Inkjet-Printed Conductive Patterns on Electrospun Substrates for the Modular Fabrication of Nonplanar Circuits

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The article is well written, and suggestions for improvement are provided below:

In the introduction, please provide reasoning for the selection of PCL as the material for the electrospinning substrate. Specifically, the substrate was heated to 80°C, while PCL melts at around 60°C. Intuitively, this does not appear to be the most suitable choice for a transfer material.

Clarify where FIB (Focused Ion Beam) was used. The images do not appear to show cross-sections.

In Figure 9, please explain the significance of the red and black circles.

The proposed mechanism assumes that the majority of the ink remains on the surface of the PLA. However, earlier explanations mention that the ink seeps through the PCL layer. Does this seeping ink contribute to higher thermal conductivity, leading to the melting of PCL fibers onto the PLA 3D print while protecting the ink? To better distinguish between the silver ink and the PCL fibers, it is recommended to supplement the current imaging with backscattered electron (BSE) imaging. This would improve material contrast. Additionally, providing both secondary electron (SE) and BSE images side-by-side could significantly enhance visual interpretation.

Include details about the temperature settings used during ink transfer to PET and TPU flexible substrates.

The conductivity, quality of print transfer, and mechanical stability under rubbing are all strongly influenced by the thickness and layering of the printed material. A cross-sectional view of the sample, rather than only surface imaging, would provide critical insights into layer uniformity, adhesion quality, and material penetration. Including such cross-sectional data would significantly strengthen the conclusions of the study.

Comments on the Quality of English Language

English can be improved.

Author Response

In the introduction, please provide reasoning for the selection of PCL as the material for the electrospinning substrate.

PCL has low melting point and can be adhered to PLA when melted. A sentence explaining this fact including references has been added to the introduction.

Specifically, the substrate was heated to 80°C, while PCL melts at around 60°C. Intuitively, this does not appear to be the most suitable choice for a transfer material.

Thank you for your valuable feedback.

While it's true what the reviewer stated, intuitively, 60°C (the melting point of PCL fibers) should be the ideal temperature for transfer, some things must be taken into account. Initially, the part is heated in an oven, removed, and once outside, the transfer step proceeds.

Choosing a temperature too close to melting leads to poor fiber adhesion due to heat loss. It should be kept in mind that the PLA printed part is hollow, so the heat loss upon removal from the oven is more noticeable.

Pilot tests were conducted to find the optimal transfer temperature, and it was found that at 80°C there was a balance between thermal contraction and adhesion.

If the system were to be automated later, it would probably be best to work with a hot press system, which would likely reduce the working temperature.

 

Clarify where FIB (Focused Ion Beam) was used. The images do not appear to show cross-sections.

Thank you for your comment. We understand the confusion.

Our group usually uses the FIB to make cuts that allow for in-depth evaluation of the materials we produce. The FIB phrase remained because we had originally intended to do so, but unfortunately, the FIB is out of service and couldn't be fixed. We removed the reference to FIB experiments in the experimental part and included the actual microscope that we used for image acquisition (FEI Quanta 250 FEG) for clarity.

 

Figure 9, please explain the significance of the red and black circles.

Indeed, the caption doesn't clearly state what the red and black circles are. The caption was updated to explain the presence of red and black circles in the figure.

 

 

 

The proposed mechanism assumes that the majority of the ink remains on the surface of the PLA. However, earlier explanations mention that the ink seeps through the PCL layer. Does this seeping ink contribute to higher thermal conductivity, leading to the melting of PCL fibers onto the PLA 3D print while protecting the ink? To better distinguish between the silver ink and the PCL fibers, it is recommended to supplement the current imaging with backscattered electron (BSE) imaging. This would improve material contrast. Additionally, providing both secondary electron (SE) and BSE images side-by-side could significantly enhance visual interpretation.

