# Bio-based silver conductive ink for flexible printed electronics, and in-mold electronics

Tony Gerges, Thomas Guérin, Vincent Semet, Philippe Lombard, Guy Masse, Jean-Yves Charmeau, Bruno Allard, Michel Cabrera

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Tony Gerges, Thomas Guérin, Vincent Semet, Philippe Lombard, Guy Masse, et al. Bio-based silver conductive ink for flexible printed electronics, and in-mold electronics. Flexible and Printed Electronics, 2025, 10 (4), pp.045012. 10.1088/2058-8585/ae2139. hal-05385650

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# Bio-based silver conductive ink for flexible printed electronics, and in-mold electronics

Tony Gerges¹,*, Thomas Guérin¹,³, Vincent Semet¹, Philippe Lombard¹, Guy Masse³, Jean-Yves Charmeau², Bruno Allard¹ ID and Michel Cabrera¹

¹ INSA Lyon, Université Claude Bernard Lyon 1, Ecole Centrale de Lyon, CNRS, Ampère, UMR 5005, 69621 Villeurbanne, France

² INSA Lyon, IMP, UMR 5223, 01100 Bellignat, France

³ Encres Dubuit, 1 Rue Isaac Newton, Mitry-Mory 77290, France

* Author to whom any correspondence should be addressed.

E-mail: tony.gerges@insa-lyon.fr

**Keywords:** flexible printed electronics, in-mold electronics, silver ink, screen printing, poly(lactic acid), Cyrene

Supplementary material for this article is available online

## Abstract

The development of environmentally friendly electronic circuits offers a sustainable alternative to conventional printed circuit board (PCB) manufacturing, which relies on chemically intensive and polluting processes. Screen printing of conductive inks is an effective and widely used method for fabricating flexible printed electronics (FPEs), and particularly well-suited for in-mold electronics (IMEs) applications. Screen printing of electronic circuits can be carried out on nearly all types of substrates, including recyclable and biodegradable polymers, thereby reducing the environmental impact. This paper presents the formulation of a silver conductive ink with stretchable properties, using poly(lactic acid) as the binder and Cyrene as the solvent, which are both biobased and biodegradable. Inks were formulated using silver flakes of different sizes (2.2 µm–5.6 µm) at a fixed content of 60%. Rheological behavior of the inks was measured to determine their printability, and resolution of printed patterns was tested using a scanning electron microscope. Thermal curing and photonic curing were used on the inks to study their electrical characteristics as functions of curing temperature and conditions. Among the latter tested formulations, the ink with a mixture of fine and large particles featured the best performance with an electrical resistivity of 34.77 ± 3.29 µΩ·cm after heat treatment at 120 °C and 9.39 ± 0.17 µΩ·cm after photonic curing. This ink was also highly stretchable, with bending cycle tests showing an increase of electrical resistance only 8% after 1000 cycles, and the electrical continuity still intact even after 5000 cycles. Thermoforming tests confirmed electrical continuity at strains exceeding 30%. Demonstrators in FPEs and IME highlight the potential of these inks. This work shows that the developed ink exhibits electrical and stretchability properties equivalent to or even better than those of inks from previous research or commercial products, while reducing the environmental impact and enhancing user safety.

## 1. Introduction

As technological development accelerates, minimizing the environmental impact of electronic device manufacturing has become a key priority. This concern is particularly relevant because the production of printed circuit boards (PCB) involves several polluting steps, like chemical etching, and recycling PCBs is especially challenging [1]. As possible alternatives,

additive manufacturing techniques for electronic circuits, such as plastronics, are emerging [2, 3].

Plastronics involves technologies that allow the prospect of integrating electronic circuits onto polymer supports: films in the case of flexible printed electronics (FPEs) [4] and in-mold electronics (IMEs), or three-dimensional (3D) substrates in the case of 3D Plastronics, by selective deposition of conductive networks, either metallic [2] or ink-based [5].
This reduces the number of manufacturing steps, thus should lower the environmental impact.

In FPE, the electronic circuit is typically prin-
ted on a flexible film substrate, whereas IME aims to
develop 3D electronic circuits. In both processes, con-
ductive traces are created by ink deposition prior to
device attachment, and in the IME process, this is fol-
lowed by thermoforming into the final 3D shape. The
developed 3D thermoformed electronic device can be
overmolded to create an IME system, where electronic
circuits and components are embedded directly into
molded polymer parts. This approach reduces man-
ufacturing and assembly time, enabling the produc-
tion of lightweight, durable, and ergonomic devices
for consumer, automotive, and medical applications
[6, 7].

Various printing techniques have been explored for the fabrication of electronics on polymer substrates, including laser-induced forward transfer [8], meniscus-guided printing [9], cold-spray printing [10], aerosol jet printing [11], inkjet printing [12], pad printing [13], and screen printing [5]. Among these techniques, screen printing offers a simple, cost-effective approach with very high printing speed, and excellent suitability for IME applications, enabling conductive patterns to be printed on a wide range of substrate surfaces. It can provide an accurate deposition of conductive inks and can allow selective printing of different kinds of inks, including decorative and dielectric inks. Screen printing is also well-suited for large-scale production, making it an attractive option to include electronics in a wide range of applications from consumer electronics, wearable technology and within the automotive industry.

To ensure optimal circuit functionality, screen
printing inks used in FPE and IME must offer
both mechanical adaptability, such as stretchability
or thermoformability, and high electrical conductiv-
ity. Achieving this balance minimizes resistive losses
thus excessive heat dissipation, thereby improving the
overall performance and reliability of the final device.

Conductive inks used in screen printing typ-
ically contain four components. Conductive fillers
are responsible for providing electrical conductiv-
ity. Polymer binders both contain the conductive
particles in a unified form and also ensure adhesion
to the substrate along with flexibility, durability, and
resistance to environmental exposure. Solvents lower
the viscosity of the inks to allow appropriate flow
through the screen, and evaporates after the print-
ing process to allow the formation of inks patterns.
Additive compounds may be included to adapt cer-
tain properties such as drying time, stability, or print-
ability. The proportion of each component depends
upon the target application and substrate.

When inks are loaded with metallic particles, they
exhibit significantly higher electrical conductivity

compared to carbon based, polymer-based or ionic
conductive inks [14]. Silver is particularly favored for
applications requiring high electrical conductivity.
The market is largely dominated by silver microflake-
based inks [15], which offer several advantages,
including lower cost than nanoparticles, mainly due
to their ease of processing, and higher electrical
conductivity compared to inks containing spherical
particles [16]. The typical flake size ranges from 1 to
20 µm.

To achieve optimal electrical performance, con-
ductive inks require thermal curing to evaporate the
solvent and fully develop their conductive proper-
ties. Curing is often carried out at high temperat-
ures in a convection oven, typically between 100 °C
and 200 °C depending on the thermal stability of
the substrate, but can also be performed using IR or
UV treatment. Alternatively, photonic curing enables
extremely rapid heating to high temperatures, poten-
tially reaching several hundred degrees Celsius within
very short time intervals [17]. This approach enables
rapid curing of the inks and enhances their electrical
performance, while avoiding damage to the polymer
substrate due to the localized and very short heating
pulses. Recent research has highlighted the benefits of
photonic curing in the processing of both high and
low temperature substrates intended for IME, even
with silver microflake-based ink [18].

