Table of Contents

Scope

The Lexicographic Data Seal of Compliance (LexSeal) is a proposal for a community-based certificate of compliance with best scholarly practices to be awarded to individual lexicographic datasets in recognition of their creators' self-assessed and well-documented adherence to the principles of trustworthiness, interoperability, stewardship, citability, reciprocity and openness. 1

These six dimensions of LexSeal taken together describe the infrastructural fitness-forpurpose of a given lexicographic dataset, i.e. the degree to which the dataset aligns with best practices and technical standards within a larger network of machine-and humanreadable lexicographic data.

What is a lexicographic dataset?

A lexicographic dataset is a machine-readable lexicographic resource. Machine readable here means that the text of the dataset can be reliably processed, extracted and manipulated as text. Scanned images of lexicographic resources lie outside the scope of LexSeal.

What is a lexicographic resource?

A lexicographic resource is a type of information resource which lists and/or describes some or all the words in one or more languages, regardless of its specific purpose (documenting a historical, contemporary or an endangered language; helping learners learn a new language, enabling part-of-speech tagging and other kinds of NLP annotations etc.)

Who awards the certificate?

The certificate shall be awarded by a cross-institutional governing body, preferably under the auspices of ESFRI Landmark Research Infrastructures DARIAH and CLARIN, as well as the future ELEXIS Association.

Why certify in the first place?

The creation and curation of digital datasets, in general, and lexicographic datasets, in particular, requires time, effort and professional skills. Exploring existing datasets is an integral part of research discovery, while sharing one's own lexicographic datasets can lead to better returns on one's own professional and institutional investment by establishing connections and encouraging collaboration with other dataset creators. Certification as a quality assessment mechanism has a double purpose: it facilitates successful, efficient and productive data-sharing practices, and functions as a reward system for dataset creators. 2

LexSeal is a trust-based system aimed at three groups of stakeholders: creators and dataset providers, funders and dataset users. Each group can benefit from certification in different ways:

• creators and dataset providers can show to both their users and their funders that an independent authority has validated, evaluated and endorsed the overall infrastructural fitness-for-purpose of a given dataset; the visibility and citability of certified datasets could contribute to their creators' scholarly and professional prestige, whereas the certification process itself could become a mechanism for intellectual exchange on best practices in the domains of lexicography and lexicographic infrastructures; • funders can use the certification mechanism as a way of ensuring that the funded lexicographic datasets meet a set of community-based best practices and criteria, and, by extension, that their investment was well spent, resulting in the creation of interoperable resources that are well-documented; and, finally, • dataset users can more easily assess the extent to which a given dataset meets their particular scholarly or professional needs, the degree to which it adheres to best practices and the conditions under which it can be reused; in addition, they can find out who to get in touch with in case they have questions or suggestions.

Why certify in the long run?

The proliferation of community-certified lexicographic datasets can have a positive effect on the solidification and expansion of a sustainable data-sharing ecosystem by giving additional visibility to existing resources and by encouraging best practices in the creation of new resources.

What LexSeal is not?

• LexSeal is not a repository certification system. Those already exist (see Background below). We focus on individual datasets because we want to facilitate access to and reuse of specific lexicographic datasets while recognizing their inherent richness and diversity. Stable hosting of lexicographic datasets is only one of the parameters to be evaluated as part of the LexSeal certification process. • LexSeal is not a data representation standard. It is a certification mechanism that builds upon the existing standardization activities (TEI, ISO, W3C etc.) and evaluates them in the context of overall accessibility, reusability and sustainability of lexicographic datasets. • LexSeal passes no judgement on the methodological soundness of a given lexicographic resource, the quality of its linguistic content or the professional expertise of its creators. As such, it is no replacement for the scholarly assessment of lexicographic resources which one encounters in the scholarly literature (journal articles, book reviews etc.), but rather complementary to it.

Background

LexSeal has been long in the making. The discussions around a certification mechanism for lexicographic datasets were initiated by Toma Tasovac and Laurent Romary in the context of the DARIAH Working Group "Lexical Resources" against the background of a number of existing initiatives and trends in the scholarly communities and research infrastructures ranging from standards and FAIR principles to certified data repositories and the Heritage Data Reuse Charter.

Standards

Various standards and data formats are increasingly affecting the workflows of researchers and end users, and not only those directly involved in the development and application of computational methods. Professional and amateur lexicographers do not have to be computational linguists or NLP experts to use generic dictionary writing systems, such as Lexonomy, or dictionary viewers, such as LEX2 , but they do need to be aware of what 3 4 structured data is, why it matters, how to create it and how to use it. They also need reliable information about lexicographic datasets so that they can more easily assess their usefulness in a given context.

FAIR Principles

The FAIR principles for scientific data management and stewardship provide guidelines for 5 improving machine-actionability (i.e. the capacity of computational systems to process and interact with data with no or minimal human intervention) in terms of their findability (via machine-readable metadata), accessibility (via the use of standardized communication protocols), interoperability (by being open to integration with other data or workflows for analysis, storage or processing); and reusability (via well-described metadata attributes and clear data usage licensing). While the FAIR principles have received wide political support 6 and are well-established in science, technology and innovation domains, there is still 7 significant room for improvement: • FAIR Data principles are generic and discipline-agnostic principles that have emerged in the context of natural sciences. On a day-to-day level, researchers and 8 end users are looking for data not in the abstract, but always from a very specific disciplinary angle. For example, to properly assess the fitness-for-purpose of a lexicographic dataset, lexicographers and researchers need specific information that goes beyond generic metadata formats and focuses on the specific questions of linguistic scope, structural model, markup granularity etc. FAIR Data on its own is not sufficient for such queries. • FAIR Data principles focus more on metadata than content , which is why the 9 implementation of FAIR Data principles does not always facilitate actual content reusability. In fact, studies have shown that there is a significant gap between the self-identified adherence to the principles of FAIR, and concrete implementations .

10

• FAIR Data principles focus on end products rather than workflows. That is why the more dynamic principles of provenance and stewardship are needed to account for different life cycles of datasets in order to guarantee their sustainability. workflow whereby data repositories provide evidence that they are sustainable and trustworthy by conducting an internal self-assessment, which is then peer-reviewed. We have adopted a similar approach in the development of the LexSeal but with a focus on individual lexicographic datasets. Stewardship and Trustworthiness) have been selected in order to articulate mutual commitment to clearly articulated conditions of reuse, data sharing formats, standards, processes and protocols, hosting and maintenance responsibilities, as well as the richest possible track of documentation and provenance information. Because data reuse is high on the list of priorities of research infrastructures and projects such as ELEXIS, the proposed Lexical Data Seal of Compliance has been aligned with the principles of the Heritage Data Reuse Charter.

Certified repositories

The Heritage Data

LexSeal in context

LexSeal promotes the use of standards and standard data formats, while acknowledging the reality that not all datasets are indeed standardized and that proper documentation can go a long way in assuring the reuse of all datasets.

LexSeal grounds generic FAIR Principles in the disciplinary realities of lexicographic research and practice. As a domain-specific implementation, LexSeal is more substantial than FAIR, but nonetheless situated in a minimally intensive workflow based on selfassessment questionnaires that should be relatively easy for dataset creators and providers to respond to. A LexSeal-certified lexicographic dataset is a FAIR dataset, but the domainspecific information required for LexSeal compliance makes it eminently richer and more useful. Unlike FAIR, LexSeal is a truly community-governed initiative from and for experts who work directly with lexicographic datasets.

The current proposal is a product of ongoing discussions between the DARIAH WG "Lexical Resources", the ELEXIS Standards Committee and the ELEXIS Integration and Sustainability Committee.

Challenges Evaluation culture

The challenge of providing and adopting clear evaluation criteria and procedures for digital publications in general have been highlighted both by individual scholars as well as Each approach comes with its own set of challenges. The traditional pre-publication peerreview solved the problem of the publication and dissemination costs in the age of print: a selection process had to be established because it was too expensive to print everything. But this kind of pass/fail selection process is completely outdated in the digital age: it takes too long, and has questionable implications for innovation. Certification mechanisms, on 18 the other hand, often require significant organizational investment and may not seem at first necessarily appropriate for individual datasets: "it takes more than checking boxes like "TEIbased", "Open Access" and "Long Time Archiving" to conceive such a certification model for digital scholarly editions in order for it to be truly useful." 19 The proposed Lexical Data Seal of Compliance is an attempt to create a community-run certification mechanism based on the peer-review of not datasets themselves, but selfassessment reports provided by their creators. The proposed mechanism is based on shared responsibilities and trust-building among the involved stakeholders. Like standardization efforts in general, certification is also a social construct: for its value to be established, a critical mass of users and endorses will be needed.

TRUSTWORTHINESS

The trustworthiness of the lexicographic resource is assessed by the degree to which the given dataset is documented in terms of its scope, content, authorship and provenance.

Baseline Criteria Advanced Criteria

Expression of scope

INTEROPERABILITY

The interoperability of the lexicographic dataset is assessed by the degree to which a given dataset is documented in terms of its structure, format, reusability and licensing

Baseline Criteria Advanced Criteria

Text format

STEWARDSHIP

The stewardship of a given lexicographic dataset is assessed in terms of the commitment to stable curation and hosting of the lexicographic dataset.

Baseline Criteria Advanced Criteria

• Is somebody in charge of curating the lexicographic dataset? • Is the information provided in this questionnaire available to the users as part of explicit metadata or in some other form of documentation (for instance, in a README file)? • Is there stable institutional hosting and provisions for the long-term availability of the dataset?

• Is the dataset hosted in a CoreTrustSeal compliant repository

LexSeal+ Assessment Categories

CITABILITY

The citability of a lexicographic dataset is assessed in terms of the degree to which the user is made aware of the preferred way of referring to the dataset.

Baseline Criteria Advanced Criteria

• Is there a clear statement on how to refer to the dataset in a human-legible fashion (in scholarly publications, for example)?

• Is there a clear statement on how to refer to parts of the dataset (for instance, an individual entry, or an individual sense) in a human-legible fashion? • Are there technical means to refer to the resource or parts thereof (PIDs, querying mechanisms etc.)?

