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Na et al. Soft Sci. 2026, 6, 25 Page 9 of 22

PTE effect
The PTE effect converts absorbed light into a temperature gradient, generating a Seebeck voltage and
thermally driven photocurrent for zero-bias operation with bandgap-independent spectral response .
[49]
Although the thermally driven response is slower than photovoltaic detectors, such intrinsic temporal delays
are beneficial for emulating biological synaptic plasticity, particularly gradual potentiation and relaxation.
Recently, this concept has been extended from conventional electron-driven Seebeck systems to
ion-conductive materials, utilizing heterostructures engineered with distinct photothermal properties and
Seebeck coefficients [Figure 4Ai] [43-45] . Unlike Schottky or heterojunction systems that rely primarily on
electronic carrier trapping, light-induced thermal gradients drive asymmetric ion migration, establishing
directional ionic transport that modulates photoconductance without external electrical bias [Figure 4Aii].
Following illumination, slow thermal relaxation, progressive ion redistribution, and defect-assisted
thermoelectric modulation induce persistent conductance changes analogous to long-term synaptic behavior
[Figure 4Aiii]. The zero-power optoelectronic synapses based on PTE effect using ionogel heterostructure
offer inherent mechanical softness, flexibility, and stretchability. Thus, the PTE effect-based zero-power
optoelectronic synapses exhibit stable synaptic functionality under bending, stretching, and repeated
mechanical deformation, highlighting their robustness for soft and body-interfaced applications compared to
Schottky junction- and heterojunction-based synapses. The soft PTE-effect-based zero-power optoelectronic
synapses are particularly promising candidates for wearable neuromorphic platforms.

Qian et al. reported an artificial neuromorphic vision skin based on a silver nanoparticle-doped ionogel
heterostructure exhibiting zero-power operation, self-healing, mechanical flexibility, and photonic synaptic
processing [Figure 4B] . Localized surface plasmon resonance in the embedded nanoparticles converted
[43]
light into heat, establishing a temperature gradient that drove directional ion migration through the PTE
mechanism. The ion flux produced photosynaptic behaviors such as EPSC, PPF, and gradually accumulated
responses under repeated illumination, while reversible hydrogen bonds within the polymer network
provided mechanical resilience [Figure 4C]. When assembled into a 5 × 5 photosynaptic array, the ionogel
skin sensed, memorized, and reconstructed light patterns on both flat and curved surfaces, functioning as a
conformal neuromorphic imager [Figure 4D].

Luo et al. realized a bionic hemispherical retinomorphic eye, in which an elastomeric retina composed of an
ionogel heterojunction pillar array acted as photosynaptic photoreceptors [Figure 4E] . Driven by the PTE
[44]
effect, doped and undoped ionogel regions generated temperature differences under broadband illumination
(365 to 970 nm), inducing asymmetric Li /bis(trifluoromethanesulfonyl)imide anion (TFSI ) migration and a
+
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built-in potential that produced neuroelectric signals with pronounced synaptic plasticity without external
electrical bias [Figure 4F and G]. The soft ionogel retina conformally laminated onto arbitrary curved
surfaces and was further assembled into a 5 × 5 transplantable photosynaptic patch to restore photocurrent
in defective regions, mimicking retinal repair [Figure 4H and I]. The integrated system exhibited
neuromorphic image learning, forgetting, and real-time motion tracking using spatiotemporal voltage
mapping [Figure 4J and K].

CHALLENGES
Despite the rapid progress in zero-power optoelectronic synapses, several scientific and engineering
challenges remain before these technologies can evolve into practical wearable neuromorphic systems. These
limitations stem not only from the intrinsic constraints of materials and device architectures but also from
the requirements unique to zero-power, body-interfaced systems, such as operation under low-intensity
illumination, mechanical deformation, and spatiotemporally varying environments. The realization of
energy-autonomous, mechanically compliant, and long-term stable systems requires advances across
multiple dimensions, ranging from light-matter interactions and synaptic plasticity to structural design and