The proposed mechanism assumes that the inks seeps into the electrospun PCL layer. However, when the pores are saturated with ink, the remaining ink can form a solid layer on top of the PCL-ink composite. This could be verified by profilometry data (Figure 7) and SEM images (Figures 5 and 6). Moreover, fibers are always visible, and the deposited ink can be distinguished as brighter zones (due to the fact that the silver ink is composed of nanoparticles). Saturated pores can be clearly distinguished in Figure 5D. SEM images of higher magnification are provided in order to provide a clear picture of the silver nanoparticles.

Regarding the melting of PCL during transfer process, it occurs independently of the presence or absence of silver ink within the pores. Moreover, the presence of ink prevents the coalescence of neighboring fibers. Additional SEM images were added to the supplementary material in order to depict this fact.

With this additional information provided (all images were taken using the SE detector) we consider that BSE imaging does not provide any additional information.

Include details about the temperature settings used during ink transfer to PET and TPU flexible substrates.

Transfer and curing conditions were the same for all materials. The conditions are described in the experimental part.

 

The conductivity, quality of print transfer, and mechanical stability under rubbing are all strongly influenced by the thickness and layering of the printed material. A cross-sectional view of the sample, rather than only surface imaging, would provide critical insights into layer uniformity, adhesion quality, and material penetration. Including such cross-sectional data would significantly strengthen the conclusions of the study.

We agree that SEM images cross sections would have provided a very clear picture of the silver ink penetrating the PCL electrospun mat. We have made many cross-section images of different materials (including electrospun PCL) for our previous publications and this case wouldn’t have been the exception if the focused ion beam at our institution wasn’t out of service since quite a large period. We have considered cryofracture, but we were not able to achieve satisfactory results. However, SEM images of both sides of the printed electrospun mats (Figure 5) and the sequential transfer of different printed patterns achieving electrical contact are sufficient evidence of ink penetration through the electrospun PCL layer. Moreover, adhesion has been tested by rub resistance and SEM images of Figure 9 show clearly that the PCL mat with embedded silver was torn off from PLA mainly at the grooves of the printed PLA piece between the fused filaments at the surface. In order to clarify this, we have modified Figure 10.  

Reviewer 2 Report

Comments and Suggestions for Authors

The study titled “Inkjet-printed conductive patterns on electro-spun substrates for the modular fabrication of non-planar circuits” describes a three-step workflow in which PCL nanofibers are electro-spun onto release paper, Ag nanoparticle ink is ink-jet printed and the fibre/ink stack is thermally fused onto complex PLA, PET or TPU geometries. The authors show that tracks survive light crock-meter abrasion, conform to concave cavities and elastomeric films, and can be connected in successive transfer steps. These results are encouraging, yet the manuscript needs a stronger quantitative frame and clearer methodological details to stand firmly against the recent literature.

1. Please add a compact table that lists your minimum trace width, sheet resistance and processing temperature alongside a few up-to-date conformal-printing techniques for quick comparison (e.g. 10.1038/s41528-024-00340-0; 10.1126/sciadv.adi0357; 10.1038/s41598-023-32044-2; 10.1038/s41467-025-57959-4).
2. Beyond abrasion, please include peel-strength, cyclic-bending and basic ageing measurements in order to benchmark the durability of the fused-PCL interface against endurance-oriented approaches. As a reference see e.g. 10.1038/s41467-024-52873-7 and 10.1002/adem.202300907.
3. Add a short paragraph that quantifies the silver loading (mg cm⁻²) and places it within the broader palette of printable conductors, such as low-cost metallic and hybrid alternatives (e.g. 10.1038/s41528-024-00331-1, 10.1021/acsaelm.3c01175); carbon-based inks (e.g. 10.1002/adfm.201807659, 10.1002/smsc.202400282); MXene inks (e.g. 10.1038/s41467-024-47700-y); printed PEDOT:PSS (e.g. 10.1039/d3tc04485h) and liquid-metal circuits (e.g. 10.1002/adma.202307632)  
4. For every data set, state the sample size, report mean ± SD and apply at least one significance test when comparing layer counts or substrates.
5. Minor points:
   - Correct typographical errors (e.g. “trough” to “through”) and add missing units in Figure 3.
   - Provide raw profilometry/SEM images in the Supporting Information.