In principle, conductive screen printing inks tra-
ditionally rely on petrochemical-based binders such
as polyester [6], polyurethanes, polyvinyl butyral,
or phenoxy resins to provide mechanical stabil-
ity and adhesion properties [14]. The latter bind-
ers are dissolved with solvents such as toluene,
xylene, acetone, tetrahydrofuran (THF), N-methyl
pyrrolidone (NMP), N,N-dimethylformamide
(DMF) or cyclohexanone [19]. While effective, these
solvents are toxic and often damaging to health and
the environment, necessitating a transition to greener
and safer alternatives under pressure of emerging
legislation and lessen impact on the environment.

Several studies have explored the development
of conductive screen printing inks using bio-based
matrices, employing binders such as poly(ethylene)
oxide [20], cellulose [21–23], cellulose acetate [24],
ethyl cellulose [25], polycaprolactone [26] or chitosan
[27].

Among environmentally friendly binders used
in ink formulation, poly(lactic acid) (PLA) offers
several advantages, particularly for sustainable and
eco-conscious applications. Derived from renew-
able resources such as corn starch or sugarcane,
PLA is biobased, biocompatible and biodegradable,
providing a reduced environmental footprint com-
pared to conventional petrochemical-derived bind-
ers. Furthermore, its degradation products, such as
lactic acid, are relatively non-toxic, further enhancing
its appeal as a green material. The latter characteris-
ics position PLA as a promising candidate for bio-
source ink formulations, especially in applications
where environmental sustainability is a priority. At
the end of its life cycle, PLA can be degraded or
recycled in several ways, such as industrial compost-
ing, thermophilic anaerobic digestion, or chemical
processes like hydrolysis, alcoholysis, and enzymatic
degradation. It can also be mechanically recycled by
melting and reshaping [28]. These options make PLA
an environmentally friendly material that is easy to
break down and reuse.

To date, some studies have investigated PLA as a
binder, although it is typically dissolved in hazard-
ous solvents like THF [29] or in less toxic altern-
atives such as anisole [30]. However, these PLA-
based inks exhibited moderate electrical conduct-
ivity, and are mainly suited for deposition tech-
niques like syringe needle dispensing or rod coating.
Only one study has reported the use of PLA with
environmentally friendly solvents, such as ethyl acet-
ate and benzyl alcohol [31]; however, the resulting
graphene-based ink was specifically designed for air-
brush spraying. To our knowledge, the formulation
of biobased conductive inks combining high electrical
conductivity, PLA as a binder, and compatibility with
screen printing for FPE and IME has not yet been
reported.

This study presents the formulation of a highly
conductive ink specifically tailored for screen printing
applications using silver flakes as the conductive filler
due to their excellent electrical properties, PLA as a
biobased and biodegradable binder, and dihydrolevo-
glucosenone (commonly known as Cyrene) as a green
solvent.

Cyrene is a dipolar aprotic solvent made from
renewable raw materials such as cellulose, lignin or
glycerine [32]. It features good solubility properties
for a wide variety of organic compounds, with prop-
erties similar to solvents such as NMP and DMF
widely used in inks and paints. It is considered as
a practical non-toxic solvent thanks to its interest-
ing biocompatibility, non-toxic and non-mutagenic
properties and is also biodegradable [33, 34].

The research investigates the influence of sil-
ver flake dimensions on the ink's rheological beha-
vior, surface morphology, electrical conductivity, and
mechanical performance.

Electrical resistance is evaluated as a function of
both thermal curing temperature and photonic cur-
ing. Ink flexibility is assessed through cyclic bend-
ing tests, and a FPE demonstrator was developed as
proof of concept. The ink's stretchability was also
tested after thermoforming, with conductivity mon-
itored under different elongation levels. Finally, a fully
functional 3D demonstrator is presented, highlight-
ing the ink's potential for integration into complex,
3D electronic devices.

## 2. Materials and methods

2.1. Inks formulation

PLA granulates (Renew 212, Futerro) were used as
a binder due to their bio-based and biodegradable
properties. This grade was selected due to its high
D-isomer content (12.9%) which exceeds 8% and
therefore results in a mainly amorphous polymer [35,
36]. This characteristic improves its thermoformabil-
ity and minimizes shrinkage during processing [37].
They were dried at 60 °C under vacuum for 24 h.
Cyrene (Sigma-Aldrich) was selected as the solvent
for its sustainable and bio-based formulation. Silver
flakes with particle sizes of 5.6 µm and/or 2.2 µm
(Technic Inc.) were used as conductive fillers. These
flake sizes, in the range of a few micrometers, and a
silver loading of ~60% are commonly used in IME
inks [5, 38], and were selected for their low cost and
ease of processing [15].

First, the PLA granulates were dissolved in the
solvent in a sealed container by gradually increasing
the temperature from 50 °C to 85 °C, over 4 h with
continuous stirring using magnetic agitator to ensure
complete dissolution. The resulting solution was then
degassed under vacuum to eliminate all air bubbles.

Subsequently, three inks (Ink A, Ink B, and Ink C)
were prepared by incorporating silver flakes of differ-
ent sizes into the solution. Mechanical dispersion was
performed using an overhead stirrer (VELP LH 40l).
The composition, filler content, and particle size dis-
tribution of each ink are summarized in table 1.

2.2. Screen printing and curing of the conductive
inks

The three latter inks were screen printed onto
polycarbonate (PC) film substrates (Covestro
Makrofol DE 1–4) with a thickness of 375 µm using
a semi-automatic screen printing machine (Fritsch
Semi-automatic stencil printer printALL210) and a
screen with a polyester mesh with 90 threads/cm and
wires with a diameter of 40 µm. This fine-to-medium
mesh provides a suitable balance between resolution
and ink deposition, while also ensuring good print-
ability of inks containing micron-sized particles. The
distance between the mesh and the film was set to
1.5 mm. Constant pressure and speed were applied to
the Shore 75 squeegee, which was positioned at a 45°
angle between the screen and substrate. All samples
were produced by applying a rubber squeegee twice
over the film to ensure the best quality and consist-
ency of the print.

To test the electrical and mechanical perform-
ances of the formulated inks, straight lines, each
measuring 160 mm in length and 1 mm in width (see
supplementary information SI1), were screen prin-
ted with these conductive inks onto PC films. The
films were then submitted to convective heat curing
<table><caption>**Table 1.** Composition, filler content, and particle size distribution of each ink.</caption><thead><tr><th></th><th>PLA (wt%)</th><th>Cyrene (wt%)</th><th>Silver (wt%)</th><th>Silver flakes size</th></tr></thead><tbody><tr><td>Ink A</td><td>6%</td><td>34%</td><td>60%</td><td>5.6 µm (100%)</td></tr><tr><td>Ink B</td><td>6%</td><td>34%</td><td>60%</td><td>5.6 µm (50%)/2.2 µm (50%)</td></tr><tr><td>Ink C</td><td>6%</td><td>34%</td><td>60%</td><td>2.2 µm (100%)</td></tr></tbody></table>

in a box oven (Memmert UF30) at temperatures ranging from room temperature up to 150 °C, with a treatment duration of 20 min in each temperature condition. Photonic curing was also performed using 30 kW NovaCentrix PulseForge Invent with the following parameters: frequency (10 Hz), pulse duration (1 ms) and number of pulses (40). These settings were used to process the samples after screen printing and prior to performance evaluation.

## 2.3. Placement of the components and curing of the conductive adhesive

Due to the low number of components used for the demonstrators, surface mount devices (SMD) were manually placed. The ME902 adhesive from Dupont (USA) was utilized to establish electrical conductivity between the electronic SMD components and the screen-printed substrate. To enhance the adhesion of the SMD components onto the polymer film, SMA electrolube (Farnell) was applied as a structural adhesive between the components and the film. The assembled films were then subjected to convective heat curing in a box oven (Memmert UF30) at 120 °C for 20 min to fully dry both the conductive and structural adhesives.

