MarekBukovský(s192844)Supervisor:AlirezaDolatshahi-Pirouz(DTU)Co-supervisor:FirozBabuKadumudi(DTU)Examiner:LarsWågberg(KTH) Flexibleandrecyclableelectronicsmadefromnanoreinforcedsilk TechnicalUniversityofDenmarkKTHRoyalInstituteofTechnology

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Technical University of Denmark KTH Royal Institute of Technology

Flexible and recyclable electronics made from nanoreinforced silk

Marek Bukovský (s192844)

Supervisor:

Alireza Dolatshahi-Pirouz (DTU) Co-supervisor:

Firoz Babu Kadumudi (DTU) Examiner:

Lars Wågberg (KTH)

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Abstract

As the research area of wearable electronics is still relatively new, material science with this focus opens plenty of unexplored fields. That is why a study characterizing the unexplored composite system of silk fibroin and MXene (Silk/MXene) was conducted. These two biocompatible materials are complementary with regard to the requirements for wearable electronics materials. Silk fibroin dispose an ionic conductivity and solid flexibility, while MXene brings mechanical strength and significant increase of electrical conductivity. The reinforced hydrogel materials were studied at two concentrations of fillers, 1%

and 5% and compared to pristine silk fibroin. All three materials were studied from the point of view of their structure, mechanical properties, behaviour in aqueous environment, biodegradability and electrical conductivity, both static and dynamic. Nanocomposite systems of silk fibroin and MXene have shown a potential for being used in the intended application area, as Silk/MXene 5% film displays good stability, conductivity with high and stable Gauge factor.

Sammanfattning

Forskningsområdet för bärbar elektronik är fortfarande relativt ungt och det finns ett stort behov av utveckling av nya material inom området. Olika typer av kompositer är mycket intressanta och de ska uppvisa såväl hög hållfasthet som goda ledande egenskaper. I detta avseende är silkes fibroin och MXene mycket intressanta utgångsmaterial eftersom silkestrådarna kan ge en struktur med god jonledningsförmåga och god flexibilitet och MXene kan bidra med hög styvhet och god elektrisk ledningsförmåga. Med detta som bakgrund beslöts att undersöka om kompositer av silkestrådar och MXene kan användas i kompositer som kan användas i bärbar elektronik. 3 olika typer av hydrogeler studerades och de innehöll silkes fibroin med 0, 1 och 5% MXene. De

egenskaper som utvärderades var struktur, mekaniska egenskaper, stabilitet i vatten, bionedbrytbarhet och både statisk och dynamisk ledningsförmåga. Resultaten visar att de

tillverkade nanokompositerna har lovande förutsättningar inom området eftersom en kombination av silkes fibroin med 5 % MXene har god stabilitet, konduktivitet och en hög och stabil Gauge- faktor.

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Acknowledgements

First of all, I would like to express my profound gratitude to my family for their never ending support, that gave me a chance to study.

I would also like to acknowledge my supervisor, Prof. Alireza Dolatshahi-Pirouz, for the possibility of working on this interesting project and the subsequent guidance.

My big gratitude belongs also to Dr. Firoz Babu Kadumudi for plenty of inspiration, help and knowledge, he has shared with me. Moreover, I would like to thank to all the members of our scientific group for their help with my experiments.

Also, I would like to express my gratitude to Prof. Lars Wågberg for proofreading and evaluating my thesis.

Lastly, I would like to thank to Arnau Hernández Orrit for a huge help during writing the thesis, that helped me to save a lot of time, energy and nerves.

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List of Figures

1 Schematic depiction of composites. Brown – fillers; black – thermoplastic

polymer matrix.^[13] . . . 4

2 Difference between a thermoplastic polymer (left) and a thermoset (right).^[15] 5 3 Chitosan structure.^[18] . . . 6

4 Stress-strain curves of PAM/Chitosan composite for repeated loading.^[20] . 7 5 Observation of a resistance change with a body motion for PAM/Chitosan composite.^[20] . . . 7

6 Repeating unit of Polyacrylamide.^[23] . . . 8

7 Values of fracture toughness for interfaces between PHEMA, PAM/alginate composite and PVA with various materials.^[28] . . . 9

