circuits by us.”"}, {"round": 3, "reviewer_comments": {"reviewer_1": "The authors have added comparisons with commercial sensors, crosstalk experiments, and extended discussions, which help better position the strengths of the work. However, the following issues, if addressed, would better meet the standards of the NC journal:\n1.Quantitative reconstruction errors for Fx, Fy, and Fz are still missing. Classification accuracy (96.5%) alone does not seem sufficient to demonstrate reliable three-axis force decoupling. Although Gaussian fitting equations are proposed for force decoding, how do the decoded forces quantitatively compare with ground-truth measurements?\n2.The supplementary experiments show strong interference when two contact points are close, but quantitative metrics are lacking, such as systematic error as a function of inter-contact distance. In addition, the porous layer architecture appears to suffer from channel interference under simultaneous loading, which may contradict the claim of “high-resolution three-axis imaging.” The author should revise some overly assertive statements in the article.\n3.A spatial resolution of 1.5 mm and a dynamic range of 0–8 N are not particularly competitive relative to the 2024–2025 state of the art, and scalability does not seem to be unique to the PIT framework.\n4.Improve the quality of the group images, especially Fig. 1(sequential placement), Fig. 2k(Add explanatory labels),Fig.4g, Fig.5(Layout is unclear from top to bottom)."}, "author_response": "Reviewer 1\n1. The authors have added comparisons with commercial sensors, crosstalk experiments, and\nextended discussions, which help better position the strengths of the work. However, the\nfollowing issues, if addressed, would better meet the standards of the NC journal.\nReply: We sincerely appreciate your positive feedback on our revised manuscript and\nacknowledgement on the suitability for NC journal. We have further refined the manuscript in\naccordance with your suggestions.\n2. Quantitative reconstruction errors for Fx, Fy, and Fz are still missing. Classification accuracy\n(96.5%) alone does not seem sufficient to demonstrate reliable three-axis force decoupling.\nAlthough Gaussian fitting equations are proposed for force decoding, how do the decoded\nforces quantitatively compare with ground-truth measurements?\nReply: We appreciate your insightful question. We agree that 96.5% classification accuracy by\nimages is different from actual force decoupling. Thus, we conducted the Gaussian fitting for\nX, Y, and Z directions, which were calibrated with ground-truth forces (Figure 4j and 4k), The\nMethods part provided the details, which we simplified the measurements by controlling the\nFy as 0.\n3. The supplementary experiments show strong interference when two contact points are close,\nbut quantitative metrics are lacking, such as systematic error as a function of inter-contact\ndistance. In addition, the porous layer architecture appears to suffer from channel interference\nunder simultaneous loading, which may contradict the claim of “high-resolution three-axis\nimaging.” The author should revise some overly assertive statements in the article.\nReply: Thank you for carefully checking. As we mentioned in our previous response, similar to\nother reported/commercially available 3D tactile sensors, our PIT-based tactile sensor was also\ndesigned for single-point 3D force detection, thus the inter-contact force/distance was\nrecognized as combined single-point force.\nRegarding the term “high-resolution three-axis imaging”, we searched out the manuscript, but\nwe cannot find such claim. In fact, we just claimed high-resolution imaging for our newly\ndeveloped PIT imager, not the tactile sensor.\n4. A spatial resolution of 1.5 mm and a dynamic range of 0–8 N are not particularly competitive\nrelative to the 2024–2025 state of the art, and scalability does not seem to be unique to the PIT\nframework.\nReply: Thank you for your comments. Compared to two recent 3D tactile sensor works on\npiezoresistive array, our PIT-based tactile sensor is either exhibit high dynamic range (8 N VS\n0.6 N) or high resolution (1.5 mm VS 20 mm). Compared to one recent work based on hall\nsensor, our PIT-based tactile sensor better resolution (1.5 mm VS 6 mm) and no electromagnetic\ninterference like hall-effect ones. In short, the design for high dynamic range and high\nresolution adopts different strategies, it is unrealistic for all performance metrics of PIT imager-\nbased to 3D tactile sensor lead simultaneously.\n5. Improve the quality of the group images, especially Fig. 1 (sequential placement), Fig. 2k\n(Add explanatory labels), Fig. 4g, Fig. 5 (Layout is unclear from top to bottom).\nReply: We sincerely appreciate your valuable suggestions. As advised, we refined Fig. 2k, Fig.\n4g, and Fig. 5 in the revised manuscript. As the Fig. 1c is large, after careful consideration, we\nhave chosen to retain the sequential placement of Fig. 1."}]

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