Author Response

 

1. Please add a compact table that lists your minimum trace width, sheet resistance and processing temperature alongside a few up-to-date conformal-printing techniques for quick comparison (e.g. 10.1038/s41528-024-00340-0; 10.1126/sciadv.adi0357; 10.1038/s41598-023-32044-2; 10.1038/s41467-025-57959-4).

We have included a table as a comparison of our results to recent literature in the main text.

1. Beyond abrasion, please include peel-strength, cyclic-bending and basic ageing measurements in order to benchmark the durability of the fused-PCL interface against endurance-oriented approaches. As a reference see e.g. 10.1038/s41467-024-52873-7 and 10.1002/adem.202300907.

We have performed a peel-strength test with a new figure, an experimental procedure and a discussion in the main text.

Bending test cannot be performed on these samples since the PLA piece is rigid. Regarding ageing, we were able to measure resistivity and verify electrical continuity in samples printed two years ago and stored in air. We have included this finding in the main text by updating the table 5.

1. Add a short paragraph that quantifies the silver loading (mg cm⁻²) and places it within the broader palette of printable conductors, such as low-cost metallic and hybrid alternatives (e.g. 10.1038/s41528-024-00331-1, 10.1021/acsaelm.3c01175); carbon-based inks (e.g. 10.1002/adfm.201807659, 10.1002/smsc.202400282); MXene inks (e.g. 10.1038/s41467-024-47700-y); printed PEDOT:PSS (e.g. 10.1039/d3tc04485h) and liquid-metal circuits (e.g. 10.1002/adma.202307632)

A short paragraph comparing silver loading for inkjet-printed silver nanoparticle inks has been added to the main text. Moreover, a table were added to the supplementary section. The comparison with other conductive printing materials has been avoided since the novelty of the work consists in the design of the electrospun substrate for flatbed modular printing and transfer to nonplanar pieces rather than saving ink, and the same approach could be easily applied to other printing materials such as carbon or conductive polymer inkjet inks.

 

1. For every data set, state the sample size, report mean ± SD and apply at least one significance test when comparing layer counts or substrates.

Although each graph has a +/- SD, we understand that they are not very clearly visualized, so we have added a table in the supplementary material for the graphs of figures 3 and 7. The number of samples and measurements were indicated in the caption for each case.

Regarding the significance test, and perhaps due to the lack of clarity shown in the graph of resistance versus number of printed layers, we find it pointless to perform it since the populations in some cases have orders of magnitude differences in the mean value and standard deviation. The only exception is the resistance and thickness measured for 3 and 4 layers, which are treated as equal since there is no significant difference between them.

minor points:
- Correct typographical errors (e.g. “trough” to “through”) and add missing units in Figure 3.
- Provide raw profilometry/SEM images in the Supporting Information.

Raw profilometry data and uncropped SEM images were added to the supplementary material.

 

Reviewer 3 Report

Comments and Suggestions for Authors

 

The manuscript titled "Inkjet-printed conductive patterns on electrospun substrates for the modular fabrication of nonplanar circuits" presents a novel method for fabricating conductive patterns on electrospun polycaprolactone (PCL) substrates using inkjet printing. The method allows for the transfer of these printed patterns onto planar and nonplanar surfaces, creating modular, flexible, and conductive circuits. This approach has potential applications in wearable electronics, flexible sensors, and smart surfaces.

The study is generally well-structured and provides a detailed experimental methodology. The authors systematically investigate the influence of the number of printed layers on the electrical, mechanical, and transfer properties of the conductive patterns, offering valuable insights for the optimization of this process. However, the manuscript has several critical and minor issues that should be addressed to enhance its quality.

General comments:

Good work overall, Needs a bit of work.

The resolution of the micrographs should be improved

Minor language issues

 

Specific  comments and questions:

1. The manuscript lacks a clear scientific hypothesis or rationale at the beginning of the introduction. Try to emphasize why this method is expected to be better existing approaches (e.g., direct inkjet printing on nonplanar surfaces).