## 2.4. 3D shaping by thermoforming

The PC screen-printed film, incorporating conductive ink and electronic components, was mounted onto a 270 × 240 mm frame of a vacuum forming machine (1820 CR Clarke, UK). The process was carried out at 190 °C, with the film vacuum formed against an aluminum mold under a negative pressure of 1 bar to create a 3D electronic circuit. Two molds were used in this study (see supplementary information SI2). The first was a stepped conical mold with curvature radii from 5 mm to 30 mm, designed to apply different levels of strain and assess the electrical and stretchability properties of the inks under elongations up to 35%. The second mold, shaped like a small car, was used to thermoform a functional demonstrator.

## 2.5. Thermoforming simulation

At first, the strain-stress behavior of PC was measured at different temperatures (from 100 to 190 °C) and at elongation rates ranging from $10^{-4}$ to $10^{-1}$ s$^{-1}$, using a universal testing machine (Shimadzu AGSX). The results are consistent with those reported in [39]. A finite element model of the experiment was developed using ABAQUS/Explicit software, based on an Ogden

strain energy function (hyperelastic model, Ogden 6). The model incorporates the material's viscoelastic behavior, represented by a Prony series, coupled with a WLF (Williams-Landel-Ferry) equation to account for the effect of temperature near and above the glass transition. Our ABAQUS model is a fully 3D thermomechanical explicit model. It includes the temperature-dependent thermal properties of each component: thermal conductivity and specific heat for both the aluminum mold and the PC film. We also account for convection and a conductive heat transfer coefficient of 500 W m$^{-2}$ K during contact between the mold and the PC film [40, 41]. The finite penalty contact method was employed, with a temperature-dependent Coulomb friction coefficient ranging from 0.2 to 5 [40]. For the PC film, C3D8T elements were used, with an element size of approximately 100 µm, appropriate for the film thickness of 375 µm, to accurately capture temperature variations during thermoforming. The simulation consists of five steps. In the first step, the polymer sheet is heated until its temperature reaches 185 °C. Next, the mold is displaced at a speed of 0.1 m s$^{-1}$. After 0.1 s, compression is applied at the top of the polymer film, increasing linearly to a pressure of $10^{-5}$ Pa at 0.3 s. The simulation results were validated by measuring the thickness evolution of the PC film using an optical microscope (Hirox RH-2000) (see supplementary information SI3).

## 2.6. Characterization

**Viscosity:** Rheological measurements were carried out using a coaxial cylinder rheometer (Viscotester iQ Air) over a shear rate range of 0.1–1000 s$^{-1}$. Prior to testing, the inks were pre-sheared at 1 s$^{-1}$ for 30 s. All measurements were conducted at 25 °C. To assess the influence of the inks' rheological behavior, the screen printing process was simulated based on [42]. Tests were thus conducted in three steps: (1) 0.1 s$^{-1}$ for 30 s to imitate the ink state before printing; (2) 500 s$^{-1}$ for 30 s to reproduce the squeegee movement; and (3) 0.1 s$^{-1}$ for 2 min to simulate structural recovery after printing. Recovery rate is calculated by dividing the viscosity at the third step by the viscosity at the first step.

**Surface morphology:** The surface morphology and roughness of the polymer films and the screen printed circuits were examined using optical microscopy (Hirox-RH-2000) and a chromatic confocal profilometer (Nano Point Scanner, NP3). Three measurements per sample over a 5 mm distance
were performed to obtain the average roughness (Ra) value. Scanning electron microscopy (TESCAN MIRA-3) was also performed at 20 KV to characterize the surface morphology, size, and distribution of the silver flakes.

**Electrical resistivity:** The resistance R of the formulated conductive inks was measured on the screen-printed lines on the PC films at 20 °C after ink curing, using a sourcemeter (Keithley 2450). The measurements were taken on the edges of the lines, over a distance of 160 mm. Moreover, the cross-sectional area A of the conductive ink track was measured using the confocal profilometer. The resistivity ρ in μΩ cm was calculated using the Ohm's Law for Resistivity:

$$ \rho = \frac{R * A}{l} \qquad (1) $$

with R as the measured resistance, l the terminal spacing, and A the cross-sectional area of the cured silver line. The reported values represent the average value taken from 20 different measurements, performed on 20 screen-printed lines for each of the three ink samples and for each temperature condition. Data are presented as mean ± standard deviation.

**Electrical resistance:** After thermoforming, the electrical resistance along the conductive tracks was measured using a Keithley 2000 ohmmeter. One probe was connected to the end of the track, while the other was moved along its length. This movable probe was mounted on a Keyence XM-5000 3D measurement system, allowing its spatial position to be determined with a precision of less than 10 µm. This setup enabled measurement of the variation in electrical resistance along the tracks.

**Adhesion tests:** To evaluate the conductive tracks adhesion, a standard scotch-tape-test following the ISO 2409 test procedure was firstly conducted on a 20 × 30 mm screen printed surface using the formulated inks. A sharp razor blade was used to cut a grid pattern across the sample. A 2525 Scotch adhesive tape was pressed against the sample and peeled off rapidly and vertically. The percentage area of ink removed or deformed was used as an indicator of adhesion, providing the corresponding adhesion quality from 0 to 5 (supplementary information SI4).

**Bending tests:** Bending tests were performed using an automated laboratory-built compression bench operating at a speed of 30 mm s⁻¹. The bending radius of 10 mm, corresponding to the smallest achievable curvature due to the rigidity of the PC film, was measured manually at the beginning of the test and checked every 1000 cycles. The electrical resistance of the screen-printed inks, in the form of lines, was measured 5 times with a two-point probe using a Keithley 2450 SourceMeter.

## 3. Results

### 3.1. Rheology: viscosity and screen printing simulation

Rheological characterization plays a critical role in assessing the printability and stability of conductive inks, particularly in screen printing where sheath-thinning behavior and viscosity control are essential. During screen printing, the viscosity of the ink must be sufficiently fluid under the shear applied by the squeegee to transfer through the mesh and form a uniform coating on the substrate. After printing, the ink viscosity also needs to rapidly recover to maintain the fidelity of the pattern. To represent the above behavior, a screen printing simulation was conducted by varying the shear rate to mimic the printing steps. Figure 1 shows how viscosity was influenced over time, the ink's viscosity dropped significantly over time as the shear rate increased from 0.1 s⁻¹–500 s⁻¹. High shear causes a breakdown in the internal structure of the ink which results in a lower viscosity allowing the ink to flow freely through the screen printing mesh. Upon the shear rate returning to 0.1 s⁻¹, the viscosity rapidly recovered, indicating the internal structure of the inks was reformed. This allows the ink to retain its form after the printing and provides well-defined patterns. For this reason, a sufficiently viscous ink is desired to prevent any ink flow after printing. Table 2 shows the viscosity change in each of the inks over the three stages of the screen printing process. In addition, the recovery rate was calculated two minutes after stopping the shear. The results for Ink A and Ink B showed an approximate 60% recovery of their original viscosity, whereas Ink C recovers only of about 50%. The lower viscosity recovery for Ink C may result in less defined printed patterns and potential deterioration of electrical properties.