8 Silk fibroin chemical structure.^[32] . . . 10

9 Schematic depiction of silk fibroin extraction.^[31] . . . 10

10 Tensile tests results of various silk fibroin/graphene systems.^[8] . . . 11

11 Graphene structure.^[36]. . . 12

12 Nanoclays structure.^[47] . . . 14

13 Influence of MXene yield on Gauge factor for PVA/MXene hydrogels.^[53] . . 15

14 Human motion sensing of PVA/MXene hydrogels.^[53] . . . 16

15 General comparison of FTIR spectra for the films with all the MXene concentrations before (a) and after (b) the methanol treatment. The dashed lines are (from left to right) referring to the absorption peaks for amide I (1650 cm^-1), amide II (1510 cm^-1) and amide III (1230 cm^-1) groups. . . 22

16 Deconvoluted FTIR spectra for the films with all the MXene concentrations before and after methanol treatment. The vertical lines are (from left to right) referring to the borders of the β sheets with β turns region (1696 cm^-1), β turns with α helices (1662 cm^-1), α helices with random coils (1655 cm^-1) and random coils with β sheets (1637 cm^-1). . . 23

17 Yields of various types of secondary structures for silk materials with different MXene concentrations, before and after methanol treatment. . . 24

18 a, b, c: SEM images of 10 000 magnitude for Silk/Mxene films cross- sections with filler concentrations of 0%, 1% and 5% respectively (scale bar: 5 µm). d, e: SEM images of 20 000 magnitude for Silk/Mxene films cross-sections with filler concentrations of 1% and 5% respectively (scale bar: 2 µm). . . 25

19 EDX spectra for Silk/Mxene 0, 1 and 5% films (a, b, c respectively). . . 26

20 EDX spectra for Silk/Mxene 0% film. . . 27

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23 Stress-strain curves of Silk/MXene 0%, 1% and 5% films. . . 29 24 a, b, c: Values of tensile strength, strain at break and Young’s modulus

obtained from tensile testing. . . 30 25 Results of water content (a), water degradation (b) and swelling (c) studies. 30 26 Ionic conductivity values of measured samples. . . 32 27 Dependence of electrical resistance on increasing extension rate. Applied

maximal strain for all extension rates and samples: 10%. . . 33 28 Dependence of electrical resistance on increasing strain. Applied exten-

sion rate for all maximal strains: 10 mm/min. . . 33 29 Dependence of resistance on repeated extensions. . . 34 30 Dependence of Gauge factor on different concentrations of MXene for var-

ious extensions. . . 34 31 Dependence of Gauge factor on increasing extension for various concen-

trations of MXene. . . 35 32 Before (left) and after (right) photos depicting the degradation of Silk/Mxene

films in LiBr solution. . . 36

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List of Tables

1 Elemental composition of Silk/Mxene films. . . 27

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1 Introduction

In the first part of the publication, several topics covering the focus of the project are going to be introduced. Firstly, the concept of the group of devices called wearable ionics is going to be described, starting from their basic description and applications, going through their functional requirements, leading to the challenges from the perspective of materials science. As these devices have specific requirements, traditional single component materials are not sufficient. The chapter is leading to a choice of hydrogels, that are reinforced by a filler, creating a composite. In the following section, basic concepts of composite materials are described for a better understanding of a material that is examined in the experimental part of the thesis. Concluding from the previous chapters, in the last two sections of the theoretical part, several commonly used matrices and fillers for biocompatible composites are going to be described, including silk fibroin and MXene, the components used for tested material. Also, for each of the matrices and fillers, an example of a recent scientific progress in applications is shown. All the topics in the theoretical part are written in order to give an overview of this interesting research area and explain a motivation for a study and characterization of the investigated material.

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2 Theoretical part

2.1 Wearable electronics and ionic conductivity

The wearable electronics are a relatively new group of devices, that are developed to overcome certain limitations of human bodies by building a bridge between nature and man-made technologies. By a proper connection between a tissue and an electronic gadget, a device is able to receive a signal and subsequently convert the signal towards a response and give a feedback to its user. That is why wearables do have a largest usage in biomedicals, mostly as sensors. These sensors are able of monitoring a heart rate, blood pressure, body temperature and other human body characteristics, which opens a door towards more precise and effective self-diagnosis or even self-treatment. More- over, it can enable a better data collection for doctors as the devices can be used permanently for a long period of time, giving a much more complex overview of observed characteristics.^[1,2]Another large area of usage is sport, or more specifically a science of exercise, where these sensors are able to get information of an athlete’s response on an exercise. This feedback gives an opportunity to the athletes and their trainers to adapt a training, resulting in better physical performance.^[3] The main driven force for a research in the area of wearable electronics is a rapid increase of demand for these gadgets from both previously mentioned fields of usage. Speaking about specific numbers, in 2018, around 100 million of healthcare monitors were sold and sales of wearable technologies in sports has reached around 2,5 billion US dollars in the same year.^[2,3]