2. The authors present results for different substrates (kraft paper, sulphite paper, baking paper),but  a detailed discussion comparing these substrates in terms of their surface properties and  how these characteristics influence ink diffusion should be somehow added to the manuscript

3. I believe factors influencing the substrate adhesion, such as temperature, surface energy, and ink composition, should be discussed.

4. Interestingly ,the authors present  only rub resistance tests. I sthere a reason why standard mechanical assessments such bending fatigue tests were not performed ?

5. Can the  relationship between the number of printed layers and electrical resistivity be quantified ? Can the authors explain why there is such an increase in thickness (as seen in figure 7) at the 6 printed layers?

6. Sentecnes such as "DMF and chloroform were reagent grade and used straight from the bottle. " should be avoided as they are inprofessional. Also be specific e.g. DMF (anhydrous, 99,8 % Merck)

7. Add any availbel information for the Baking paper, kraft paper and sulphite paper. Was the baking paper, silicon baking paper? What was the surface density of the kraft paper (g/m2)

8. The quality and resoltion of the figures should be improved.

 

 

 

 

 

 

 

Comments on the Quality of English Language

The overall language of the manuscript is acceptable. Several spelling and grammatical issues should be resolved, for example:

"Ink buildup allows both sides of the electropun PCL mats to be conductive" (should be "electrospun").

"Intensivity of the diffusion process depends" (should be "Intensity").

"Moreover, the printed tracks could be transferred to nonplanar surfaces along sharp edges retaining electrical connection." (Should be rephrased for clarity).

Author Response

1. The manuscript lacks a clear scientific hypothesis or rationale at the beginning of the introduction. Try to emphasize why this method is expected to be better existing approaches (e.g., direct inkjet printing on nonplanar surfaces).

 

The introduction begins with a general description of a problem followed by a more specific description of the current approaches to this problem and finally the formulation of a hypothesis that would lead to a novel solution. This is what we always intend to do when we write a paper. We have added an additional sentence in the beginning and a brief explanation for the selection of PCL as a material for the fabrication of this transfer system that may provide a clearer picture.

 

2. The authors present results for different substrates (kraft paper, sulphite paper, baking paper),but a detailed discussion comparing these substrates in terms of their surface properties and how these characteristics influence ink diffusion should be somehow added to the manuscript

 

This query is answered jointly with the next one.

 

3. I believe factors influencing the substrate adhesion, such as temperature, surface energy, and ink composition, should be discussed.

 

The ink that we used is commercially available. It is described as a dispersion of silver nanoparticles in triethyleneglycol monomethyl ether with a surface tension of 35 - 40 dyn/cm. The solvent does not dissolve the electrospun mat and the ink can penetrate the mat’s pores without any additional treatment. If the ink wasn’t able to penetrate the pores, a plasma/corona treatment could have been performed on the electrospun mat to increase its wettability. Plasma treatment is a well-known method for improving adhesion and printability that we use routinely and was not necessary at all for this work.

The opposite approach was employed for the layer underneath the electrospun mat. We chose baking paper as an affordable and ubiquitous material of low surface tension that helps to keep the ink in the electrospun mat and can be easily detached during the transfer process. The same result could be achieved with other materials of low surface tension such as PDMS (in fact, we tested it, but we discarded it because the process with baking paper was much more straightforward).

In order to clarify, we have added a couple of sentences to the discussion regarding these issues.

4. Interestingly, the authors present only rub resistance tests. Is there a reason why standard mechanical assessments such bending fatigue tests were not performed ?

Bending testing is a great option in cases where the base material is flexible. In our case, the base material is a rigid part 3D printed using FDM technology in PLA. We understand that the inclusion of transfers on PET and TPU may have caused confusion, but it was a proof of concept demonstrating that the transfer system allows the use of other receiving substrates besides PLA.

In the case of TPU and PET, the tracks could be printed directly onto the material without major inconveniences, also reducing the number of steps required to have a functional track on the substrate.