In this study, the rheological properties of the three silver-based inks were analyzed to evaluate the influence of particle size and distribution on their flow behavior. All three inks exhibited sheath-thinning, thixotropic behavior, characterized by a decrease in viscosity with increasing shear rate (table 3). The use of large silver flakes in Ink A resulted in higher viscosity values under all shear rates, due to stronger interparticle interactions and the formation of a more interconnected network within the ink matrix. Ink C, containing fine silver particles, showed the lowest viscosity, while Ink B, formulated with a blend of fine and large particles, displayed intermediate viscosity values.

### 3.2. Morphology analysis after screen printing

The quality of screen printed pattern achievable with the latter inks can significantly influence the performance of the subsequent electronic circuit, particularly regarding electrical conductivity and the resolution of the printed conductive tracks. Optical microscopy revealed that the tracks obtained using Ink A exhibit
<figure><img src="image_4.png" /><figcaption>**Figure 1.** Measured viscosity behavior during screen printing.</figcaption></figure>

<table><caption>**Table 2.** Measured viscosity (Pa·s) at shear rates corresponding to the three key steps of the screen printing process: before (0.1 s⁻¹), during (500 s⁻¹), and after the squeegee action (0.1 s⁻¹).</caption><thead><tr><th rowspan="2"></th><th colspan="3">Three steps</th><th rowspan="2">Recovery rate</th></tr><tr><th>0.1 s<sup>-1</sup></th><th>500 s<sup>-1</sup></th><th>0.1 s<sup>-1</sup></th></tr></thead><tbody><tr><td>Ink A</td><td>1600 ± 20</td><td>8.6 ± 1.4</td><td>1006 ± 15</td><td>62.8%</td></tr><tr><td>Ink B</td><td>1400 ± 13</td><td>8.1 ± 1.1</td><td>863 ± 13</td><td>61.6%</td></tr><tr><td>Ink C</td><td>960 ± 5.5</td><td>5.3 ± 1.2</td><td>489 ± 8.4</td><td>50.9%</td></tr></tbody></table>

well-defined patterns (figure 2(a)). Nevertheless, the high viscosity of Ink A leads to the retention of mesh imprint marks from the screen printing process on the surface of the printed tracks (figure 2(d)), which could potentially impact the uniformity of the conductive pathways. The arithmetic average roughness of the deposits formed by Ink A was measured at 3.5 µm ± 0.5, indicating a relatively rough texture surface.

Similarly, the tracks made using Ink B display well-defined patterns (figures 2(b)–(e)), which indicates better ink flow and leveling characteristics, consistent with lower viscosity. As a result, the average roughness was reduced to 1.94 ± 0.2 µm, indicating a smoother printing, which may improve electrical performance.

In contrast to Ink A and Ink B, the patterns printed with Ink C have a tendency to spread down the edges of the printed patterns, creating less well-defined features (figures 2(c)–(f)), likely due to lower viscosity. However, owing to its smaller particle size, Ink C achieves a reduced average roughness of 1.56 ± 0.15 µm, which is lower than that observed for the other two inks.

Additionally, preliminary adhesion tests for Ink B, performed using the ISO 2409 tape test on a

screen-printed surface (figure 2(g)), showed that the printed squares remained intact after tape removal (figure 2(h)), with no visible detachment compared to the original surface. The results correspond to a category 0 classification according to the ISO 2409 standard. This indicates that Ink B exhibits excellent adhesion to the PC film surface. These values are comparable to those obtained for a commercial ink used in IME [5]. Similar results were also observed for Ink A, however, the film deposited using Ink C exhibits surface cracks that compromise the morphological integrity and, consequently, degrade the printing quality (supplementary information SI5). This behavior is likely attributed to the high proportion of fine flakes, which hinder the formation of a uniform and continuous network during solvent evaporation and thermal curing.

### 3.3. Electrical performances of the inks

Electrical conductivity is a critical parameter in evaluating the performance of conductive inks. In this context, the size of the flakes plays a significant role, as it has an impact on how the flakes connect with each other in the ink matrix [38]. Thus, the size of the flakes can lead to greatly different overall electrical resistivity of the printed films.

Scanning electron microscope (SEM) analysis reveals that Ink A (figure 3(a)), composed exclusively of large flakes, forms well-stacked and interconnected particles across much of the printed surface. However, some regions display gaps between particles of several micrometers wide (figure 3(d)), potentially increasing electrical resistivity and limiting the ink's mechanical flexibility under bending or thermoforming.
<table><caption>**Table 3.** Measured variation of viscosity (Pa·s) with the shear rate.</caption><thead><tr><th></th><th>0.1 s<sup>-1</sup></th><th>1 s<sup>-1</sup></th><th>10 s<sup>-1</sup></th><th>100 s<sup>-1</sup></th><th>1000 s<sup>-1</sup></th></tr></thead><tbody><tr><th>Ink A</th><td>4280 ± 89</td><td>1220 ± 45</td><td>217 ± 32</td><td>16.4 ± 5.8</td><td>1.12 ± 0.3</td></tr><tr><th>Ink B</th><td>1310 ± 51</td><td>250 ± 21</td><td>33 ± 5.6</td><td>7.9 ± 2.5</td><td>1.07 ± 0.1</td></tr><tr><th>Ink C</th><td>410 ± 22</td><td>81.1 ± 2.5</td><td>28.7 ± 3.4</td><td>12.1 ± 3.8</td><td>0.215 ± 0.0</td></tr></tbody></table>

<figure><img src="image_5.png" /><figcaption>**Figure 2.** Characterization of screen printed inks. Optical images of conductive tracks using (a) Ink A (thickness = 5.01 µm ± 0.25, width = 1 mm ± 0.1), (b) Ink B (thickness = 4.96 µm ± 0.23, width = 1 mm ± 0.1), and (c) Ink C (thickness = 4.81 µm ± 0.18, width = 1 mm ± 0.2). SEM images of tracks with (d) Ink A, (e) Ink B, and (f) Ink C. (g) Screen-printed PC film with Ink B. (h) Adhesion tests performed on tracks with Ink B using the ISO 2409 tape test.</figcaption></figure>

In contrast, Ink B, consisting of a mixture of large and fine flakes, presents a more dense and continuous structure (figure 3(b)). When Ink B is printed, the finer flakes fill the voids between larger flakes, resulting in a more homogeneous, densely packed network on the substrate (figure 3(e)). This increase in packing density leads to less possible, potentially non-conductive, gaps leading to potentially increased electrical conductivity.

Finally, Ink C, made only of fine flakes, shows a uniform particle distribution upon deposition (figure 3(c)). Nonetheless, several areas lack inter-particle connectivity (figure 3(f)), which may elevate electrical resistivity and reduce the ink's capability to endure deformation.

To evaluate the electrical resistivity of these inks, resistance measurements were performed on screen printed lines using all three inks. At first, thermal curing was conducted, followed by comparison with photonic curing.