OPENNESS

The openness of a lexicographic dataset of a lexicographic dataset is assessed in terms of the provision of free, unrestricted access to the content.

Baseline Criteria Advanced Criteria

• Are parts of the lexicographic dataset available for free, unrestricted access, download and reuse (for instance, a lemma list, or partial/specific entry components such as alternative spellings, phonetic transcriptions, sense divisions, synonyms/antonyms, usage information, definitions, examples, translation equivalents, MWEs etc.)?

• Is the whole lexicographic dataset available for free, unrestricted access, download and reuse?

RECIPROCITY

The reciprocity of a lexicographic dataset is assessed in terms of the provider's openness to receive feedback from the users.

Baseline Criteria Advanced Criteria

• Is there a contact person that the user can submit questions about the resource to?

• Is there a possibility for users to submit suggestions for improvement of the resource (corrections, enrichment etc.)?

  Acknowledgements ................................................................................................................................ Scope ...................................................................................................................................................... Background ............................................................................................................................................. Challenges ............................................................................................................................................... EvaluaTon culture ............................................................................................................................... CompeTng interests .......................................................................................................................... Dynamic landscape ........................................................................................................................... Required effort .................................................................................................................................. CerTficaTon .......................................................................................................................................... Levels ................................................................................................................................................ Self-assessment ................................................................................................................................ LexSeal Assessment Categories .................................................................................................... LexSeal+ Assessment Categories .................................................................................................. Review process ................................................................................................................................. Governance .......................................................................................................................................... Future perspecTves .............................................................................................................................. Short term (2021-2022) .................................................................................................................... Medium term (2022-2025) ............................................................................................................... Long term (2025 and beyond) ..........................................................................................................

  research institutions and scholarly associations. Broadly speaking, the available 15 approaches can be grouped into two broad categories: • traditional peer-review, exemplified, for instance by RIDE: A Review Journal for Digital Editions and Resources and 16 • certification systems such as the DINI-Zertifikat for Open Access Publication Services, provided by the Deutsche Initiative für Netzwerkinformation or the already 17 mentioned DataSeal and CoreTrustSeal initiatives.

  See also Laurent Romary and Erzsébet Tóth-Czifra. 2019. Open Access guidelines for the arts and humanities: Recommendations by the DARIAH European research infrastructure consortium. ⟨halshs-02106332⟩

	1	These principles have been put forward and validated by communities around the Heritage Data Reuse Charter https://
	datacharter.hypotheses.org/.

  Stable and trustworthy hosting of datasets has been recognized as a key infrastructural challenge for providing long-term access to various types of data. Community-based initiatives have led to the development and implementation of important core-level certification mechanisms for data repositories such as the Data Seal of Approval (2008-2018) and the CoreTrustSeal. Core certification involves a minimally intensive

	11	12

  DARIAH to agree on The Heritage Data Reuse Charter as a set of principles and mechanisms for improving the conditions for the use and re-use of cultural heritage data by researchers. These high-level principles (Reciprocity, Interoperability, Citability, Openness,

		13
	10	For instance, a survey of 100 datasets in journals that publish ecological and evolutionary research with a strong
	personal digital archiving policy found that 56% of them were incomplete, and 64% were archived in a way that partially or
	entirely prevented reuse. See Roche DG, Kruuk LEB, Lanfear R, Binning SA. 2015. Public Data Archiving in Ecology and
	Evolution: How Well Are We Doing? PLoS Biol 13(11): e1002295. https://doi.org/10.1371/journal.pbio.1002295.
	11	Developed in 2008 by DANS (Data Archiving and Networked Services) in response to the requirement from two Dutch
	funding organizations KNAW and NWO to create a seal of approval that would help ensure that archived data can still be
	found, understood and used in the future. In the first quarter of 2009, the DSA was handed over to an international board.
	DSA was focused on the certification of data repositories through peer-reviewed self-assessment. Aimed at data
	providers, funders and end-users, DSA assured the reliability, accessibility and reusability of stored data. See Leeuw, L.
	2019. Data Seal of Approval (DSA). DANS. https://doi.org/10.17026/dans-28z-njxq

Reuse Charter

A number of European organizations such as APEF, CLARIN, Europeana, E-RIHS and projects such as Iperion-CH and PARTHENOS joined forces under the leadership of On the dominant impact on STEM on FAIR and the need for domain-specific implementations, see Deniz Beyan, Oya, 8 Chue Hong, Neil, Cozzini et al. (2020). Seven Recommendations for Implementation of FAIR Practice. Zenodo. http:// doi.org/10.5281/zenodo.3904140. See, for example European Commission Expert Group on FAIR Data (2018), Turning FAIR into Reality: Final Report 9 and Action Plan https://ec.europa.eu/info/sites/default/files/turning_fair_into_reality_1.pdf. doi: 10.2777/1524. In 2018, the DSA was merged with the ICSU Word Data System (WDS) into a CoreTrustSeal, an international, 12 community based, non-governmental, and non-profit certification organization promoting sustainable and trustworthy data infrastructures. To manage its finances, CoreTrustSeal is a legal entity under Dutch law (CoreTrustSeal Foundation Statutes and Rules of Procedure) governed by a Standards and Certification Board composed of 12 elected members representing the Assembly of Reviewers. Since 1 February 2018, an administrative fee of EUR 1,000 is charged for the review of a CoreTrustSeal certification. See https://www.coretrustseal.org/apply/administrative-fee/

14

  Erzsébet Tóth-Czifra, Laurent Romary. 2020. The Heritage Data Reuse Charter: from principles to research workflows. In Digital Technology and the Practices of Humanities Research, edited by Jennifer Edmond. Open Book Publishers. https://doi.org/10.11647/ OBP.0192.06.

	13
	⟨halshs-02475692⟩
	14	See, for instance, Baillot, Anne. 2016. 'A Certification Model for Digital Scholarly Editions', October. https://
	halshs.archives-ouvertes.fr/halshs-01392880; Eve, Martin. 2020. 'Violins in the Subway: Scarcity Correlations, Evaluative

Cultures, and Disciplinary Authority in the Digital Humanities'. In Digital Technology and the Practices of Humanities Research, edited by Jennifer Edmonds. Cambridge: Open Book Publishers; Takats, Sean. 2013. 'A Digital Humanities Tenure Case, Part 2: Letters and Committees'. Quintessence of Ham. http://quintessenceofham.org/2013/02/07/a-digitalhumanities-tenure-case-part-2-letters-and-committees/; and Zundert, Joris J. van, Smiljana Antonijević, and Tara L. Andrews. 2020. '6. "Black Boxes" and True Colour -A Rhetoric of Scholarly Code'.

  See for instance, the criteria developed by the Institut für Dokumentologie und Editorik: Patrick Sahle et al. 2014.

	Self-assessment
	LexSeal Assessment Categories
	humanities. In Digital Scholarly Editing: Theory, Practice, Methods, Université d'Anvers, Oct 2016, Anvers, Belgium.
	⟨halshs-01392880⟩
	10

Acknowledgements

The authors of this proposal would like to express their thanks tо members of the DARIAH Working Group "Lexical Resources", the ELEXIS Standards Committee and the ELEXIS Integration and Sustainability Committee for their feedback on earlier versions of this document.

Competing interests

As a knowledge domain, lexicography is of interest to both the scholarly and the commercial sector. Researchers have a vested interest in having full access to lexical data, while 20 commercial providers have a vested interest in controlling access to their intellectual property. The two interests are in fundamental opposition to one another and impossible to reconcile in every single instance.

Even within the public sector itself, the landscape is varied and full open access has not yet become the established norm in every part of the world. Some publicly funded institutions may provide access to their lexicographic content via graphical user interfaces such as online portals or dedicated applications, but they may still not make the source data available for download or via APIs.

Yet, regardless of the degree of access which the data provider is ready to open up to, all the stakeholders have an interest in having access to clear and transparent information about lexicographic datasets. A dataset provider may not wish to grant direct access to the content of a given resource, but would have nothing to lose from granting access to the highquality metadata about the resource, including but not limited to the questions of possible licensing models. A researcher, on the other hand, may prefer full access, but would also find it useful, in cases when such access is not open and free, to learn about the resource itself and the attached licensing opportunities.

Discussions around this topic included proposals to develop a multi-tiered LexSeal certification system (gold, silver and bronze) which would address the gamut running from closed, commercial datasets to fully open datasets, but this approach was abandoned for practical reasons: a certification system which is based on the hierarchical assessment of openness would not be very attractive to commercial providers, because they would know from the beginning that they could attain only the "lowest" version of the LexSeal. At the same time, limiting certification eligibility to only publicly available datasets would paint an unrealistic picture of the domain.

For LexSeal to be maximally attuned to the realities in which lexicographic datasets are subject to competing interests and legitimately diverse levels of openness, we propose to provide two types of certification:

1. LexSeal would certify the information about the resource itself, the degree to which it is well-documented and aligned with best practices; and 2. LexSeal+ would, in addition, certify the degree to which the resource adheres to the principle of open access and collaborative knowledge creation.

A large portion of dictionaries for general users that are produced today are aimed at the mass market and produced by 20 commercial publishers. On the notion of a "dictionary for general users," see Henri Béjoint.

Dynamic landscape

The proposed certification system for lexical datasets should be capable of evolving over time. Technologies change, new standards may appear, new business models for providing access to lexical resources may be developed. We do not want LexSeal to solidify the current state of affairs. Instead, we want LexSeal to be able to move with the times.

To address this challenge, we propose that the LexSeal certificates be versioned objects.

The first official release of the LexSeal, which we expect to come into being after the official constitution of the Governing Body, will be versioned as LexSeal 1.0. Similar to, for instance, Creative Commons licenses, which have gone through four versions between December 2002 and November 2013, we propose that LexSeal be built, from the ground up, as a versionable system.

The criteria for making minor and major updates shall be left to the future Governing Body.

Required effort

It is very difficult to predict the scope of the institutional infrastructure needed for running and maintaining the LexSeal as a community-based certification system. Can this process be incorporated into the day-to-day business of the governing bodies without much disruption? Will the institutional overhead of maintaining the quality control of the process be manageable? And, finally, will the community buy-in in terms of peer-review volunteer contributions be significant enough to ensure a smooth operation?