To sufficiently accomplish the functions mentioned in the previous paragraph, a device has to fulfill several needs. Firstly, to ensure a right connection between the device and a human body, a high quality gadget-skin interface has to be secured. Thus, the bio- electronics are able to react on a dynamic character of a human skin, which can undergo plenty types of deformation, such as stretching, bending, folding or twisting. These ne- cessities are invoking a huge claims for a device in terms of flexibility, stretchability and compressability.^[1]Another reason for the need for these three attributes is also a fact, that wearable electronics are designed in a way that their user should be able to wear them permanently (for instance for observation of heart functions).^[2] Therefore the equipment needs to be designed in a way, that it causes no discomfort or tension to its user. The other key requirement for wearable electronics is biocompatibility. As it was previously stated, a device is in a close contact with a body for a long time, thus it cannot cause any biological harm, not even after a long exposition.^[2] Third, a very apparent attribute that a good device has to display is an electrical conductivity in a big range of voltages and frequencies, in order to be able to transfer an electric signal from a body.^[4]Moreover, this conductivity has to be maintained also in large strains, due to the previously mentioned skin deformations.^[1,2,4]Another desirable parameter of wearable electronics is an optical transparency, but this parameter is not essential for all the applications.^[2]

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Concluding from the requirements for wearable ionics, the field is facing a significant challenge. As it is known, electronics are mostly based on solid materials and this is not really matching with previously mentioned need for flexibility and stretchability. Even though there exist plenty of stretchable conductors based on electronic conductivity, such as carbon nanotubes, graphene sheets, carbon grease, saline solutions and others, these conductors are limited in their function.^[4] First of all, these conductors are not solid, but also they are not able to fulfill all the other demands like biocompatibility, functionality at high voltages and frequencies or not too high resistance at large strains.^[1] That is why in the previous years, the scientific focus was increased towards materials, whose electric transfer is built on a mechanism of ionic conduction.^[1,4] As the field of wearable electronics needs to overcome a challenge of interface body-electronics and the transfer between these two layers, it is more than favourable to use this principle, as it is commonly used at biosystems. Generally, ionic conduction can be devided into two categories by their state: solid-state and liquid-state ionics. Solid-state ionic conduction transfers an electrical current by transitions of ions via defects in a crystal lattice. These defects are voids, vacancies in the lattice that are mobile and thus enable conduction. Solid-state ionic conduction is a very important mechanism, used mostly for semiconductors. Liquid- state ionic conduction, on the other hand works on a principle of ion transport in solvents created either by an external electric field or by a concentration gradient.^[1,5]

A problem that occurs for liquid-state ionics, considering their usage for wearable electronics, is their dimension stability, as liquid materials are just not suitable for electronics application. A good alternative, that meets all the criteria are hydrogels, semi-solid matters, that are dimensionally stable (or at least unstable but controlled) and supply liquid- state ionic conductivity.^[1] As stated before, hydrogels are semi-solid materials, build of polymer chains that are intermolecularly connected, creating a 3-D network. This 3-D network ensures not just an ability of keeping the shape, but also a significant capability of water absorption. By nature, hydrogels are also very flexible and stretchable, making them a very promising candidate for wearable electronics. Nevertheless, as for all the cases, also hydrogels have several disadvantages. By far their most important drawback are low stress levels that hydrogels can sustain when in hydrated state and high brittle- ness (low stretchablity) when in dried state.^[6] As hydrogels appear in a wet state for the wearable electronics application due to the need for flexibility, a question that requires a solution from the perspective of material science is an increase of mechanical properties.

Several attempts for increasing the maximal stress, that hydrogels can sustain were performed, mostly by a synthesis control. This variation in synthesis is represented by mod- ifications in monomers, monomer ratio and polymerization conditions.^[1] By these inter- ventions, only moderate improvements can be achieved. For ensuring really significant

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to this group of materials, their basic principles and several examples that are used for similar aplications.