Thermal curing was completed at different temperatures from 25 °C to 150 °C for a duration of

20 min. The data shown in figure 3(g) indicate a clear trend of decreasing electrical resistance with increasing curing temperature for all inks. Ink B had the lowest initial track resistivity, which continuously decreased with increasing curing temperature reaching 34.77 ± 3.29 µΩ·cm at 120 °C and a minimum of 14.68 ± 2.4 µΩ·cm at 150 °C demonstrating the best electrical performance across the range. Besides, Ink A and Ink C had similar track resistivity with Ink A exhibiting a slightly better performance than Ink C when heated to a higher temperature. As an important note, it must be mentioned that Ink A exhibits variable individual results, with track samples not being uniform during the curing process. The track resistivity recorded for Ink A was 67.01 ± 12 µΩ·cm at 120 °C and a minimum of 25.45 ± 4.6 µΩ·cm at 150 °C, while Ink C reached a higher value of 71.88 ± 5.2 µΩ·cm at 120 °C and 37.92 ± 4 µΩ·cm at 150 °C. However, both remain less performant than Ink B, which demonstrated the best track electrical behavior with a minimum resistivity of 14.68 ± 2.4 µΩ·cm at 150 °C.
<figure><img src="image_6.png" /><figcaption>**Figure 3.** Characterization of the inks after curing. SEM images of the ink after curing at 120 °C for 20 min: (a), (d) Ink A, (b), (e) Ink B, (c), (f) Ink C. Measured electrical resistivity (20 screen-printed lines per condition) of the tracks as function of (g) heat curing temperature, and (h) photonic curing.</figcaption></figure>

This illustrates the importance of curing temperat-
ure and ink composition when considering electrical
performance of printed films. The electrical resist-
ance measurements and the cross-sectional area data
used to calculate the electrical resistivity of the inks
are provided in supplementary information SI6.

Regarding photonic curing, the obtained track
resistivity values are slightly better than those meas-
ured under thermal curing at 150 °C for all three
inks (figure 3(h)). Ink B still demonstrates the
best performance, achieving a track resistivity of
9.39 ± 0.17 μΩ · cm, compared to 14.68 ± 2.4 μΩ · cm
at 150 °C. The SEM images (see supplementary
information SI7(c) and (d)) after photonic curing
revealed a markedly different microstructure com-
pared to thermal curing (see supplementary inform-
ation SI7(a) and (b)), with the silver particles appear-
ing densely packed and uniformly distributed across
the surface. The particles retained their flake-like
morphology and exhibited stronger interparticle con-
tact, resulting in a more compact and tightly bonded
network. Ink A also shows an improvement, with res-
istivity decreasing to 13.83 ± 0.54 μΩ · cm, while Ink
C features a track resistivity of 27.61 ± 5.2 μΩ · cm.
These results indicate that photonic curing not only

enhances conductivity but could also serve as a prom-
ising curing method for these inks, offering faster pro-
cessing and better compatibility with heat-sensitive
substrates.

**3.4. Suitability of the inks for FPEs**

The mechanical performance of the conductive tracks
under deformation was evaluated through basic
bending tests to assess the potential of the developed
inks for FPE applications. PC films with straight
screen-printed tracks were cut into type 5A speci-
mens in accordance with ISO 527, to study their com-
pressive behavior (figure 4(a)). The electrical nor-
malized resistance (R/R₀) as a function of the num-
ber of bending cycles was measured and reported for
three samples of each ink (figure 4(b)). The track elec-
trical resistance of whatever the ink increases pro-
gressively with the number of bending cycles, indic-
ating mechanical and structural degradation under
repeated deformation. However, Ink B consistently
exhibits tracks of lower resistance increase of 8% after
1000 bending cycles and 80% after 5000 bending
cycles, compared to 110% and 130% for Ink C and Ink
A respectively, suggesting better mechanical stability
<figure><img src="image_7.png" /><figcaption>**Figure 4.** Characterization of the flexibility of conductive track printed on PC film into type 5A specimens in accordance with ISO 527 directive. (a) Samples. (b) Bending fatigue test of screen-printed tracks after 5000 cycles under 10 mm bending radius. (c) Flexible electronic demonstrator manufactured using Ink B. (d), (e) Device operating under two different bending conditions.</figcaption></figure>

and adhesion to the substrate. Measurements could not be performed beyond 5000 cycles due to the fracture of the PC film.

To illustrate the flexible device fabrication capability using Ink B, an 8-bit microcontroller-based circuit with LEDs was designed and fabricated (figure 4(c)). Figures 4(d) and (e) show that the circuit works even when bent in two different directions, which is consistent with the results obtained from the bending tests. The small experimental vehicle reveals promising results towards the development of reliable, FPE products using Ink B, which could lead to a wide variety of sustainable wearable and bendable technologies.

**3.5. The impact of thermoforming on electrical performances**

Screen-printed films using Ink A, Ink B and Ink C were also vacuum-formed over the stepped conical mold (supplementary information SI2) to evaluate the stretchability of the inks during the thermoforming process and their impact on electrical conductivity. Figure 5(a) shows the strain distribution of the PC film after thermoforming, obtained using finite element method simulation. This deformation introduces mechanical strain to both the polymer substrate

and the printed conductive tracks, which may affect the adhesion and electrical performance of the inks. To accurately assess this deformation, thermoforming simulations of the PC substrate are performed, as the elongation of the conductive tracks is directly linked to the stretching of the thermoformed PC substrate, an aspect that is difficult to measure experimentally. While thermoforming simulations have been extensively studied [43–46], the coupling between thermoforming and the evolution of the electrical resistance of conductive tracks in the context of IME, has only been addressed by a few studies [47–49].

Next, the strain along the white line shown in figure 5(a), corresponding to the conductive track undergoing the highest elongation, was extracted as a function of the curvilinear coordinate. The resulting curve was fitted to obtain an analytical expression of strain versus curvilinear coordinate, with the origin set at the top of the thermoformed part (figure 5(b)). Figure 5(c) shows the linear resistance (Ω mm⁻¹) versus curvilinear coordinates (mm) for the three inks. For Ink A, the resistance rises up to 200 Ω mm⁻¹, and the track breaks at the point of highest elongation, after which electrical continuity resumes. A similar phenomenon is observed for Ink C, with a break occurring at the same location, although the variation
<figure><img src="image_8.png" /><figcaption>**Figure 5.** Thermoforming simulation and resistance evolution. (a) Simulation of the nominal strain distribution in the PC film; (b) strain distribution as a function of curvilinear coordinates. Evolution of the linear resistance of the tracks (Ω mm⁻¹) for the three inks as a function of (c) curvilinear coordinates (mm), and (d) strain.</figcaption></figure>

in resistance is less pronounced. Finally, Ink B maintains electrical continuity even under high elongation, showing only a slight variation in resistance. Using the strain versus curvilinear coordinate function, we were able to plot the linear resistance versus strain for each ink (figure 5(d)). The results clearly show that the resistance of Ink A varies significantly with strain, even at low strain levels. Ink C exhibits a noticeable variation, especially at high strain. In contrast, Ink B demonstrates the best performance, with a much smaller resistance variation compared to the other two inks, even under high strain.

The resistance measurements were carried out on all printed tracks (figure 6(a)) and for each ink. The variation in electrical resistance of the printed tracks was analyzed as a function of their strain, determined by their position on the 3D cone (figure 6(b)). Ink A exhibited early electrical failure, with open circuits observed at strain levels as low as 15%. Ink C showed relatively better resistance to mechanical strain; however, electrical discontinuities were still detected at strain levels below 30%. In contrast, Ink B demonstrated superior mechanical and electrical stability, maintaining continuous conductivity across all tracks, even at strain levels exceeding 30%. The red-highlighted region in the figure 6(a) corresponds to the high-strain zone of

the cone. In this area, significant cracking and electrical discontinuities were observed for Ink A and Ink C (figure 6(c)), indicating mechanical failure under high strain. In contrast, although minor cracks were also visible with Ink B, they were less severe and did not disrupt electrical continuity, highlighting its superior stretchability and electrical stability. Figure 6(d) shows a screen-printed film of an electrical circuit using Ink B, with components attached, while figure 6(e) shows the same film after thermoforming, demonstrating that the ink maintains its electrical functionality and mechanical integrity even after being shaped into a 3D form.