To answer these and similar questions, we propose the following mitigating measure: after completing the ELEXIS deliverable but before negotiating a possible formal agreement with DARIAH and/or CLARIN, the ELEXIS team behind the LexSeal will run a trial review process. The process take several steps:

• drafting the full Self-Assessment Questionnaire based on the recommendations expressed in this report (see below); • inviting members of the DARIAH Working Group "Lexical Resources", ELEXIS partners and, potentially, ELEXIS Observers to volunteer a small number of lexicographic datasets for trial evaluation; • inviting members of the DARIAH Working Group "Lexical Resources", interested ELEXIS partners and, potentially, colleagues from the ELEXIS Observer Organizations to participate in a trial review process as reviewers. • conducting a trial review process; and • evaluating the process once it has been completed.

The trial period will be used to gain real-life experience and identify possible shortcomings in both the workflow and the drafted topics for the self-assessment questionnaire. The institutions volunteering for the trial period shall be made aware that no Lexical Seal will be awarded at the end of the trial period and that the future Governing Board may require that they resubmit or update their self-assessment questionnaires, should LexSeal become an officially recognized certification mechanism.

Certification

The Lexicographic Data Seal of Compliance is established through a three-stage process:

• a self-assessment questionnaire by creators and providers of lexicographic datasets; • a review of the self-assessment questionnaire by members of the LexSeal Assembly of Reviewers • approval of the LexSeal review by the Governing Board

Levels

We propose two levels of certification: LexSeal and LexSeal+.

LexSeal is awarded to lexicographic datasets which meet the LexSeal baseline criteria of compliance in every LexSeal assessment category.

LexSeal+ is awarded to lexicographic datasets which, in addition to meeting the baseline criteria for compliance in every LexSeal assessment category, also meet the advanced criteria of compliance in three out of five LexSeal assessment categories and at least one LexSeal+ assessment category.

LexSeal assessment categories are:

LexSeal+ assessment categories are:

• Reciprocity • Openness A lexicographic dataset can be checked against the baseline and advanced criteria of compliance in each assessment category, with the exception of trustworthiness, which only has one required set of baseline criteria that all LexSeal-certified datasets must meet.

Each answer provided in the self-assessment questionnaire is assessed as satisfactory or unsatisfactory. A reviewer will mark the dataset as compliant in a given category if he or she considers all the answers in the given category and at a given criterium level as satisfactory.

Review process

Reviewers are members of the Assembly of Reviewers, which is not a fixed scientific committee but a fluid body, similar to the pools of reviewers that review scholarly articles or conference submissions.

The initial pool or reviewers are self-nominated and approved by the LexSeal Governing Body. A call for reviewers can be made to the wider lexicographic community via mailing lists (incl. but not limited to CLARIN, DARIAH, ELEXIS, incl. ELEXIS Observers). The Assembly of Reviewers shall be recruited dynamically: representatives of the reviewed lexicographic data providers are subsequently invited to become reviewers themselves.

Reviewers receive a certificate of recognition for their role in reviewing datasets, which they can mention on their CVs the same way that they highlight the journals or conferences that they write reviews for. Members of the Assembly of Reviewers can grant permission for their names to be listed on the LexSeal website.

Submissions shall be managed through an instance of Sciencesconf.org or a similar platform hosted by the DARIAH WG "Lexical Resources."

Resubmissions shall be allowed if:

• a previously evaluated resource has undergone substantial editorial changes which merit a new review (and potentially a "better" evaluation) • a resource has been rejected and the authors can document concrete improvements to the resource and its documentation since the last evaluation

Governance

The LexSeal shall be administratively anchored in the DARIAH WG "Lexical Resources" to ensure sustainability beyond the end of ELEXIS as a funded project.

We propose for LexSeal to be overseen by the LexSeal Governing Board, consisting of:

• 2 members from DARIAH, nominated by the DARIAH Board of Directors • 2 members from CLARIN, nominated by the CLARIN Board of Directors • 2 members from the ELEXIS legal entity which is expected to be formed in 2022

The role of the LexSeal Governing Board shall be:

• to vouch for the integrity of the review process • to appoint reviewers • to approve completed reviews • potentially, to adopt possible changes to the procedures • potentially, to prepare new versions of the LexSeal and offer them to the community for public consultations • potentially, to officially adopt new versions of the LexSeal after a round of public consultations New versions of the LexSeal will not be frequent, so it is unlikely that serving on the LexSeal Governing Board will consume an exorbitant amount of time. At the same time, we shouldn't minimize the effort required. The proposed trial review period (see above) will help us understand better the extent to which the Governing Board will need to be consulted in the review process.

Future perspectives

Short term (2021-2022) Medium term (2022-2025)

• Analysis of the community uptake • Infrastructural consolidation: discussions on the possibility of building a registry of LexSeal certified resources • Explore integration potential with SSHOC Marketplace, the TRIPLE discovery platform and, generally, EOSC • Scanning the landscape for possible project funding opportunities for implementing such integrations

Long term (2025 and beyond)

• Depending on the outcomes of the community uptake analysis, the assessment of the opportunities for infrastructural consolidation, and the identification of possible funding sources, work toward robust, technologically innovative solutions for disseminating information about LexSeal-certified datasets; and providing federated access to LexSeal+ certified datasets.


Dr Kätlin Rohtlaid

Prof Giao T M F Nguyen

Dr Caroline Soyer

Eric Cattan

Frederic Vidal

Cedric Plesse
email: cedric.plesse@u-cergy.fr

Frédéric Vidal

Cédric Plesse

Dr C Vidal

Lppi Plesse

Ea2528

Prof E Cattan

4-ethylenedioxythiophene):poly(styrene sulfonate)/polyethylene oxide electrodes with improved electrical and electrochemical properties for soft microactuators and microsensors

Keywords: layer-by-layer fabrication, microactuators, microsensors, PEDOT:PSS

de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Soft actuators, also called artificial muscles, are polymer-based materials or devices that are able to generate work when electrically stimulated, similar to biological muscles. [1,2] Electronically conducting polymers (ECPs), such as polypyrrole (PPy), polyaniline, and poly(3,4-ethylenedioxythiophene) (PEDOT) have been intensively investigated for this purpose not only due to their low operating voltages and large strains, but also, because they are soft, lightweight, easily processed and manufactured. [3][4][5][6] These polymers undergo reversible dimensional changes in response to electrical stimuli through redox process in the presence of the electrolyte. [1] Additionally, they can also operate in reverse mode as strain sensors, converting mechanical stimulation into electrical response. [7] The concept and possible designs of micro electromechanical actuators based on ECPs were demonstrated by Baughman in 1991. [3] Smela et al. demonstrated the fabrication of millimeter scale polymer fingers, made of PPy/gold bilayer strips in 1993. [8] Over the next years, Jager et al. reported the fabrication of different microstructures as microfluidic system, cell clinic, and microactuators for different biomedical applications, able to operate in different environments (salt solutions, blood plasma, urine, and cell culture medium). [9][START_REF] Pettersson | st Annual Int. IEEE-EMBS Special Topic Conf. on Microtechnologies in Medicine and Biology (Cat. No. 00EX[END_REF][START_REF] Jager | [END_REF][12][13] Even though these bilayer microactuators have been operated in special environments, they were still restricted to operation in liquid electrolyte, limiting their use in different applications.

The operation in open air can be obtained using trilayer configuration of the actuator, where two electroactive electrodes are separated by solid polymer electrolyte (SPE), playing the role of ion reservoir. The air-operating ECP-based microactuator was demonstrated first by Alici et al. in 2009. [14] The fabrication method was adapted from the macroscopic process and downscaling of the 155 µm-thick device was performed using laser ablation. Another way of realizing air-operating ECP-based

The development of microsystems is a rapidly evolving field which enables a wide range of applications for electroactive materials. Microelectromechanical actuators based on electronically conducting polymers are elaborated with an up-scalable process. Commercial poly(3,4-ethylenedioxythiophene):poly-(styrene sulfonate) (PEDOT:PSS) is chosen as an easily processable electrode to ensure the reproducibility for further technological production and practical applications. First, the improvement of electrical, electrochemical, and mechanical properties of the PEDOT:PSS electrodes is described by incorporating reactive additives, that is, glycol-based monomers (mPEG) as polyethylene oxide (PEO) network precursors. Moreover, the custom fit layer-by-layer (LbL) process to integrate these PEDOT:PSS/PEO composite electrodes into trilayer microactuators is presented. The incorporation of PEO within PEDOT:PSS improves significantly the electromechanical performances of the resulting microactuators with significant strain (0.82%) and high output forces (472 µN) compared to similar PEDOT based or pristine PEDOT:PSS microactuators. This work provides also the first demonstration that mechanical strain sensing behavior, extensively studied at macroscale, still occurs at microscale for these trilayer systems. Additionally, to this proof of concept, it highlights that output signal is significantly enhanced by downsizing the devices compared to similar macroscale samples. These results open promising perspectives in the development of numerous applications for soft and scalable microactuators and microsensors. microactuators was introduced by Khaldi et al. in 2011. [15] The microactuator was fabricated with interpenetrating polymer network (IPN) configuration, where two PEDOT electrodes were interpenetrated through chemical oxidative polymerization on both sides of a 10 µm-thick SPE membrane. Owing to the low thickness, the collective patterning of the actuators was achievable by photolithography and reactive ion etching (RIE). Most of the reported air-working microactuator fabrication methods required manual handling at some step of the process which makes it difficult and time-consuming to fabricate and downscale these devices. To minimize any manual handling, a layerby-layer (LbL) synthesis was recently developed by Maziz et al. by fabricating and operating trilayer microactuators directly on a substrate. [16] This process is based on sequential layer stacking on top of previous layers. The PEDOT layers were obtained from successive vapor phase polymerization (VPP) on the substrate and on the in situ polymerized SPE layer. This work also demonstrated the possibility of integrating bottom gold electrical contact with top-down approach on a flexible substrate. More recently, the electroactivity of the PEDOT electrodes was greatly improved by incorporating glycol-based monomers (mPEG) during the VPP. [17] These PEDOT electrodes were then used to fabricate microactuators with LbL synthesis and resulted in the improvement of the microactuator's performances with maximum strain of 0.5% and maximum output force of 11 µN. However, this process requires washing step in order to remove the oxidant and the residuals of the EDOT monomer after VPP synthesized electrodes, is poorly compatible with microsystem process and susceptible to promote lift-off from the substrate, arising in problems during the fabrication process.