2.2 Composites

Composites are a group of materials consisting of at least two different materials that are mechanically separable and have clear and distinctive interface. These components are mixed in a way that one of them is dispersed in the other in a more or less controlled way in order to gain required properties. The final properties are an atributes’ combination of each of the matters, with a benefit, that their values are mostly superior to a simple number average of the values of each component. During preparation or manufacturing of a composite, strong influence of processing and composite design on the final properties have to be taken into consideration. In other words, two composite materials, consisting of the same components in the same ratio, processed or designed in a different manner can have different qualities, sometimes even very significant.^[9-12]

Figure 1: Schematic depiction of composites. Brown – fillers; black – thermoplastic polymer matrix.^[13]

Generally, composites are made of two basic types of components: matrix and fillers, as depicted at Figure 1. The matrix mostly makes up a majority of both, volume and mass of a composite and its main function is binding and fixing filler units in order to ensure the dimensional stability and stiffness of a material, transfer of a load between the filler particles and protection of reinforcements towards external environment. Based on the main functions, material requirements for a matrix can be determined. First of all, the matrix needs to be a continuous matter capable of a changing from a liquid to a solid state in order to surround and bond filler particles sufficiently when in liquid state and to ensure a dimensional stability and protection when in solid state. The other useful qualities of a matrix are low density and viscosity in a liquid state, good chemical resistance (oxygen, polar and non-polar solvents) and also good resistance towards physical influences, such as temperature, pH, UV-radiation, moisture etc. Due to these requirements, polymers, both thermosets and thermoplastics, are very common group of used materials, as many of them fulfill the previously mentioned needs. Thermosets that are used as a matrix are mostly epoxy resins, phenol and amino plastics or unsaturated polyesters; materials that are liquid but after curing, they create 3-D network of covalent bonds, making them tough, hard and solid. Thermoplastics on the other hand are not cross-linked and their change of parameters from liquid to solid is caused by a glass transition. Strength and stiffness

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of these polymers are derived from the entanglements of polymer chains, when these physical interactions substitute a role of covalent bonds at the cross-linked thermosets.

Existence of entanglements is dependent on the length of chains (too short chains cannot entangle), therefore during designing a composite, emphasis has to be put on choosing a material with high degree of polymerization and molecular weight.^[9-12,14]Silk fibroin, that is a matrix for the materials that are tested in the experimental part of this project is also a thermoplastic polymer.

Figure 2: Difference between a thermoplastic polymer (left) and a thermoset (right).^[15]

Unlike for matrices, matters that are used as fillers consists of a very broad scale of chemical composition, dimensions, sizes, geometries and formations, depending on their price, area of use and requirements. By a structural composition, we can distinguish six basic types of reinforcements: continuous fibres, short fibres, particulates, lamellar structures, skeletal networks and multicomponential reinforcements. Generally, the fillers ensure mechanical properties of the materials, mostly its rigidity and strength, but they can be also a multifunctional and beside mechanical improvements, fillers can enrich the composite also in electrical (higher conductivity) or thermal properties (fire and heat resistance) and many others. Moreover, fillers have also an irreplaceable role in prevention of crack propagation. Depending on a geometry, concept of anisotropy might have come into consideration. Most apparent is an effect of anisotropy for fibres (1-D geometry), where the axial and non-axial properties, such as Young’s modulus, differ dramatically. On the other hand, the least apparent effect of anisotropy can be seen at particulates, where the crys- tals create a 3D structure.^[9-12] MXene, a filler for the composite materials of this project exists in a form of small 2-D plates, making it suitable for fabrication of films and sheets.

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2.3 Matrices for wearable ionics composites

Properties of matrices used for biocompatible wearable ionics are derived from what requirements are placed on the final material. As it was previously mentioned, flexible bio- electronics have to ensure proper dynamic contact with a human body, dispose a good ionic conductivity even when bended or stretched and be absolutely harmless for a human body. That is why a matrix has to show a good flexibility, high toughness, biocompatibility and also an ionic conducitvity, Another useful property for wearable electronics and therefore also for a composite matrix is an optical transparency. The property that is mostly listed as the last one, even though it is desirable for the mass production is a price of the material.^[1,2,4]

Concluding from the previous paragraph, several commonly used polymer matrices for wearable electronics can be listed:

Chitosan

Chitosan is a polysaccharide obtained by deacetylation of chitin. Chitin is a natural polymer occuring in the cuticle (outer organic layer of living organsims made for their protection) of crustaceans and some insect species. By a process of incrustation, driven by minerals, chitin is transfomed into a very rigid and hard material, that is protecting the animal from a damage.^[16,17]

The repeating unit of citosan consists of N-acetyl-D-glucosamine linked to D-glucosamine by β 1-4 glycosidic bond, as can be seen at Figure 3.^[16,17]

Figure 3: Chitosan structure.^[18]

As it was already mentioned, chitosan is prepared by a deacetylation of chitin. This is performed by the reaction with sodium hydroxide in aqueous environment. Concluding from this, it can be assumed, that chitosan is a cheap material, as the exoskeletons of crustaceans, such as crabs, lobsters and shrimps are a waste material in food industry.