## 4. Discussion

In this study, we reported the formulation of an environmentally friendly, silver-based conductive ink designed for FPE and IME. PLA polymer, dissolved with the green solvent Cyrene, served as an effective biobased and biodegradable matrix, enabling homogeneous dispersion of conductive silver flakes.

Three ink formulations using silver flakes with 60% loading were tested, utilizing either large (Ink A), fine (Ink C), or both particles (Ink B). Among them, the Ink B exhibited an optimal viscosity of 250 Pa·s at a shear rate of 0.1 s⁻¹, allowing for the printing
<figure><img src="image_9.png" /><figcaption>**Figure 6.** Tracks deformation and electrical properties after thermoforming onto a conical shape. (a) Picture of a thermoformed screen-printed film. (b) Variation in electrical resistance of silver tracks using Ink A, Ink B, and Ink C as a function of conductive track number. (c) Microscopic images of silver tracks from Ink A, Ink B, and Ink C in the high-strain zone. Demonstrator of an electronic circuit using Ink B, (d) after screen printing, and (e) after thermoforming.</figcaption></figure>

of well-defined lines. This viscosity provided a favorable balance between printability and pattern resolution, outperforming the Ink C (less viscous) and Ink A (more viscous).

Ink formulation strongly dictates the rheological behavior of the inks, which in turn governs both printing precision and electrical performance. Large-flake inks (Ink A) form a highly interconnected particle network due to their irregular shaped [50], resulting in high viscosity and slow flow; this produces well-defined patterns but leaves a rough surface due to mesh imprint retention and large flakes dimension. On the other hand, fine-flake inks (Ink C), flow easily under shear but tend to spread and form cracks upon deposition, leading to less precise patterns and potential gaps that increase resistivity. The mixed-flake ink (Ink B) balances these effects: fine particles fill the voids between larger flakes, achieving intermediate viscosity, rapid thixotropic recovery after shear, and a smooth, well-defined printed track. Screen-printing simulations confirm that this balance between shear-thinning and recovery rates is crucial for pattern fidelity. As a result, Ink B achieves the lowest resistivity, demonstrating that optimizing particle size distribution directly links ink formulation, rheological properties, and the resulting precision and conductivity of screen-printed circuits.

Furthermore, the ink demonstrated good adhesion to PC substrates, confirming its suitability for printed electronics applications. Ink B also demonstrated excellent electrical performance, achieving conductive tracks of resistivity of $34.77 \pm 3.29 \ \mu\Omega \cdot \text{cm}$ at 120 °C, a temperature supported by most common substrates used in printed electronics, and $14.68 \pm 2.4 \ \mu\Omega \cdot \text{cm}$ at 150 °C, typically supported by high-performance substrates in the field. These values are comparable to, and in some cases even surpass, those reported for conventional commercial inks formulated with petrochemical-based matrices (table 4). Ink B also showed compatibility with photonic curing, leading to an even lower resistivity of $9.39 \pm 0.17 \ \mu\Omega \cdot \text{cm}$, what highlights its potential for processing on low temperature polymers (such as PLA instead of PC [5, 18]) and for applications requiring rapid curing processes compatible with large-scale production.

Its superior conductivity compared to the other two formulations is basically due to the presence of fine flakes alongside larger ones, which effectively fill the gaps between particles and enhance interparticle connectivity. These results are consistent with previous studies showing that incorporating flake-shaped conductive fillers is an effective strategy for achieving high electrical conductivity in stretchable
<table><caption>**Table 4.** Comparison of the results of the present work with those of previous research on silver conductive inks used in IME.</caption><thead><tr><th>Reference</th><th>Silver filler</th><th>% Filler</th><th>Curing conditions</th><th>Resistivity (μΩ·cm)</th></tr></thead><tbody><tr><td>Present work</td><td>Ink B: flakes 2.2–5.6 μm</td><td>60%</td><td>120 °C–20 min</td><td>34.77 ± 3.29</td></tr><tr><td>Present work</td><td>Ink B: flakes 2.2–5.6 μm</td><td>60%</td><td>Photonic curing</td><td>9.39 ± 0.17</td></tr><tr><td>[5]</td><td>Commercial ink ME603-Dupont</td><td>&lt;57%</td><td>120 °C–20 min</td><td>31–54</td></tr><tr><td>[52]</td><td>Nanoparticles</td><td></td><td>120 °C–20 min</td><td>101.6</td></tr><tr><td>[53]</td><td>Commercial ink E550-chimet</td><td></td><td>120 °C–10 min</td><td>&lt;175</td></tr><tr><td>[38]</td><td>AgAg1: commercial inks E550–E555</td><td>60.1%</td><td>120 °C–20 min</td><td>65</td></tr><tr><td>[51]</td><td>Fr-2: 20 nm–10 μm-nanowire</td><td>&gt;70%</td><td>150 °C–10 s</td><td>110</td></tr><tr><td>[51]</td><td>G: 20 nm–10 μm</td><td>&gt;70%</td><td>150 °C–10 s</td><td>145</td></tr><tr><td>[51]</td><td>Commercial ink (NSX, Namics)</td><td></td><td>150 °C–10 s</td><td>50</td></tr><tr><td>[54]</td><td>Particles 3–15 μm</td><td>&gt;60%</td><td>150 °C–20 min</td><td>80</td></tr><tr><td>[54]</td><td>Particles ≤5 μm</td><td>70%</td><td>120 °C–30 min</td><td>58.82</td></tr></tbody></table>

applications [38]. Furthermore, the combination of small and large particles has also been demonstrated to improve the conductivity of inks [51]. In addition, the low glass transition temperature of PLA (Tg ~55 °C–65 °C) does not appear to compromise the ink’s adhesion or stability after thermal curing at elevated temperatures.

Under bending tests, Ink B enables conductive traces with strong durability and adhesion to the polymer substrate. It continued to show better performance than the other two inks with an increase in resistance of only 8% after 1000 cycles. Similarly, after thermoforming, Ink B was the only ink to keep the electrical conductivity after elongation greater than 30%.

Finally, a FPE demonstrator and a thermoformed demonstrator dedicated to IME were presented as proof of concept, confirming that Ink B represents a promising option for the fabrication of flexible circuits and IME devices.

The applicability of Ink B is not restricted to a single polymer substrate as PC; other biobased or biodegradable polymers may also be suitable. For instance, its expected compatibility with PLA could enable the development of fully biobased and bio-degradable electronic devices. Furthermore, PLA has previously been identified as a promising substrate for IME applications [5, 18]. For applications requiring high electrical conductivity, silver-based fillers will likely remain the preferred choice. However, carbon-based fillers derived from renewable sources could be considered as alternatives for applications requiring moderate electrical conductivity, further reducing environmental impact and promoting more sustainable, eco-friendly formulations.

In addition to the substrate and conductive ink, conductive adhesives also represent a key component to consider when targeting fully bio-based electronic devices. Some studies have explored formulations based on bio-sourced materials for conductive adhesives [55, 56], highlighting their potential contribution to the development of environmentally friendly electronics.

## 5. Conclusions

Using sustainable materials such as PLA and Cyrene, this study presents a possible alternative to commercial petrochemical-based inks. An environmentally friendly silver-flake ink was successfully formulated and screen-printed onto PC films. The addition of both large (5.6 μm) and fine (2.2 μm) silver particles enhanced the ink’s properties, particularly electrical conductivity even under high stretchability. The developed Ink exhibited electrical resistivities of 34.77 ± 3.29 μΩ·cm at 120 °C, compatible with most technical polymers used in electronics, and 9.39 ± 0.17 μΩ·cm under photonic curing, making it well-suited for use on temperature-sensitive polymer substrates.