Poly (3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) is a commercially available conducting poly mer dispersion. This aqueous dispersion has the advantage of easy processing through various methods, such as drop casting, spray coating, spin coating, and ink-printing techniques. [5,[18][19][20][21][22][23] Although PEDOT:PSS is known as a promising candidate for many applications due to its remarkable properties, it also has shortcomings when it comes to electrical and electrochemical properties. To overcome these issues, many research groups around the world have reported the enhancement of the electronic conductivity of PEDOT:PSS by thermal, light, acid, organic solvent, ionic liquid, and surfactant treatments. [24][25][26][27][28][29][30][31] Such improvements have made PEDOT:PSS a front-line player in organic electronic devices, such as solar cells and light-emitting diodes, [32][33][34][35][36] especially when stretchability is involved. However, most of the studies on PEDOT:PSS are focused on the improvement of electrical conductivity but not on their electrochemical properties. The improvement in electrochemical properties has been sparsely reported for Li-Ion batteries by the inclusion of lithium salts or by increasing the temperature of the electrochemical cell [37,38] or for energy storage devices, such as supercapacitors, by the incorporation of ethylene glycol and Triton-X surfactants. [39,40] Concerning electroactive soft actuators, the use of PEDOT:PSS as an electrode material has tremendously increased in recent years due to its flexibility and high thermal and chemical stability. Different research groups have reported the fabrication of PEDOT:PSS electrodes with different additives, such as ionic liquid, poly(ethylene oxide) (PEO), single-walled carbon nanotubes, and multi-walled carbon nanotubes to improve the performances of the resulting actuators. [41][42][43][44][45] Only a minor research has been reported for PEDOT:PSS microactuators. Taccola et al. demonstrated the fabrication of PEDOT:PSS/ SU-8 bilayer fingers but the limiting factor of this work was the operation in liquid electrolyte. [46] Põldsalu et al. reported the fabrication of ink-jet printed PEDOT:PSS microactuators where bending actuation was demonstrated in air. [47] Unfortunately, the process appears insufficient for microactuator fabrication, requiring flipping the substrate to obtain the PEDOT:PSS electrodes on both faces of the ionic membrane.

In this work, we propose a simple and effective fabrication method for PEDOT:PSS microactuators with LbL process. The polyethylene glycol methacrylate derivatives (mPEG) are incorporated into the PEDOT:PSS dispersion and its effect on electrical and electrochemical properties of the electrodes will be discussed. The Young's modulus, electrochemical behavior, bending actuation, and blocking force of the microactuators will be presented. Moreover, the mechanical strain sensing ability, widely described and studied for these types of materials at macroscale, has never been described on microscale devices. In this work, we demonstrate for the first time that the sensing properties of conducting polymer actuators are still occurring at microscale and present higher output voltage signal compared to similar type macroactuators.

Results and Discussion

The general procedure for the LbL fabrication and patterning of microactuators based on PEDOT:PSS/PEO composite electrodes is presented in Figure 1. Typically, it consists of successive stacking of electrodes and ionic conducting membrane followed by a final polymerization step to obtain a trilayer configuration device without any manual handling.

PEDOT:PSS/PEO Composite Electrodes

High electronic conductivity (σ) is necessary for obtaining fast and homogeneous bending along the length of the actuator. It is also found that the bending amplitude is ruled by the strain of electrically conducting polymer (ECP) (ε) which is linked to the strain to charge ratio α (m 3 C -1 ) and the volumetric charge density ρ(C m -3 ) of the conducting polymer layers by an empirical relationship: ε = αρ. [48] Therefore, the higher is the charge density, the higher will be the actuation strain due the number of inserted/expelled ions during the redox process. As a consequence, these parameters have to be maximized. Based on these principles, the study of electrical and electrochemical properties of the electrodes was carried out in the absence and in the presence of PEO network precursors (mPEG). In the latter, a radical polymerization initiator is introduced in order to perform mPEG polymerization leading to a PEO network within the PEDOT:PSS electrodes.

Since this work is dealing with the fabrication of microactuators, relatively thick PEDOT:PSS/PEO layers of about ten microns thick were targeted. The PEDOT:PSS electrodes with PEO network precursors (mPEG) were fabricated using casting method. [22,23] The PEO network precursors (expressed vs final mass of the electrode), containing 50 wt% poly(ethylene glycol) dimethacrylate (PEGDM) and 50 wt% poly(ethylene glycol) methyl ether methacrylate (PEGM) were added to the commercial Clevios PH1000 aqueous dispersion with water soluble radical initiator ammonium persulfate (APS, 3 wt% vs mPEG). The resulting solution was casted (0.08 ml cm -2 ) into previously prepared microscope slide molds, which were then placed on a heating plate at 50 °C to evaporate the water and to form freestanding PEDOT:PSS electrodes. A final heat treatment was carried out under argon during 3h 45 min at 50 °C and 1h at 80 °C to promote the free radical polymerization of mPEG within the PEDOT:PSS electrodes. These obtained PEDOT:PSS/PEO composite electrodes were immersed into ionic liquid (EMImTFSI) prior to the characterizations.

Effect of PEO Content

The effect on electrical and electrochemical properties of the PEDOT:PSS electrodes was studied as a function of the PEO content (0-60 wt%). The electrodes were fabricated as described in the previous section. Thickness, electronic conductivity and electroactivity of each electrode are characterized when swollen by electrolyte (EMImTFSI).

The thickness of each PEDOT:PSS/PEO layer was obtained by multiple scans (3 to 5) across the width of the casted layers (2 cm) and at different locations along the length (7 cm) (Figure S1, Supporting Information). Data and error bars demonstrate that, except the plate edges, casting method provides satisfying thickness homogeneity of the layers. Figure 2a presents the increase in thickness when the content of PEO is increased. The thickness is increasing from 7.3 µm (0% PEO) to 15.1 µm (60% PEO) due to the presence of higher solid content in the casting solution. Moreover, the presence of PEO precursors enhances the electronic conductivity of the PEDOT:PSS electrodes (Figure 2b). It must be mentioned that conductivity measurements have been made over distances close to the dimensions of our transducers (7 mm) and must be considered as an average value over these distances. The electronic conductivity of the pristine PEDOT:PSS electrode was 1 S cm -1 and reached the maximum value of 163 S cm -1 at a concentration of 40% PEO in the electrodes. Even though the electrode with 50% PEO in the electrodes presented slightly higher electronic conductivity, then some phase separation occurred, leading to partly wet and nonhomogeneous surface. As a consequence, the electrode composition with 40% PEO in the electrodes was considered as the maximum content to fabricate reproducible electrodes. Although insulating PEO phase is diluting the conductive part of the PEDOT:PSS/PEO electrode, the electronic conductivity is increased by two orders of magnitude. Similar improvement was observed for PEDOT:PSS-high molecular weight PEO composite thin film demonstrating that the beneficial effect of PEO derivative is observed not only in thin films, but also for thicker films. [33] As another important parameter for efficient bending type actuator, the influence of the PEO content on electrochemical properties of the PEDOT:PSS/PEO composite electrodes was studied. Figure 3 depicts the cyclic voltammograms of the electrodes with an inset of the volumetric charge density ρ. The cyclic voltammetry demonstrates a progressive increase in current when higher content of PEO is incorporated to the electrodes. The PEDOT:PSS electrode with 40% PEO presents high current density with clearly defined oxidation and reduction peaks. The presence of PEO as an additive allows the redox process to take place more efficiently within the PEDOT:PSS electrodes and then, allows the ion exchanges necessary for the volume variation of the layer. PEO is known to be a good material for ion transport and is probably helping ions to access the PEDOT domains within the composite electrodes. The same behavior was observed in the case of VPP PEDOT electrodes, where incorporation of PEO improved both, the electrical and electrochemical properties. [17] The volumetric charge density ρ allows to evaluate quantitatively the electroactivity of the PEDOT:PSS electrodes. The inset in Figure 3 confirms the increase in electrochemical properties when PEO is incorporated as an additive into the PEDOT:PSS solution: ρ increases from 8.0 × 10 6 to 4.5 × 10 7 C m -3 when 40% PEO is incorporated into the electrodes, demonstrating a sixfold increase compared to the pristine PEDOT:PSS electrode.

In conclusion, it was found that adding PEO as an additive into the PEDOT:PSS casting solution significantly increases both, the electrical and electrochemical properties of the electrodes up to a maximum threshold of 40% PEO in the electrodes. As a result, the PEDOT:PSS electrode containing 40% PEO was considered as the composite electrode for following investigation and is assigned as PEDOT:PSS/PEO electrode.

PEDOT:PSS Electrodes with 40% PEO

The morphology of the PEDOT:PSS electrodes was studied using Atomic Force Microscopy (AFM) in order to understand the effect of the additives in the electrodes. The pristine PEDOT:PSS electrode and the PEDOT:PSS/PEO electrode were analyzed as fabricated. The surface of the PEDOT:PSS film is modified from granular to fibrous structure by the addition of PEO network (Figure 4a,b). These morphological changes are also known to improve the conductivity of the PEDOT:PSS electrodes. [49][START_REF] Worfolk | Proc. Natl. Acad. Sci[END_REF][START_REF] Raj | [END_REF] The addition of PEO can induce phase separation between the hydrophobic PEDOT and the hydrophilic PSS, creating a better connected network between PEDOT fibers and allowing an improved electronic charge transport along the electrode. Moreover, the resulting fibrous and interconnected morphology is also most likely the reason for improved electrochemical properties of the PEDOT:PSS electrodes. Additionally to the presence of PEO, which should improve significantly the ionic conductivity within the PEDOT:PSS layer, [52] such morphology is possibly providing higher accessibility of PEDOT domains to ionic species.