Also, chitosan is biobased, biocompatible and biodegradable material, soluble in water solutions of acidic pH, disposing the ionic conductivity when hydrated. Moreover, this material is also flexible and relatively strong, with Young’s modulus around 300-350 MPa and tensile strength rising up to 3 MPa. Another beneficial quality of this material is its antibacterial properties and transparency.^[16,17,19]

Chitosan is an alternative for packaging or it is used also as a material for bioprinting.^[16]

Due to its antimicrobial and antifungal potential, it is also used in medicine, food industry

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or water treatment.^[16,17]

Chitosan can be used also as a biobased reinforcement, as Duan et al. reported in 2016, where chitosan fibres were reinforcing a PAM hydrogel. The material has reached interesting values of strain (626%) and tensile strength (0,879 MPa), which was the highest strain for E-Skin (wearable electronic films attached to a human skin) materials reported in that time. Very interesting results were observed also for repeated loading and unload- ing the film to almost 200% strain, when after the first, significant hysteresis, the material showed similar stress-strain curves for approximately 100 cycles (Figure 4).^[20]

Figure 4: Stress-strain curves of PAM/Chitosan composite for repeated loading.^[20]

Moreover, its resistance response on a human body motion is also very suitable for usage in stretchable body sensors, as shown in the Figure 5.

Figure 5: Observation of a resistance change with a body motion for PAM/Chitosan composite.^[20]

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Polyacrylamide (PAM)

PAM is a synthetic polymer, created of acrylamide subunits, as depicted at Figure 6.^[21,22]

Figure 6: Repeating unit of Polyacrylamide.^[23]

This material is synthesised by a chain growth polymerisation, obtaining mostly linear chains, that can be subsequently cross-linked. By its nature, PAM is non-ionic, but when immersed in strong inorganic acids or in aqueous solutions of inorganic salts, the material gets ionically conductive character. Towards its usage as a matrix, its water absorptivity is also a desirable property. Moreover, another characteristic supporting the usage of PAM as a hydrogel substrate for an application in wearable ionics is its non-toxicity and biocompatibility. It is true that its monomer, acrylamide, is toxic and carcinogenic, but in PAM, it is present only in very low concentrations.[21,22,24-26]

Main usage of this material (mostly occuring in copolymers) is in paper industry and water and soil treatment, due to its water absorbing potential, that enables a more efficient flocculation. PAM gels are also used in chemical laboratories, mostly as a medium for capillary electrophoresis.^[21,22,27]

Wirthl et al. have studied this material (in combination with alginate), together with PHEMA (Polyhydroxyethylmethacrylate) and PVA (polyvinyl alcohol), in a perspective of its interface with other materials, ranging from soft ones, such as rubbers and leather, to hard ones like PMMA (polymethyl methacrylate), PET (polyethylene terephtalate) or a bone.

This interface is very strong, due a diffusion of polymer chains through a cyanoacrylate adhesive. The values of a fracture toughness ranged around 1,5 kJ/m² for all the surfaces (Figure 7), making this material a promising possibility as a matrix for biocompatible composites used for body sensing. On the other hand, the fracture toughness of the PAM/alginate hydrogels was only around 30 J/m², but this issue can be overcomed by an incorporation of fillers.^[28]

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Figure 7: Values of fracture toughness for interfaces between PHEMA, PAM/alginate composite and PVA with various materials.^[28]

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Silk fibroin

Silk fibroin is a biobased protein fiber, created by silkworm (Bombyx mori), when forming a cocoon which is protecting the larvae. This cocoon is not consisting only of silk fibroin, but also from sericin (20-30%), a protein that is binding the fibroin fibers together, leading to a cocoon being hard and stiff. From this perspective, the cocoon is also a composite material, where sericin is a matrix and fibroin fibres are a reinforcement.^[29-31]

Figure 8: Silk fibroin chemical structure.^[32]

As only silk fibroin is used as a polymer matrix for composites, sericin needs to be extracted from the cocoons. This extraction is depicted in detail in Figure 9.^[31]

Figure 9: Schematic depiction of silk fibroin extraction.^[31]

Silk fibroin has many remarkable advantages as a matrix material for wearable electronics. First of all, this material can be obtained for a reasonable cost, as the silk cocoons are relatively cheap (8-10 €/kg) and the fibroin can be extracted as shown in Figure 9.^[33]

The cocoons are cheap due to their extensive cultivation, as silk is desired cloth for textile