The ink combines satisfying electrical and mechanical performance with a composition that reduces environmental impact. Its rheological properties are suitable for screen printing enabling the production of uniform, sharp-edged patterns. Additionally, the ink demonstrated excellent stretchability and the ability to maintain electrical continuity during both bending cycle tests and thermoforming processes. These results pave the way for a new generation of eco-friendly, highly conductive inks compatible with industrial screen printing processes and suitable for flexible and IME applications, suggesting they are excellent candidate as bio-based substitutes for conventional conductive inks.

Finally, long-term reliability testing is required to fully assess the performance, durability, shelf life, and optimal storage conditions of the inks under long-term operational conditions, assessing the electrical performance over time, resistance to mechanical fatigue, and behavior during various environmental treatments, such as humidity, temperature, and UV and chemical exposure.

Further optimization of ink rheology (solid loading, polymer ratio, additives) and printing settings (mesh selection, printing parameters) is also underway to improve line definition for higher-resolution features. Ongoing work is focusing on improving
print resolution, deposit uniformity, and dimensional stability of finer features to address these remaining limitations.

## Data availability statement

All data that support the findings of this study are included within the article, and supplementary file.

## Acknowledgments

This work was financially supported by the CIFRE PhD fellowship No. 2020/0979, granted by the French ANRT (Agence Nationale de la Recherche et de la Technologie) to Thomas Guérin. The authors also gratefully acknowledge the accessory financial support provided by the Agence Nationale de la Recherche (AMBRE project, ANR-22-CE05-0015-03), the Auvergne-Rhône-Alpes Region (BioAntenna project, convention 20 00955101–22464), the GIP CNFM (ANR-23-CMAS-0024), and INSA Lyon through a strategic grant. Their valuable contributions are sincerely appreciated. We would like to thank H. Cauchy-Clerc from Ampere/IMP for performing screen printing.

## Conflict of interest

Thomas GUERIN was employed by Encres Dubuit as an investigator in this study. Guy MASSE is currently employed by Encres Dubuit. The authors declare no other competing interests.

## ORCID iDs

Tony Gerges ID 0000-0002-6208-2916

Bruno Allard ID 0000-0001-8547-5483

## References

[1] Manikkampatti Palanisamy M, Myneni V R, Gudeta B and Komarabathina S 2022 Toxic metal recovery from waste printed circuit boards: a review of advanced approaches for sustainable treatment methodology *Adv. Mater. Sci. Eng.* **2022** 1–9

[2] Gerges T et al 2023 Rapid 3D-plastronics prototyping by selective metallization of 3D printed parts *Addit. Manuf.* **73** 103673

[3] Franke J 2014 *Three-Dimensional Molded Interconnect Devices (3D-MID)* (Carl Hanser Verlag GmbH & Co. KG)

[4] Yue C, Wang J, Wang Z, Kong B and Wang G 2023 Flexible printed electronics and their applications in food quality monitoring and intelligent food packaging: recent advances *Food Control* **154** 109983

[5] Goument C, Gerges T, Lombard P, Lakhdar H, Arli M, Gilmus VM, Lambert S A, Allard B, Charmeau J Y and Cabrera M 2023 In-mold electronics on poly(lactic acid): towards a more sustainable mass production of plastronic devices *Int. J. Adv. Manuf. Technol.* **125** 2643–60

[6] Irfan M et al 2025 Thermoforming approach for in-mold LED arrays utilizing screen printing process with silver paste *Int. J. Precis. Eng. Manuf.* **12** 1749–62

[7] Campos-Arias L, Peřinka N, Vicente J, Gomes Correia V M, Mendes-Felipe C, Vilas-Vilela J L and Lanceros-Méndez S

2025 Fully printed 3D-thermoformed electroluminescent capacitive system for human-machine interface applications *Adv. Mater. Technol.* **10** 2402116

[8] Florian C and Serra P 2023 Printing via laser-induced forward transfer and the future of digital manufacturing *Materials* **16** 698

[9] Lee G-H, Lee Y R, Kim H, Kwon D A, Kim H, Yang C, Choi S Q, Park S, Jeong J-W and Park S 2022 Rapid meniscus-guided printing of stable semi-solid-state liquid metal microgranular-particle for soft electronics *Nat. Commun.* **13** 2643

[10] Akin S, Jo S and Jun M B-G 2023 A cold spray-based novel manufacturing route for flexible electronics *J. Manuf. Process.* **86** 98–108

[11] Elsersawy R, Rahman A, Sakib-Uz-Zaman C and Khondoker M A H 2025 Multifunctional inks in aerosol jet printing: performance, challenges, and applications *Front. Manuf. Technol.* **5** 1558209

[12] Tilford T, Stoyanov S, Braun J, Janhsen J C, Patel M K and Bailey C 2021 Comparative reliability of inkjet-printed electronics packaging *IEEE Trans. Compon. Packag. Manuf. Technol.* **11** 351–62

[13] Jaafar A, Schoinas S and Passeraub P 2021 Pad-printing as a fabrication process for flexible and compact multilayer circuits *Sensors* **21** 6802

[14] Camargo J R et al 2021 Development of conductive inks for electrochemical sensors and biosensors *Microchem. J.* **164** 105998

[15] O'Brien C and Howley J 2025 *Conductive Inks Market 2024-2034: Technologies, Applications, Players* IDTechEx (available at: www.idtechex.com/en/research-report/conductive-inks/1013)

[16] Kim S, Yang G, Park K-I and Min Y Effect of silver filler morphology on the conductivity of screen-printable silver inks *J. Korean Inst. Electr. Electron. Mater. Eng.* **38** 436–41

[17] Schroder K A 2011 Mechanisms of photonic curing™: processing high temperature films on low temperature substrates *Nano- Technol.* **2** 220–3 (available at: https://briefs.techconnect.org/wp-content/volumes/Nanotech2011v2/pdf/1234.pdf)

[18] Gerges T, Cauchy-Clerc H, Lombard P, Charmeau J-Y, Allard B and Cabrera M 2025 In-mold electronics with high electrical conductivity circuits on poly(lactic acid) and polycarbonate using photonic curing *Int. J. Adv. Manuf. Technol.* (https://doi.org/10.1007/s00170-025-16977-8)

[19] Ke S, Liu Y, Chen F, Ni X and Ma Y 2024 Recent advances in conductive materials for printed electronics and printed technology? *Polym. Adv. Technol.* **35** e6581

[20] Shukla D, Liu Y and Zhu Y 2023 Eco-friendly screen printing of silver nanowires for flexible and stretchable electronics *Nanoscale* **15** 2767–78

[21] Brooke R, Fall A, Borras M, Yilma D B, Edberg J, Martinez-Crespiera S, Aulin C and Beni V 2021 Nanocellulose based carbon ink and its application in electrochromic displays and supercapacitors *Flexible Print. Electron.* **6** 045011

[22] Edberg J, Brooke R, Hosseinaei O, Fall A, Wijeratne K and Sandberg M 2020 Laser-induced graphitization of a forest-based ink for use in flexible and printed electronics *npj Flexible Electron.* **4** 17

[23] El Baradai O, Beneventi D, Alloin F, Bongiovanni R, Bruas-Reverdy N, Bultel Y and Chaussy D 2016 Microfibrillated cellulose based ink for eco-sustainable screen printed flexible electrodes in lithium ion batteries *J. Mater. Sci. Technol.* **32** 566–72

[24] Zappi D, Varani G, Cozzoni E, Iatsunskyi I, Laschi S and Giardi M T 2021 Innovative eco-friendly conductive ink based on carbonized lignin for the production of flexible and stretchable bio-sensors *Nanomaterials* **11** 3428