In order to evaluate the dimensional stability of the PEDOT:PSS-based electrodes in the presence of PEO, the obtained electrodes were immersed into water. The effect of each precursor or initiator (mPEG or APS) on PEDOT:PSS electrodes was studied separately and the results are presented in Figure S4, Supporting Information. Figure 5a,b shows that the pristine PEDOT:PSS film broke down into small pieces after 10 min. On the other hand, the PEDOT:PSS/PEO electrode was stable and remained as a strong film even while removing from the water. It was still possible to handle and to stretch the electrode without breaking it into pieces (Figure 5c). These results demonstrate the beneficial effect of additives to fabricate PEDOT:PSS electrodes with improved electrical but also electrochemical and mechanical properties which is an asset for the elaboration of robust and efficient electromechanical microdevices.

PEDOT:PSS/PEO Based Microactuators

The synthesis of the trilayer actuators was performed according to LbL synthesis, described elsewhere. [16] The process was tailored to be compatible with PEDOT:PSS as an electrode material.

Microactuators with pristine PEDOT:PSS and PEDOT:PSS/PEO composite electrodes were fabricated in order to evaluate the effect of PEO on the performances of the resulting PEDOT:PSS microactuators.

Fabrication of PEDOT:PSS/PEO Microactuators

The fabrication process of the trilayer microactuators (Figure 6a) is described in detail in the experimental section, but shortly, it combines casting of the PEDOT:PSS-based layers and spin-coating and polymerization of the semi-interpenetrating poly mer network (semi-IPN) precursors. The semi-IPN layer is composed of PEO network and nitrile butadiene rubber (NBR). These steps allow precise control in thickness of every layer because the volume of the casted solution and the rotation speed of spin-coater can be easily adjusted. In the last step, the microactuators are obtained via laser ablation technique and are swollen in EMImTFSI before further characterizations. Resulting laser patterned PEDOT:PSS-based microactuators are illustrated in Figure 6b. The length and the width of the obtained microbeams were 6.8 and 1.2 mm, respectively. The thicknesses were 29.7 and 34.6 µm for the pristine PEDOT:PSS and PEDOT:PSS/PEO microactuators, respectively.

Mechanical Properties

Stiffness of the beams was measured during mechanical bending experiments. The stiffness is calculated from the relationship between the applied displacement and resulting forces, allowing to determine the Young's modulus of the microbeams. The PEDOT:PSS microactuator has relatively high Young's modulus (2.8 GPa) compared to the PEDOT:PSS/PEO microactuator (0.9 GPa) (Table 1). This trend has also been reported in the literature, suggesting that incorporating additives dilutes the casting solution and helps to form more fibrous PEDOT:PSS with less dense structure and therefore lower Young's modulus. [49] The equation below is then used to derive Young's modulus of the electrodes separately: ). [53] The derived results for the electrodes are brought in Table 1. The pristine PEDOT:PSS electrode presents really high Young's modulus (5.8 GPa), which is also higher compared to the values reported in the literature for PEDOT:PSS films (≈2.5 GPa). [54,55] The derived modulus of the PEDOT:PSS/PEO electrode is 1.6 GPa. Overall, these results demonstrate softening of the microactuators and electrodes when PEO is incorporated to the PEDOT:PSS layers.

Electrochemical Properties

The electrochemical properties of the PEDOT:PSS microactuators were determined using cyclic voltammetry (± 1.0 V, ± 2.0 V, 20 mV s -1 ). Figure 7 illustrates the cyclic voltammetry of the microactuators with an inset of volumetric charge density ρ. Both of the microactuators present double-layer capacitor-like behavior at a potential window of ± 1.0 V. The curve of the PEDOT:PSS microactuator remains similar also at wider potential window (± 2.0 V), indicating that the resistive behavior of pristine PEDOT:PSS electrodes prevents efficient redox process to take place. The PEDOT:PSS/PEO microactuator shows a redox process occurring at higher potential range (± 2.0 V). These results are consistent with the work of Okuzaki et al. where it was found that IL/PU/PEDOT:PSS actuators present capacitor behavior at lower voltages and a redox behavior at voltages higher than 1.5 V. [41] The inset of volumetric charge densities confirms the increasing trend of electroactivity at higher potentials and even further when PEO is incorporated to the PEDOT:PSS electrodes. The resulting values of ρ for PEDOT:PSS microactuator are 9.0 × 10 6 and 2.6 × 10 7 C m -3 at voltage windows of ± 1.0 and ± 2.0 V, respectively. The corresponding values for PEDOT:PSS/PEO microactuator are 2.7 × 10 7 and 7.7 × 10 7 C m -3 at voltage windows of ± 1.0 and ± 2.0 V. The volumetric charge density of the PEDOT:PSS/PEO microactuator is three times higher at both voltage windows compared to the pristine PEDOT:PSS microactuator. Higher electrochemical performances of the PEDOT:PSS/PEO microactuators can be related, as for electrode study, to the fibrous structure of the PEDOT:PSS electrode and the presence of PEO (AFM image, Figure 4), promoting good ion transport in the PEDOT phase and improving therefore the charge transport in the electrodes.

Actuation Characterization

The actuation characterizations of the microactuators were performed as a function of square wave potential between 0.4 and 2.2 V at a frequency of 0.1 Hz. It can be seen from Figure 8, that applied potential has an effect on the strain difference of both microactuators. For both samples, the strain difference is increasing with the voltage and seems to reach a plateau beyond 1.6 V, indicating that the maximum deformation of the device is reached. Regardless of the applied potential, the strain difference is notably higher in the presence of PEO. The pristine PEDOT:PSS microactuator reached the maximum value of 0.19% at the highest applied potential of 2.2 V. This value is relatively low and can be explained by both, low electronic conductivity and low volumetric charge density of the pristine PEDOT:PSS electrodes. The PEDOT:PSS/PEO microactuator reached the maximum value of 0.82% also at 2.2 V. It confirms that the incorporation of PEO is also beneficial for the strain performances, most likely related to higher volumetric charge density and therefore a better charge transport in the composite electrodes, but also to the lower Young's modulus which facilitates their bending. Finally, it should be mentioned here that the bending direction during oxidation of the working electrode correspond to a volume contraction. In other words, during oxidation the EMI + cations are expelled from the oxidized PEDOT electrodes, which is consistent with the cation mobile mechanism previously reported for PEDOT macroactuators using EMITFSI as electrolyte. [56]

Blocking Force Characterization

The blocking force measurements were carried out as a function of applied potential at 0.1 Hz (Figure 9). At low potential (0.4 V), the generated forces for pristine PEDOT:PSS and PEDOT:PSS/PEO microactuators are 120 and 208 µN, respectively. As observed previously for strain, the output force is increasing with the increase of the potential for both microactuators until 1.6 V before reaching a plateau. The maximum values at 1.6 V for the PEDOT:PSS and PEDOT:PSS/PEO microactuators are 301 and 472 µN, respectively. This indicates that the output force is only improved by a factor 1.5 between pristine PEDOT:PSS and PEDOT:PSS/PEO microactuators, while a fourfold increase was measured for strain values. This result can appear surprising taken into account the enhanced electrical and electrochemical properties, but can be explained by an electromechanical model proposed by Alici et al. showing that the output force is directly proportional to the Young's modulus of the electrodes. [57] The values of Young's modulus (Table 1) demonstrated the electrodes softening when PEO was incorporated, which consequently may decrease the generated forces of the final microbeam. Supposedly it can be a trade-off between electronic conductivity, electroactivity, and Young's modulus where the first two improve the overall performances of the PEDOT:PSS/PEO microactuator but softening of the electrodes limit the output force. Nevertheless, output forces in the range of hundreds of µN can be obtained which is considerably higher compared to previously described LbL VPP PEDOT microactuators, where the maximum force was 11 µN. [17] This large improvement can be of course attributed to the geometry factor (two times thicker and two times shorter measurement distance in the present study) but also to the difference in Young's modulus between PEDOT obtained through VPP and PEDOT:PSS/ PEO electrodes, around 20 MPa [58] and 1.6 GPa respectively.

Sensor Characterization

Depending on external stimuli (electrical or mechanical stimulation), ECP-based devices are known to act both, as actuators or as strain sensors. The mechanism behind the voltage generation in sensor mode is not fully defined yet, but there are several proposed mechanisms with experimental characterizations to understand better this voltage generation. Wu et al. introduced the "Deformation Induced Ion Flux" model to explain the estimation of the magnitude and the polarity of the sensor response. [7] Shoa et al. proposed a model, indicating that external application of stress alters the internal stress of ions, helping to insert or remove charge and as a consequence, to change the voltage. [59] More recently, the "piezoionic effect" has been introduced to explain the voltage generation through inhomogeneous ionic distribution, based on the Donnan potential. [60] All these ECP sensors were only described at macroscale, with variation of open circuit voltages (OCV) usually in the range of mV or lower. However, there is no prediction based on theoretical model and no experimental demonstration to know if such behavior still occurs for microsized samples and which voltage amplitude can be generated. In this work, mechanical bending is applied to the microactuators and OCV is recorded. The upper layer of the device was connected to the working electrode and during upward bending displacement the top electrode was then compressed. The length and the width of the characterized microbeams were 6.8 and 1.2 mm, respectively. Figure 10a presents the sinusoidal mechanical stimulation (strain amplitude ± 0.5%) applied to the PEDOT:PSS/ PEO microbeam as a function of time with the corresponding OCV variation. It demonstrates a good correlation between strain and electrical response indicating that the detection of mechanical stimulation at microscale is possible. The signal is corresponding to an in-phase response, that is, a positive strain of the working electrode corresponding to an increase of the OCV. The sign of the voltage variation is consistent with previously reported results on PEDOT macro-samples in the case of ionic liquids where it was demonstrated that the more mobile ionic species are the cations. [56] During compression/expansion of the working electrode, the mechanically induced ion flux is promoting dominant motion of cations from or towards the electrodes, respectively. In other words, we demonstrate here that the same ion motion mechanism is taking place in macro-samples and also in the microsized ones.