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industry, what opens a door for a possible mass production of silk fibroin. Beside the low price, fibroin has outstanding material qualities too, such as being biobased, biocompatible and also biodegradable, making this matter aa perfect candidate for a responsible and sustainable usage of materials. Silk fibers are also very strong, its mechanical properties are comparable to commonly used synthetic fibres, such as PA6, PES or viscose. Fur- thermore, it is simple to chemically modify silk fibroin, leading to a great diversity of final materials and their usage.^[25,26,27]

On the other hand, silk is not very resistant towards water (having water content up to 30%) and it also loses about one fifth of its strength when wet. Since the fibres are strong, they do not show a large elasticity.^[30-32]

One of the applications of silk fibroin into wearable ionics composites was studied by Ling et al., who had to overcome a challenge in incorporation of hydrophobic graphene into hydrophilic silk fibroin. This issue was solved by dissolving the silk fibroin in solvents, namely hexafluoroisopropanol (HFIP) and a mixture of CaCl2with formic acid, and incorporating graphene in the form of ink (mixture of graphene with a small yield of polylactide- co-glycolide) into these mixtures. Created suspensions were subsequently used for fabrication of films or strings. Films of many different concentrations of graphene from both suspension types were casted and tested, bringing various results, as can be seen at Figure 10.^[8]

Figure 10: Tensile tests results of various silk fibroin/graphene systems.^[8]

Ranges and combinations of properties are differing greatly, offering a potential for various applications for wearable electronics. ¨

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2.4 Fillers for biocompatible composites

In contrast with matrices, fillers have to meet different needs for being used at wearable electronics. Beside acceptable price, these needs are a good conductivity and dis- persability, high strength and of course biocompatibility.^[34] These specific requirements have shortlisted the possible materials on these usually used fillers:

Graphene

Graphene is a planar network of carbon atoms assembled to hexagones by sp² bonds (Figure 11). Due to its unique properties, graphene is considered to be one of the most interesting materials in the world.^[35]

Figure 11: Graphene structure.^[36]

As the chemical composition of graphene is simple, it can be fabricated by a large amount of methods. As Rudrpati publishes, the main procedures to obtain graphene are microme- chanical exfoliation, liquid-phase exfoliation, chemical vapor deposition, flame synthesis and pulsed laser deposition.^[35]

Relating to the thickness, this material is the strongest one in the world (300 times stronger than steel), due to its ideal structure from only carbon atoms, whose C-C bonds are very strong. Specifically, its strength reaches up around 120-140 GPa with Young’s modulus reaching a remarkable value of 1 TPa. Moreover, it is not just very strong, but also extremely light biocompatible material, as its surfacic mass is approx 0,77 mg per square meter (as graphene is only a 2-D structure, surfacic mass is used as a descriptive parameter instead of density). Talking about a surface, another characteristic to be highlighted is graphene’s extremely high surface area (2630 m²/g). It is also a perfect heat and electrical conductor, because of the system of delocalized π electrones moving only in two directions around the carbon lattice. Furthermore, graphene is also chemically stable, yet

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chemically modifiable by implication of functional groups. Lastly, resulting from its low thickness, it is nearly optically transparent too (97,7%).^[35]

Unfortunately, graphene has also its one big drawback and that is its high price ranging from 6-7 000 €/g for a single layer graphene. Nevertheless, graphene is suitable for a very broad spectre of applications, due to its properties, ranging from fotovoltaics and energy storage, over medical application and electronics, to chemical catalysis or material engineering.^[35,37]

For the wearable electronics, Ling et al. came with an approach of mixing graphene with silk fibroin and subsequent characterization of such a material, as depicted in chapter 2.2.3.^[8]

Nanoclays

A description of this type of fillers is really straight forward and simple. Nanoclays are a group of clay-like materials, whose dimensions are very small and in at least one dimension between 1 and 100 nanometers. Beside the smaller dimensions, that are adjusted for a need for nanocomposites’ fillers, these materials do not differ from ordinary clays, i.e. they are composed of layered structures which are created by either octahedral or tetrahedral sheets.^[38-46]

Two types of nanoclays are distinguished, based on their structural geometry. In 1:1 structures, each tetrahedral is linked with only one octahedral sheet, whilst in 2:1 structures, tetrahedrals are connected with one octahedral sheet on each side.^[46]