[25] Szałapak J et al 2024 Carbon-based composites with bio-degradable matrix for flexible paper electronics *Polymers* **16** 686

[26] Baumbauer C L, Gopalakrishnan A, Atreya M, Whiting G L and Arias A C 2024 Polycaprolactone-based zinc ink for high
conductivity transient printed electronics and antennas *Adv. Electron. Mater.* **10** 2300658

[27] Camargo JR, Silva TA, Rivas G A and Janegitz B C 2022 Novel eco-friendly water-based conductive ink for the preparation of disposable screen-printed electrodes for sensing and biosensing applications *Electrochim. Acta* **409** 139968

[28] Cosate de Andrade M F, Souza P M S, Cavalett O and Morales A R 2016 Life cycle assessment of poly(lactic acid) (PLA): comparison between chemical recycling, mechanical recycling and composting *J. Polym. Environ.* **24** 372–84

[29] Atreya M, Dikshit K, Marinick G, Nielson J, Bruns C and Whiting G L 2020 Poly(lactic acid)-based ink for biodegradable printed electronics with conductivity enhanced through solvent aging *ACS Appl. Mater. Interfaces* **12** 23494–501

[30] Najafi M, Zahid M, Ceseracciu L, Safarpour M, Athanassiou A and Bayer I S 2022 Polylactic acid-graphene emulsion ink based conductive cotton fabrics *J. Mater. Res. Technol.* **18** 5197–211

[31] Forestier E, Najafi M, Dussoni S, Maggiali M, Athanassiou A and Bayer I S 2023 Characterization and performance of silicone modified polylactic acid (PLA)-graphene nanoplatelet ink coatings for flexible elastomeric substrates *Prog. Org. Coat.* **174** 107251

[32] Wang Y, Dai M, Luo G, Fan J, Clark J H and Zhang S 2023 Preparation and application of green sustainable solvent cyrene *Chemistry* **5** 2322–46

[33] Kong D and Dolzhenko A V 2022 Cyrene: a bio-based sustainable solvent for organic synthesis *Sustain. Chem. Pharm.* **25** 100591

[34] Jordan A, Hall CGJ, Thorp LR and Sneddon HF 2022 Replacement of less-preferred dipolar aprotic and ethereal solvents in synthetic organic chemistry with more sustainable alternatives *Chem. Rev.* **122** 6749–94

[35] Stoclet G, Séguéla R, Lefebvre J M, Li S and Vert M 2011 Thermal and strain-induced chain ordering in lactic acid stereocopolymers: influence of the composition in stereomers *Macromolecules* **44** 4961–9

[36] Auras R, Harte B and Selke S 2004 An overview of poly-lactides as packaging materials *Macromol. Biosci.* **4** 835–64

[37] Teh N J, Conway P P, Palmer P J, Prosser S and Kioul A 2000 Statistical optimisation of thermoplastic injection moulding process for the encapsulation of electronic subassembly *J. Electron. Manuf.* **10** 171–9

[38] Miliciani M, Mendicino G M and DeMeuse M T 2023 In-mold electronics applications: the control of ink properties through the use of mixtures of electrically conductive pastes *Flexible Print. Electron.* **8** 035011

[39] Srivastava V, Chester SA, Ames N M and Anand L 2010 A thermo-mechanically-coupled large-deformation theory for amorphous polymers in a temperature range which spans their glass transition *Int. J. Plast.* **26** 1138–82

[40] Caubet B, Léonardi F and Amand S 2023 Polymer-tool friction coefficient in temperature for thermoforming numerical simulation *Int. ESAFORM Conf.* 2023 pp 1907–16 (available at: https://univ-pau.hal.science/hal-04295358v1/document)

[41] Marotta E E and Fletcher L S 1996 Thermal contact conductance of selected polymeric materials *J. Thermophys. Heat Transf.* **10** 334–42

[42] Kardashian V S and Rao Vellanki S J 1979 A method for the rheological characterization of thick-film pastes *IEEE Trans. Compon. Hybrids Manuf. Technol.* **2** 232–9

[43] Takaffoli M, Hangalur G, Bakker R and Chandrashekar N 2020 Thermo-visco-hyperelastic behavior of polycarbonate in forming of a complex geometry *J. Manuf. Process.* **57** 105–13

[44] Ghobadnam M, Mosaddegh P, Rezaei Rejani M, Amirabadi H and Ghaei A 2015 Numerical and experimental analysis of HIPS sheets in thermoforming process *Int. J. Adv. Manuf. Technol.* **76** 1079–89

[45] Sala G, Di Landro L and Cassago D 2002 A numerical and experimental approach to optimise sheet stamping technologies: polymers thermoforming *Mater. Des.* **23** 21–39

[46] O'Connor C P J, Martin P J, Sweeney J, Menary G, Caton-Rose P and Spencer P E 2013 Simulation of the plug-assisted thermoforming of polypropylene using a large strain thermally coupled constitutive model *J. Mater. Process. Technol.* **213** 1588–600

[47] Gong Y, Cha K J and Park J M 2020 Deformation characteristics and resistance distribution in thermoforming of printed electrical circuits for in-mold electronics application *Int. J. Adv. Manuf. Technol.* **108** 749–58

[48] Lall P, Soni V, Narangaparambail J and Miller S 2023 Evaluation of thermoformability of additively printed circuits printed using gravure offset printing technique and investigation of in-mold electronic circuits *ASME 2023 Int. Technical Conf. and Exhibition on Packaging and Integration of Electronic and Photonic Microsystems* (American Society of Mechanical Engineers)

[49] Gomes Correia V M, Pereira N, Perinka N, Costa P, Del Campo J and Lanceros-Mendez S 2022 Printed 3D gesture recognition thermoformed half sphere compatible with in-mold electronic applications *Adv. Eng. Mater.* **24** 2200730

[50] House A, Kuna A, Hastings D, Rodriguez N, Schoenitz M, Dreizin E L and Guvendiren M 2023 Effect of particle shape on rheology and printability of highly filled reactive inks for direct ink writing *Prog. Addit. Manuf.* **8** 1573–85

[51] Lee S Y, Jang S H, Lee H K, Kim J S, Lee S, Song H J, Jung J W, Yoo E S and Choi J 2020 The development and investigation of highly stretchable conductive inks for 3-dimensional printed in-mold electronics *Org. Electron.* **85** 105881

[52] Kulkarni S et al 2025 Additively manufactured in-mold electronics reliability under thermal cycling and sustained high temperature 2025 IEEE 75th Electronic Components and Technology Conf. (ECTC) (IEEE) pp 718–25

[53] Chimet, Polymer Thick Film (PTF) brochure, Brochure n.d. [Online] (available at: www.chimet.com/public/download/Brochure_Polymer_thick_film.pdf) (Accessed 24 July 2025)

[54] Merilampi S, Laine-Ma T and Ruuskanen P 2009 The characterization of electrically conductive silver ink patterns on flexible substrates *Microelectron. Reliab.* **49** 782–90

[55] Kayaci H U et al 2025 Waterbased electrically conductive adhesive for PERC-type shingled solar cells *Sol. Energy Mater. Sol. Cells* **285** 113525

[56] Yang C, Lin W, Li Z, Zhang R, Wen H, Gao B, Chen G, Gao P, Yuen M M F and Wong C P 2011 Water-based isotropically conductive adhesives: towards green and low-cost flexible electronics *Adv. Funct. Mater.* **21** 4582–8