For this reason, the electrical characterization after mechanical perturbation was based on rubbing resistance and did not include other types of characterizations such as bending tests for TPU and PET or tensile strength tests for TPU.

5. Can the relationship between the number of printed layers and electrical resistivity be quantified? Can the authors explain why there is such an increase in thickness (as seen in figure 7) at the 6 printed layers?

In the case of inkjet printing with inks containing metallic nanoparticles, it is difficult to establish a generalized correlation between the number of printed layers and electrical conductivity.

The final resistance of the track after curing is primarily related to the concentration of nanoparticles in the printed track. A higher concentration of particles allows the formation of conductive pathways during sintering.

The concentration of particles depends on several factors: the ink concentration, the resolution used for printing, the substrate material, and physical characteristics such as porosity and surface energy.

If, for example, the ink manages to penetrate the material on which it is being printed (is absorbed), a greater number of layers will be required to achieve an adequate concentration on the outside. A similar effect can be observed in porous materials, where some of the particles are adsorbed by the pores, and therefore a greater number of layers are required to achieve a functional sintering.

Regarding the explanation for the increase in thickness when the number of layers exceeds 4, this increase can be attributed to the first 4 layers of ink distributing themselves in the pores of the material, generating a slight change in the thickness of the transferred track.

Once the pores of the material are saturated, each layer contributes significantly to the increase in thickness. The explanation of this phenomenon is expanded in the main text.

 

6. Sentecnes such as "DMF and chloroform were reagent grade and used straight from the bottle. " should be avoided as they are inprofessional. Also be specific e.g. DMF (anhydrous, 99,8 % Merck)

Reagent grade is a standard and widespread classification for laboratory chemicals. It stands for purity higher than 95% and suitability for application in general laboratory analysis when additional requirements do not apply (i.e.: anhydrous conditions, trace analysis, etc.). Moreover, the specific requirements for reagent grade chloroform (10.1021/acsreagents.4091) and DMF (10.1021/acsreagents.4121) are clearly stablished and followed by many suppliers around the world. We have added information on purity and the commercial supplier for clarity.

7. Add any availbel information for the Baking paper, kraft paper and sulphite paper. Was the baking paper, silicon baking paper? What was the surface density of the kraft paper (g/m2)

The density of the papers used as substrates for electrospinning was added to the main text. The baking paper was a conventional baking paper treated with paraffin wax.

8. The quality and resoltion of the figures should be improved.

We have provided separate versions of the figures in order to clarify in case that pasting the figures resulted in reduced resolution.

 

Reviewer 4 Report

Comments and Suggestions for Authors

The study is very interesting; the combination of electrospinning techniques, 3D printing, and substrates presents a significant challenge for developing a new material. The following aspects should be considered:

1. It is suggested that higher-resolution SEM micrographs be presented to demonstrate pores, fiber arrangement, and cracks, since a 40-micron dimension is too large to observe aspects such as the arrangement of a fiber that will be at most 1 micron.
2. An individual characterization of each layer must be presented, because, for example, the diameter of the polycaprolactone fiber obtained is unknown.
3. Details of the 3D printing process are not described or presented; however, an indication of how the printing variables affected the materials is required.
4. A review of the adhesion between the different types of materials is requested.
5. Although mechanical friction tests were presented, it is considered that the integrity of the composite material be analyzed through tensile tests.
6. It is suggested to present a physical explanation of the results in the discussion of results, because this argument will allow us to understand the phenomena that occurred.

Author Response

1. It is suggested that higher-resolution SEM micrographs be presented to demonstrate pores, fiber arrangement, and cracks, since a 40-micron dimension is too large to observe aspects such as the arrangement of a fiber that will be at most 1 micron.

We have added images of higher magnification in the supplementary section.

An individual characterization of each layer must be presented, because, for example, the diameter of the polycaprolactone fiber obtained is unknown.

Perhaps the explanation of how the system was produced was not sufficiently clear, and this may lead to confusion.