Figure 10b presents the amplitude of OCV variation measured during mechanical stimulation of the microbeams for applied strain ranging between -0.5 and 0.5% at a frequency of 0.1 Hz. It can be observed that OCV is modified during mechanical stimulation and the variation increases linearly with the amplitude of the mechanical stimulation with a sensitivity close to 0.74 mV % -1 . These results demonstrate that conducting polymer-based microbeams are effectively behaving as strain microsensors, able to detect but also to quantify a mechanical deformation. The pristine PEDOT:PSS microsensor reaches a maximum value of 0.09 mV at a strain of 0.2%. It must be mentioned here that applying higher strain tends to break the samples, probably because of stiffer electrodes and consequently more brittle devices. The PEDOT:PSS/PEO microbeam presents almost identical response but can be stimulated up to 0.5%. For this amplitude, the OCV variation is equal to 0.42 mV. This result at a strain of 0.5% is approximately ten times higher compared to the values reported in previous work on thick trilayer macrodevices. [56] Therefore, downsizing such microdevices not only allows to still detect mechanical stimulation, but it also provides higher sensitivity. Considering that ion flux is driven by stress gradient across the thickness of the device, a decrease of the device's thickness for the same applied strain tends to proportionally increase the stress gradient and then can be the main factor of the observed sensitivity enhancement.

Figure 11 interprets the applied step displacement of -0.5% strain, that is, bending leading to the contraction of the working electrode, and the OCV variation (∆E) for PEDOT:PSS/PEO microsensor. The responses during 30 s, 1 and 5 min follow the same trend and a negative output voltage signal of approximately -0.4 mV was produced during the compression of the working electrode. However, the electrical signal evolution over time is different compared to other reported ECP-based trilayer sensors. Indeed, in the reported cases, an electrical signal relaxation is observed when maintaining constant strain, ultimately going back to initial value. [56] This typical relaxation phenomenon can be explained by the piezoionic effect introduced by Sarwar et al. [60] When applying mechanical stimulation, the mechanically induced cation flux from the compressed electrode is creating disequilibrium of ions and generating the OCV variation. However, since both, cations and anions are mobile in classical electrolytes or ionic liquids, the less mobile anions are also diffusing over time, to progressively compensate this concentration difference. As a consequence, the OCV variation is progressively going back to 0 mV. This behavior can be considered as a drawback since it allows detecting dynamic stimulation but prevents any direct relationship between the position of the device and the OCV value. Interestingly, in the case of the PEDOT:PSS/PEO electrodes, the high concentration of nonmobile PSS -anions within the electrodes probably tends to limit the relaxation compared to classical ECP electrodes. This situation seems to be similar to the behavior of IPMCs where only one type of mobile ions exist (cations), since anions are attached to the perfluorosulfonic membranes. In this case, no relaxation is also observed. [61] This result is especially interesting since it allows sensing not only dynamical mechanical perturbation but also relating the OCV value directly to the steady position of the microsensor, facilitating their further use in practical microsystems.

Conclusions

The solution casted PEDOT:PSS electrodes were fabricated counting PEO as a beneficial additive to improve the electrical and electrochemical properties of the electrodes. The incorporation of the PEO into the PEDOT:PSS electrodes resulted in high electronic conductivity (163 S cm -1 ) and high volumetric charge density (4.5 × 10 7 C m -3 ). The PEDOT:PSS-based actuators were fabricated with tailored LbL synthesis and micropatterned using laser ablation technique. Two types of microactuators with different electrode compositions (PEDOT:PSS and PEDOT:PSS/ PEO) were fabricated in order to evaluate the effect of PEO on microactuators performances. The PEDOT:PSS/PEO microactuators resulted in improvement of all the studied parameters compared to the pristine PEDOT:PSS microactuators and presented large strains (0.82%) and high output forces (472 µN). We also demonstrated for the first time that such microdevices are able to present sensing properties and that their sensitivity is even higher to that of their thick macroscopic counterparts. These results are opening promising perspectives in the fabrication of complex actuating/sensing microsystems where these easily processed electroactive materials could be combined together and interfaced with integrated electrical contacts.

Experimental Section

Materials: Poly(ethylene glycol) methyl ether methacrylate (PEGM, M n = 500 g mol -1 ), poly(ethylene glycol) dimethacrylate (PEGDM, M n = 750 g mol -1 ), ammonium persulfate (APS, 98%), and cyclohexanone (>99.8%) were obtained from Sigma-Aldrich and used as received. PEDOT:PSS aqueous solution (Clevios PH1000, solid content 1.0-1.3 wt%) was purchased from Heraeus Precious Metals GmbH & Co. Nitrile-butadiene rubber (NBR), initiator dicyclohexyl peroxydicarbonate (DCPD), and 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl) imide (EMImTFSI 99.9%) were used as supplied from LANXESS, Groupe Arnaud, and Solvionic, respectively.

Fabrication of Electrode Layers: PEDOT:PSS electrodes were fabricated using casting method. [22,23] The network of PEO precursors (mPEG) is composed of 50 wt% of PEGM and 50 wt% of PEGDM as the monomer and the crosslinker, respectively, which are added (0%-60%, vs final electrode, that is, 40-100% PEDOT:PSS, considering solid content of 1.15 wt%) to the commercial Clevios PH1000 PEDOT:PSS aqueous solution. Additionally, APS as a radical initiator for PEO precursors, was added (3 wt% vs mPEG) to the solution. The solution was stirred until dissolution and degassed. Microscope slides as the substrates were prepared for casting by covering the edges with a scotch, allowing for the solution to solidify in the "mold". The obtained PEDOT:PSS solution was casted (0.08 ml cm -2 ) into the molds and the substrates were then placed on a heating plate at 50 °C to let the water evaporate and to form free-standing PEDOT:PSS/mPEG electrodes. The heat treatment was carried out under argon, for 3h 45 min at 50 °C and 1h at 80 °C. The obtained PEDOT:PSS/PEO composite electrodes were swollen in ionic liquid (EMImTFSI) prior to the characterization.

Fabrication of Trilayer Microactuators: The synthesis of the trilayer actuators was performed according to LbL synthesis as described elsewhere. [16] This process was tailored to be compatible with PEDOT:PSS as the new electrode material (Figure 6) which does not require washing step. The PEDOT:PSS casting solutions were obtained by mixing Clevios PH1000 commercial solution, PEO precursors (0 wt% and 40 wt%), and APS (3 wt% vs PEO), as described in the previous paragraph for electrode fabrication. The solution was casted (0.08 ml cm -2 ) into previously prepared molds and the substrates were placed on a heating plate at 50 °C, to evaporate the water and to form self-standing electrodes.

The SPE, as an ion storage membrane and an insulator between two electrodes, is based on the semi-interpenetrating polymer network (semi-IPN) structure. The semi-IPN layer (PEO-NBR, 50-50 wt%) is polymerized in situ and is composed of a PEO network and a linear NBR, where the former provides the membrane with the ionic conductivity (when introduced to the electrolyte) and the latter with the mechanical properties. First, the NBR solution was prepared by dissolving NBR (20 wt%) in cyclohexanone and stirred until complete dissolution. The PEO precursors, consisting of PEGDM (25 wt% vs PEO network) and PEGM (75 wt% vs PEO network) were added to the NBR solution and stirred for 30 min. The radical initiator DCPD (3 wt% vs PEO network) was then introduced to the solution and the final solution was stirred until complete homogenization and degassed. During the next step, the reactive mixture was spin coated (the spin coating speed, acceleration, and duration were 1500 rpm, 1000 rpm s -1 and 30 s, respectively) onto the first PEDOT:PSS electrode layer and pre-polymerized in a sealed chamber under argon during 45 min at 50 °C to initiate the formation of PEO network.

The second PEDOT:PSS electrode was fabricated on top of the PEDOT:PSS/semi-IPN bilayer in the same way as the first electrode: Solutions were prepared, casted, and solidified at 50 °C by evaporating the water. Resulting trilayer actuators were then placed to the sealed chamber for the final heat treatment during 3h at 50 °C and 1h at 80 °C under argon. Finally, the fabrication process includes micropatterning of the trilayer structures into microbeams, using laser cutting (Oxford Lasers). The obtained microactuators are then immersed in ionic liquid (EMImTFSI, 72h) to incorporate the ions, necessary for the redox process. The length and the width of the characterized microbeams were 6.8 and 1.2 mm, respectively.

Thickness Measurements: Thickness measurements were performed with profilometers Dektak 150 (Veeco) and Dektak XT (Bruker). Provided values and error bars are obtained from average of multiple measurements (3 to 5) across the width and at multiple locations along the length (see Figure S1, Supporting Information).

Electrical Measurements: The electronic conductivity of the PEDOT:PSS electrodes was obtained by measuring the sheet resistance of the electrodes with in house built setup. Two gold plates (low resistivity material) were fixed onto the substrate to keep the distance (d) between the gold plates constant. Sheet resistance of the electrodes was measured with digital multimeter Model 2000 (Keithley Instruments). By knowing the distance (d) between the gold plates, the thickness of the electrode (h), the length (l x ) of the electrode that is on the surface of the gold plates and the sheet resistance of the electrode from the multimeter (R), the electronic conductivity (σ) was calculated according to the relation between resistivity and conductance from the formula:

d hl R x σ =
Electrochemical Measurements: Cyclic voltammetry (CV) of the electrodes was carried out using VMP Multipotentiostat (Biologic SA) in three-electrode configuration. The expanded stainless-steel mesh (Dexmet Corporation) was used as the counter electrode (the surface area twice as the working electrode's area), silver wire as the reference electrode, fabricated electrode as the working electrode and EMImTFSI as the electrolyte. The parameters for CV were: scan rate: 20 mV s -1 ; potential range: ± 1.0 V; cycles: 5. CV of the microactuators was performed using VMP Multipotentiostat (Biologic SA) in two-electrode configuration. One PEDOT:PSS electrode of the trilayer configuration was connected to the working electrode and the second PEDOT:PSS electrode to the reference and counter electrodes.

Integration of the current from cyclic voltammetry gives the charge (Q), which is divided by the volume of the electrode, providing volumetric charge density (ρ): where i(t) is the measured current, t is the duration of one cycle of applied voltage, l, w, h are the length, width, and thickness of the PEDOT:PSS electrodes, respectively.

Atomic Force Microscopy: AFM was used in order to investigate the topography of PEDOT:PSS electrodes. Atomic force microscopy (AFM) images were recorded in tapping mode with Nanoscope V controller, coupled with an icon microscope from Bruker.