Nanoclays are natural inorganic materials, composed of hydrous aluminium phyllosili- cates and often also from other cations, such as magnesium, iron and cations of elements from I. A and II. A group of a periodic table. In terms of structure, clays occur in a form of small tetrahedral and octahedral plates, that lay on each other, held together by Van der Waal’s forces, creating multilayer stacks, as can be seen at Figure 12.^[38-46]

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Figure 12: Nanoclays structure.^[47]

The main function of nanoclays as fillers is an increase of mechanical properties, followed by an improvement in heat and fire resistance and thermal stability. When dry, nanoclays are electrically insulating materials, but in aqueous environment they become ionically conductive, making them a possible candidate for a filler used in wearable ionics composites. Morevoer, as clays are natural materials, nanoclays are highly biocompatible and also cheaper (around 0,25-0,30 €/g).[38-46,48,49]

On the other hand, laminated nanoclays are not transparent and after being incorporated to nanocomposites, oxygen permeabitlity of final materials is significantly reduced, so they cannot be used for all the applications at the area of wearable electronics.^[50]

As already mentioned, nanoclays takes place in materials science, but they are also used due to their active surface as chemical catalysts or as viscosity modifying agents, for instance in oil industry.^[38-46]

MXenes

MXenes are, similarly to nanoclays, 2-D planar inorganic compounds, consisting of car- bonitrides, carbides and nitrides of transition metals (mostly titanium, vanadium, molyb- denum, wolfram and niobium), while surfaces of these layers are terminated by different chemical compounds, such as O, Cl and F atom, or by hydroxyl group. These functional groups makes the entire material opened for functionalization, which in combination with metallic properties coming from the transition metals, make this material unique. The thickness of an individual MXene layer is less than 1 nm, while its width and length reaches up to approx. 200 μm.^[51,52]

These materials are produced by etching of MAX phases, layered 2-D structures, composed of nitrides and ternary carbides of early transitition metals (III. to VI. B groups of

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the periodic table) and elements from III. to V. A groups of the periodic table. The etching is conducted by a combination of very strong acids, just like HCl, HF with inorganic salts, such as NH₄HF₂ and LiF, where these etching solutions cleave out the A elements from the substance.^[51,52]

MXenes are at the first place extremely high electrically conductive materials, with even better electrical properties comparing to graphene. Also, due to their metallic bonds, they are a very strong group of materials, making them a good candidates for being used at electronic devices. As MXenes are relatively new materials, not many of their properties are actually examined and some of them are only theoretically calculated and estimated.

[51,52]

Despite their advantages, a challenge in their handling arises due to their tendency towards oxidation, that negatively influences the electrical character. This might be solved by incorporating MXenes into a polymer matrix, ensuring their protection, but also en- hancing the electrical and mechanical properties of the polymer. Area of the composite system polymer-MXene is not well explored at the moment and that is why the focus of this project is aiming towards this topic.^[51,52]

In 2018, Zhang et al. camei with a study, showing groundbreaking electrical and mechanical properties of a composites consisting of MXene and a polyvinylalcohol based hydrogel. As can be seen at Figure 13, Gauge factor (ratio of relative change in electrical resistance to the tensile strain) of these composites is growing significantly with an increasing yield of reinforcements.^[53]

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Also, the reinforced material can reach huge strains, up to 3400%, that is even higher comparing to a pure hydrogel (2200%). When testing the responses of a composite film with 4,1% yield of MXene, on a human motion, the material reflected on movements by a change of a resistance with no response and with a solid sensitivity, as it is depicted at Figure 14.^[53]

Figure 14: Human motion sensing of PVA/MXene hydrogels.^[53]

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3 Materials and methods

3.1 Films preparation

For the preparation of silk fibroin for Silk/MXene films, approximately 10 g of quartered Bombyx Mori cocoons were boiled (30 minutes) in a solution of Na₂CO₃ (sodium car- bonate) in water of a concentration at 0,02 mol/L, in order to extract fibroin and eliminate sericin. After this step, the extracted solution of silk fibroin fibres had to be evaporated at room temperature for 24 hours. When dried, fibres were added to a solution of LiBr in water of a concentration of 9,3 mol/l, where it was kept for 6 hours at 60 °C. Next step of the silk fibroin solution preparation was a dialysis against demineralized water, that took 72 hours and subsequent centrifugation at 12 000 rpm for 3 x 20 minutes at 4 °C. Finally, the concentration of the solution was determined.

For mixing the two components, matrix and fillers, for casting the films, 680 mg of silk fibroin in the form of previously prepared solution, were transferred to a falcon tube and filled by water to a total volume of 25 ml.