The system consists of two parts: a receiving substrate (a 3D-printed PLA part using FDM technology). The other part is a mat of PCL fibers produced by electrospinning, onto which tracks were printed using an inkjet printer and a commercial ink containing silver nanoparticles.

Finally, using a thermal transfer system, the tracks were transferred to the PLA parts and cured to sinter the particles present in the tracks and achieve electrical conductivity.

The effect of each printed layer with the silver ink was characterized before and after transfer and before and after curing. More SEM images were added to supplementary information

Details of the 3D printing process are not described or presented; however, an indication of how the printing variables affected the materials is required.

The printing conditions are detailed in the experimental section. These are mostly standard conditions that we use for printing PLA pieces for everyday use. The objective of the work was not the optimization of the 3D printing process, but the optimization of the inkjet printing process and the transfer system to be applied to a 3D-printed piece of PLA.

1. A review of the adhesion between the different types of materials is requested.

The adhesion of PCL to PLA was described previously. A reference was added in the introduction. Moreover, SEM images after rub resistance tests show that PCL remains adhered to PLA.

Although the transfer process to PET and TPU is shown in the main text, it is only a proof of concept demonstrating the potential of this system.

Both TPU and PET could be used as printing substrates, avoiding the transfer steps. However, it is important to note that the transfer system is versatile and modular and could potentially be applied to flexible substrates.

1. Although mechanical friction tests were presented, it is considered that the integrity of the composite material be analyzed through tensile tests.

The material's tensile strength was not evaluated because the contribution of the 14-micron layer on the 7-mm PLA part is negligible.

It is important to remember that the system is not entirely 3D printed. Furthermore, adhesion is the most important property in this type of system. Tensile strength would not provide additional information on the adhesion of the transfer but rather on the behavior of the entire assembly, which, given its characteristics, will be very similar to those of the PLA part.

1. It is suggested to present a physical explanation of the results in the discussion of results, because this argument will allow us to understand the phenomena that occurred.

The phenomenon that occurs during curing is the sintering of adjacent metal particles, which join together to form a conductive path. This has been widely reported, and the ink is commercially available.

The innovation lies in the ability to print on a layer of fibers, which will then be thermally transferred and subsequently cured at a higher temperature to induce sintering of the metal particles and obtain conductive tracks. Furthermore, given the porous nature of the material, a track is obtained that contains nanoparticles on both sides of the fiber mat. This peculiarity gives the system a modular nature, allowing the joining of transferred and cured tracks in different steps.

 

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

It is recommended to include the steps taken to optimize the transfer temperature in the manuscript. 

Otherwise response is satisfactory. Paper can be accepted for publication.

Author Response

We have included the optimization procedure for the transfer temperature in the supplementary section and a short reference in the main text..  

Reviewer 2 Report

Comments and Suggestions for Authors

The authors have addressed most of the previous comments, and the manuscript has improved in clarity and overall presentation. However, I still believe that a brief comparative discussion with other conductive materials employed in printed electronics would be important to more fully contextualize the work within the relevant literature. I therefore recommend minor revision before the manuscript can be considered for publication.

Author Response

We have already added Table 3 to the main text, which compares the properties of conductive traces made of different materials such as copper and gold, and fabricated by different techniques such as sputtering, screen printing and inkjet printing. Moreover, the properties of printed tracks using our transfer method were compared to those printed using the same ink and more conventional substrates and printing parameters.  The latter comparison in terms of ink consumption was also added in a paragraph in the main text and in Table S3. We do not feel that a comparison with conductive materials such as carbon or conductive polymers would be beneficial since they are less conductive than metals and can probably be applied using the same method that we described by carefully selecting an adequate ink composition and adjusting the printing parameters.

Reviewer 3 Report

Comments and Suggestions for Authors

The authors have answered my questions. 

Author Response

Thank you for your comments.

Reviewer 4 Report

Comments and Suggestions for Authors

The authors consider the observations given, so the article is approved for publication

Author Response

Thank you for your comments.