Mechanical Characterization: Stiffness of the beams was measured during mechanical bending experiments. The stiffness is calculated from the relationship between the applied displacement and resulting forces, allowing to determine the Young's modulus of the microbeams. The experiments were carried out with the microrobotic system FT-RS1002 (Femtotools) and a microforce sensing probe FT-S1000 (Femtotools). The tip of the sensing probe was in contact with the microactuator at the distance of 3 mm. The displacement was applied and the resulting force was recorded. The Young's modulus was obtained from the relation between the displacement and the force of the microactuators and is calculated following the formula below: where k is the slope of the recorded force and displacement curve, L is the distance between the clamped end of the microactuator and the tip of the force sensor, w and h are the width and thickness of the microactuator, respectively.

The Actuation Experiments: The bending deformation of the microactuators was determined by applying square wave potential (± 0.4 to ± 2.2 V) and measuring the passing current through the trilayer microactuator while following the bending deformation/displacement of the microactuator at the frequency of 0.1 Hz. The system consists of the voltage source (NF Electronic instruments 1930) to produce a step voltage and an in-house amplifier, an ampere meter (FW Bell mA-2000) to measure the passing current, a micro camera (Dino-Lite AM7000/ AD7000) to record the bending movement of the microactuator and a laser displacement sensor LKG 32 (Keyence). The strain difference ε between the two PEDOT:PSS electrodes was calculated according to Sugino, where D is the displacement of the actuator from neutral to actuated state, h is the thickness of the actuator, and L 0 is the distance between the clamped end of the actuator and the laser beam. [62] The Blocking Force Experiments: The measurement setup consisted of voltage source (NF Electronic instruments 1930) to produce a step voltage and an in-house amplifier, microrobotic system FT-RS1002 (Femtotools) with a microforce sensing probe FT-S1000 (Femtotools). The microactuator was clamped and connected to the voltage source. The tip of the sensing probe was in contact with the microactuator at the distance of 3 mm. When potential was applied, the bending movement was then counteracted by the tip of the sensing probe and the resulting data was recorded.

Sensor Characterization: Sensor experiments were carried out by applying mechanical stimulation and measuring the responding OCV. The samples were clamped and connected to the VMP Multipotentiostat (Biologic SA). The Dual Mode Lever System (Aurora Scientific, 300C) was used to apply mechanical stimulation and the resulting OCV was recorded with the potentiostat.

Figure 1 .

 1 Figure 1. PEDOT:PSS/PEO microactuator fabrication with LbL process.

Figure 2 .

 2 Figure 2. a) Thickness and b) electronic conductivity of PEDOT:PSS electrodes in swollen state (EMImTFSI) as a function of the PEO content.

Figure 3 .

 3 Figure 3. Cyclic voltammetry with an inset of volumetric charge density of PEDOT:PSS electrodes as a function of PEO content at a scan rate of 20 mV s -1 and a voltage window of ±1.0 V in EMImTFSI.

Figure 4 .

 4 Figure 4. AFM images of PEDOT:PSS films: a) pristine PEDOT:PSS; b) PEDOT:PSS/PEO.

Figure 5 .

 5 Figure 5. PEDOT:PSS films: a) pristine PEDOT:PSS just after immersion into water; b) pristine PEDOT:PSS after 10 min water immersion; c) PEDOT:PSS/ PEO electrode after 10 min water immersion.

Figure 6 .

 6 Figure 6. a) Microactuator fabrication process based on LbL synthesis (PEDOT:PSS with 0% and 40% PEO); b) photographs of the laser patterned PEDOT:PSS-based microactuators.

  E x and t x are the Young's modulus (MPa) and the thickness (µm) of the corresponding layers. The thicknesses of the PEDOT:PSS layers are obtained from the electrode study. The Young's modulus of the PEO-NBR membrane is approximately 1 MPa and the thickness 15 µm (determined after swelling in EMImTFSI

Figure 7 .

 7 Figure 7. Cyclic voltammetry with an inset of volumetric charge density of PEDOT:PSS and PEDOT:PSS/PEO microactuators at a scan rate of 20 mV s -1 and a voltage windows of ±1.0 and ±2.0 V.

Figure 8 .

 8 Figure 8. Strain difference (%) as a function of applied potential at a frequency of 0.1 Hz of the PEDOT:PSS and PEDOT:PSS/PEO microactuators.

Figure 9 .

 9 Figure 9. Blocking force (at 3.0 mm from clamping) as a function of applied potential of the PEDOT:PSS and PEDOT:PSS/PEO microactuators. The length and the width of the microbeams are 6.8 and 1.2 mm, respectively.

Figure 10 .

 10 Figure 10. a) Open circuit voltage (∆E) response of PEDOT:PSS/PEO microsensor to the sinusoidal mechanical stimulation at a strain of 0.5%; b) Open circuit voltage (∆E) response to the sinusoidal mechanical stimulation as a function of applied strain at a frequency of 0.1 Hz of the PEDOT:PSS and PEDOT:PSS/PEO microsensors.

Figure 11 .

 11 Figure 11. PEDOT:PSS/PEO microsensor output voltage in response to the square wave mechanical stimulation with an applied strain of 0.5% during 30 s, 1 min, and 5 min.

Table 1 .

 1 The Young's modulus values of PEDOT:PSS microactuators and the calculated Young's modulus values of corresponding PEDOT:PSS electrodes.

		E trilayer [GPa]	t electrode [µm]	E electrode [GPa]
	PEDOT:PSS	2.8	7.3	5.8
	PEDOT:PSS/PEO	0.9	9.2	1.6

Acknowledgements

The research was supported by the European Union's Horizon 2020 research and innovation program under the Marie Skłodowska-Curie grant agreement No. 641822. This work was partially supported by the French Government through the National Research Agency (ANR) under program PIA EQUIPEX LEAF (ANR-11-EQPX-0025) and by the French RENATECH network.

Conflict of Interest

The authors declare no conflict of interest.


Lauréline Seurre

Kätlin Rohtlaid

Giao T M Nguyen

Caroline Soyer

Sofiane Ghenna

Sébastien Grondel

Frederic Vidal

Barthelemy Cagneau

Cedric Plesse

Eric Cattan

Frédéric Vidal

Barthélemy Cagneau

Cédric Plesse

Demonstrating full integration process for electroactive polymer microtransducers to realize soft microchips

Keywords: Polymeric microtransducer, flexible microchip, electronically conducting polymer, PEDOT:PSS

niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

INTRODUCTION

The use of organic materials to develop flexible microelectronic devices such as electronic paper, touch screens, solar cells, and wearable electronics is now widely described. However, new functional materials, being actuators and/or sensors, are still expected to be developed and fully integrated into flexible MEMS. Polymeric actuators, such as electroactive polymers [START_REF] Jung | Artificial annelid robot driven by soft actuators[END_REF], electrothermal actuators [START_REF] Nguyen | A polymeric microgripper with integrated thermal actuators[END_REF], and magnetic actuators [START_REF] Chong | Elastic magnetic membrane for improved mixing in microwells[END_REF] are then increasingly studied to be used and potentially integrated into flexible microstructures. The mechanical sensing components also need to be considered since they allow feedback control. Materials with dual functionality should be thus favored to simplify the fabrication and integration processes, as only one material would have to be considered.

For flexible electronic circuits, electronic conducting polymers (ECPs) like commercially available poly (3,4ethylenedioxythiophene):polystyrene sulfonate (PEDOT: PSS), deposited with various techniques are widely studied to enhance its electronic conductivity. Moreover, ECPs have been also considered for their potential dimensional changes under electrical stimulation. This actuation functionality is generated by the ion expulsion/inclusion motions during an oxidation and reduction process in the presence of an electrolyte [START_REF] Baughman | Conducting polymer artificial muscles[END_REF]. Additionally, they can operate in reverse mode as strain/force sensors, when mechanical stimulation is converted into an electrical response [START_REF] Wu | Soft Mechanical Sensors Through Reverse Actuation in Polypyrrole[END_REF]. ECPs with a bilayer structure have been first studied. The integration of an ECP-based bilayer actuators operating only in an electrolyte liquid is published in 2000s [START_REF] Jager | Microrobots for Micrometer-Size Objects in Aqueous Media: Potential Tools for Single-Cell Manipulation[END_REF]. To operate in air, trilayer structures combining two ECPs sandwiched a supporting polymer membrane are fabricated. Ions necessary for redox process were introduced to the system by swelling in ionic liquid. Among supporting polymer used layers we can find polyvinylidene fluoride PVDF membrane [START_REF] Khaldi | Bottom-up microfabrication process for individually controlled conjugated polymer actuators[END_REF], interpenetrating polymer networks (IPNs) like polytetrahydrofurane/poly(ethylene oxide) PTHF/PEO [START_REF] Plesse | Polyethylene oxidepolytetrahydrofurane-PEDOT conducting interpenetrating polymer networks for high speed actuators[END_REF] or nitrile butadiene rubber/poly(ethylene oxide) NBR/PEO [START_REF] Festin | Robust solid polymer electrolyte for conducting IPN actuators[END_REF]. For these configurations, PEO promotes the high ionic conductivity while elastomer (NBR or PTHF) improves the mechanical properties of the middle matrix. However, to our knowledge, no result has been obtained so far for a fully integrated ionic trilayer electroactive polymer proving actuation and sensing capabilities. All the previous studies concerning the integration of ECP-based microactuators operating in air have been reported with manual handling steps [START_REF] Jager | Patterning and electrical interfacing of individually controllable conducting polymer microactuators[END_REF][START_REF] Khaldi | Bottom-up microfabrication process for individually controlled conjugated polymer actuators[END_REF], without integrated remote electrical contacts [START_REF] Khaldi | Bottom-up microfabrication process for individually controlled conjugated polymer actuators[END_REF][START_REF] Maziz | Top-down Approach for the Direct Synthesis, Patterning, and Operation of Artificial Micromuscles on Flexible Substrates[END_REF], and without a complete integration process [START_REF] Zhong | Development of polypyrrole based solid-state onchip microactuators using photolithography[END_REF].

Moreover, the most important advantages of these microtransducers (MTs) are: lightness, low cost and biocompatibility. They can be designed with complex shapes and can operate at low voltage (1-2 V). Besides, they are versatile since they can operate in liquid, in air and even in low pressurized environments. Because of all these advantages, they are promising candidates to enable a broad range of applications for a new generation of flexible microsystems. Large strains and forces with sensing capabilities are indeed required in several fields such as microrobotics and biomedical applications.

In this work, we present the fabrication of PEDOT:PSS-based MTs fully integrated into soft microchip and their characterization as actuator and sensor.

MATERIAL AND INNOVATIVE PROCESS

Materials

The NBR/PEO IPN has been used as supporting polymer membrane. PEDOT:PSS commercially dispersion (Clevios PH1000) was chosen as an electrode material to fabricate MTs. Polyethylene glycol methacrylate derivatives (mPEG) has been used as reactive

f) SU-8 encapsulation for final flexible microchip and XeF2 vapor-phase etching of the Si wafer. The final prototype is the backside view of the previous step. g) Different prototypes incorporating conducting polymer-based MTs individually addressable in a flexible chip with gold remote contacts. h) Micro-grippers prototypes with three fingers and a wrist.

additive and directly incorporated into the PEDOT:PSS dispersion. mPEG has been chosen as additive because PEO based additives have been reported as an enhancement for electronic conductivity due to the morphological modifications of the conducting polymer layer. As a consequence, the electronic conductivity of the pristine PEDOT:PSS electrode is only 1 S.cm -1 and reaches a maximum value of 163 S.cm -1 for 40% PEO, both previously swollen in electrolyte. Moreover, the ionic conducting promotion of PEO phase allows for better ion transport and exchange between the electroactive PEDOT domains and the electrolyte 1-ethyl-3-methylimidazolium bis(trifluoro-methanesulfonyl)imide (EMImTFSI). Finally, the reactive nature of mPEG additives, via the methacrylate functions enhances the mechanical properties of the PEDOT:PSS electrodes and improves the adhesion between the electrodes and the ionic conducting membrane NBR/PEO due to covalent co-bonding. Final MTs are obtained after swelling the trilayers during 72 h in ionic liquid.

Microfabrication of flexible chip including polymeric transducer

To integrate ECP-based MTs into flexible chip, photolithography, gold evaporation and dry etching are used with 8 steps of mask. The prototypes are fabricated on 2 inches' double-side polished silicon substrates, with a thickness of 150 µm. A first gold electrode of 500 nm is obtained by evaporation and lift-off (Fig. 1a). The synthesis of the trilayer actuators is performed by stacking PEDOT:PSS/PEO, NBR/PEO and PEDOT:PSS/PEO layers with 8 µm and 10 µm thicknesses respectively, as described in [START_REF] Rohtlaid | PEDOT:PSS/PEO electrodes with improved electrical and electro-chemical properties for soft microactuators and microsensors[END_REF] (Fig. 1b). A hard mask of gold layer is patterned to define the shape of the transducer etched with a reactive ion etching (RIE) using O2/CF4 gas at 90/10 ratio [START_REF] Khaldi | Conducting interpenetrating polymer network sized to fabricate microactuators[END_REF] (Fig. 1c). Then SU-8 photoresists (2 µm) are used as insulator to avoid short-circuit between both layers of PEDOT:PSS/PEO (Fig. 1d), after the deposit of the bottom gold remote contact. Next, a last gold layer (500 nm) is deposited by evaporation for the bottom remote contact and patterned by ion beam etching (IBE) (Fig. 1e). To be manipulated, the microactuators are encapsulated with SU-8 layer with a thickness defined as a function of the desired flexibility. Finally, the microactuators are released by etching all the silicon wafer, using vapor phase XeF2 etching cycles (Fig. 1f). Remote contacts address each MT in the same microchip for actuation and/or sensing. MTs 2D geometry can be tuned at will.

The originality of the manufacturing process lies in different aspects. Firstly, the process needs to consider the hygroscopicity of the PEDOT:PSS, meaning that no waterbased medium can be used after manufacturing the MT.

Therefore, the negative SU-8 photoresist remains as the only usable photoresist. The wet chemical etching is avoided for the same reason. Secondly, a continuous electrical remote contact from the Si wafer to the top of the MTs, 26 μm higher, is performed. Owing to an intermediate SU-8 insulating layer, no short circuits were observed on our microchips. Finally, the flexible microchips are provided with self-standing MTs fully integrated, individually addressable, with different shapes like combs (Fig. 1g), membranes, micro-grippers (Fig. 1h) and could be easily hybridized to another flexible microsystem.

EXPERIMENTAL RESULTS

Young's modulus

Stiffness measurements of the MT beams were carried out with the microrobotic system FT-RS1002 and a microforce sensing probe FT-S1000 (Femtotools). The tip of the sensing probe was in contact with a beam at the distance of 2 mm from the encapsulated base of the MT. The displacement was applied and the resulting force was recorded. The Young's modulus is calculated following the formula below: E = 4kL 3 /(wh 3 ) (1) where k is the slope of the recorded force and the displacement curve, L is the distance between the clamped point of the beam and the tip of the force sensor, w and h are the width and thickness of the beam respectively. The Young's modulus of the integrated MT is measured to be 3 GPa. This value can be compared to a nonintegrated MT which is 1 GPa (cut by laser [START_REF] Rohtlaid | PEDOT:PSS/PEO electrodes with improved electrical and electro-chemical properties for soft microactuators and microsensors[END_REF]), meaning that the process modifies its mechanical properties.

Strain difference, blocking force and sensing

The actuation characterizations were performed as a function of positive voltage (Vin) step during the required time to obtain the maximum of the displacement or the blocking force. At 1.5 V the rise time of the strain difference is more than 18 min (1131 s). This value is explained by the very slow migration of ions. In comparison, for the same Vin, the rise time in blocking force is only of 108 s. The ion migration continues beyond this time but the speed is so slow that it produces variations of forces lower than the sensitivity of our microforce sensing probe (<µN). The figure 3 shows that rise time increases when the Vin decreases as expected. Beyond 1.75 V, the degradation of the performances is observed in particular when Vin is applied during a long time. The measurements are made from the highest Vin to the lowest because of the memory effect of these materials. The performances depend on the previous state of the material.

The strain difference ε was calculated according to Sugino et al equation [START_REF] Sugino | Actuator properties of the complexes composed by carbon nanotube and ionic liquid: The effects of additives[END_REF] and reaches, linearly with Vin, the value of 1.1 % at 1.75 V (Fig. 2a) for an integrated MT. For laser patterned microactuators (1374 µm of width), a value of 1.8 % is reached for the same thickness, without integration step or integrated electrical connections. Moreover, the variations of the blocking force as function of Vin is linear from 0.5 V to 1.5 V. The PEDOT:PSS-based MT beam demonstrated a maximum output force of 215 µN (Fig. 2b). These results prove that the microfabrication process does not extensively damage the resulting microbeams in comparison with MTs obtained by laser cutting without integration. Moreover, sensing capabilities were evaluated by measuring the potential difference between the two ECPs of a microbeam after applying a strain. A high-precision three-axis piezoelectric manipulation unit with integrated optical encoders (FT-RS1002) is used to measure simultaneously the displacement and the force applied with the microforce sensing probe (Fig. 4a). A maximum output voltage of 0.4 mV when a mechanical strain of 0.58 % was applied (corresponding to a force of 400 µN applied at 2 mm of the clamping point) (Fig. 4b). Such performance has never been achieved on so small and fully integrated MTs.

CONCLUSION

This paper proposes a successful microfabrication process to integrate small air operating PEDOT:PSS-based MTs for a flexible microchip. Integrated gold connections are in contact with the MT top and bottom electrodes to individually control each addressable MT. An innovative based on conventional techniques has demonstrated the integration of freestanding polymeric MTs with different shapes like combs, bridges, membranes. Microbeam actuation was successfully evaluated by a strain difference of 1 % and a blocking force of 215 µN at 1.5 V demonstrating the best achieved performance at this size. Moreover, sensing measurements were performed on the same microbeams, resulting in a maximum output voltage of 0.4 mV with a mechanical strain of 0.58 %. These are the first measurements performed on such small microbeams. The flexibility of the microchips can be tuned by changing the thickness of the last SU-8 photoresist used during encapsulation. These microchips could be easily hybridized with other flexible or non-flexible microsystems such as microfluidic channels, parts of a micro and nanorobots or in optoelectronic components.

Figure 1 :

 1 Figure 1: Schematic representation of the main steps process for the integration of PEDOT:PSS-based actuator in soft structure. a) Au top electrode on Si wafer. b) Trilayer fabrication and Au evaporation. c) IBE of Au for hard mask. d) RIE for trilayer pattern, then IBE of hard mask and SU-8 used as insulator. e) Au bottom electrode. f) SU-8 encapsulation for final flexible microchip and XeF2 vapor-phase etching of the Si wafer. The final prototype is the backside view of the previous step. g) Different prototypes incorporating conducting polymer-based MTs individually addressable in a flexible chip with gold remote contacts. h) Micro-grippers prototypes with three fingers and a wrist.

Figure 2 :

 2 Figure 2: Measurements on one microbeam (width 728 µm, thickness 24.5 µm) of an integrated PEDOT:PSS-based microtransducer. a) % strain difference and b) blocking force as a function of DC applied voltage from 1.5 to 0.25 V (error bars are smaller than the point size). In insert: microbeam embedded in SU-8 actuated with a Vin of 1.75 V applied on the remote gold contacts.

Figure 3 :

 3 Figure 3: Blocking force rise time as function of the applied Vin measured on a microbeam (w=728 µm, h=24.5 µm) after full integration.

Figure 4

 4 Figure 4: a) Force applied on a microbeam for a mechanical displacement of 500 µm and the measured sensing voltage response. b) Sensing voltage (∆V) as function of strain difference (%) or applied force (µN). Measurement at 2 mm from the SU-8 encapsulation.

ACKNOWLEDGEMENTS

This project received funding from the European Union's Horizon 2020 with project n°857263 Twinnims. This work was partly financially supported by the French National Research Agency with PIA EQUIPEX LEAF (ANR-11-EQPX-0025), RENATECH and MicroTIP (ANR-15-CE08-0032-0) projects.