The second part of preparation of the components for the films casting was obtaining the MXene (University of Twente, Netherlands). MXene (in the solution of a concentration 15 g/l) was firstly ultrasonicated for 15 minutes at the room temperature in order to get a proper dispersion of particles and then transferred (in the amounts referring to 0, 1 and 5% of yield related to a silk fibroin amount) to a falcon and filled by deionized water to a total volume of 25 ml, just as the silk fibroin.

The content from both falcons was mixed and after 3 minutes of shaking to ensure mixing of both phases, air bubbles were removed from the solution by a spatula to decrease a yield of porosity in the composite. The last step of films preparation was casting the mixed solutions of silk fibroin and MXene onto a petri dish and letting the whole system getting dried in a fume hood at 30°C for 36 hours.

After successful drying, films were treated in methanol for 4 hours for higher stability.

3.2 Fourier-transform Infrared Spectroscopy (FTIR)

A PerkinElmer Spectrum 1000 FTIR spectrometer with a diamond crystal attenuated total reflectance was used for obtaining FTIR spectra. Samples were prepared by cutting and placing in between the diamond crystal and the screw. Experiments were conducted at 25 °C in the range from 530 to 4000 cm^-1, performing 16 scans with a resolution of 4

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3.3 Scanning Electron Microscopy (SEM)

FEI Quanta 200 ESEM FEG scanning electron microscope was used for getting cross sectional and surface images of Silk/MXene films. Parameters for an emission current and an acceleration voltage of a field emission gun electron source were 0,01 A and 10 kV respectively. Samples were created by hand breaking the films and subsequent coating by a sputter deposition of 10 nm thick gold layer. After this treatment, specimen were attached on an SEM stub.

3.4 Energy-dispersive X-ray Spectroscopy (EDX)

This method was used for elemental analysis of the samples. Oxford Instruments X-Max silicon drift detector, connected to the SEM instrument, was used for creating the spectra, that were representing detection area of 80 mm².

3.5 Tensile tests

Characterization of the materials in terms of mechanical properties was done by Instron 5967 Universal Testing Systems. Samples of dimensions 30 x 3 mm were cut using a CO₂ laser and measured by a thicknessmeter with an accuracy of 1 µm, in order to determine their third dimension. Furthermore, the samples were being treated in 2 M aqueous solution of LiCl, to reach a semihydrated state of the tested material. Length of immersion varied over the concentrations of MXene, from 1 minute for 0%, 5 minutes for 1% and 60 minutes for 5% of MXene. The variation of immersion is caused by a degradation of films in aqueous environment. After this treatment, the samples were attached between the clamps, that were placed 10 mm from each other. Specimens were stretched by a 500 N load-cell at the extension rate of 1 mm/min until the break point.

When the experiment was finished, Bluehill 3 testing software was used for rendering a stress-strain curve, that served for obtaining Young’s Modulus, tensile strength and strain at break. Young’s modulus was determined using Origin software at the linear part of the stress-strain curve.

3.6 Water stability studies

Water content determination

The water content of Silk/MXene films was determined by drying samples in an oven at 60 °C for 48 hours, while weighing samples before (m₀) and after (m) this operation on an electronic scale with an accuracy of 1 mg. The moisture content was then calculated as:

M oisture content [%] = m0− m

m0 ∗ 100 (1)

Degradation study

Degradation tests of Silk/MXene films in water were performed by immersing dried (60

°C, 48 hours) and weighed (m₀) samples in demineralized water at room temperature for

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1. 3, 6 and 7 days respectively. After the exposition, samples were dried again in an oven at the similar conditions as previously. After next weighing (m_i, where i refers to a number of days), the degraded amount of Silk/MXene films was calculated as:

Amount of degraded material [%] = m₀− mi

m0 ∗ 100 (2)

Swelling studies

Results of this study were obtained by immersing the samples in demineralized water at room temperature, while weighing samples before (m₀) and at each time point (m_i), where i refers to 10, 20, 30, 60 and 120 minutes respectively. Swelling ratio was then calculated according the following equation:

W ater uptake [%] = m_i− m0

m0 ∗ 100 (3)

3.7 Ionic conductivity study

Before preparing the samples, films were immersed in 2M aqueous solution of LiCl to ensure a semihydrated state of hydrogel films, just as for tensile tests in the chapter 3.5.

Films were then cut into a square of dimensions 10 x 10 mm and placed between two steel stripes, that were attached together with a clamp in a way that the all parts together create a two electrode system, that is suitable for EIS measurements, which determines the ionic conductivity of the films. Finally, the electrodes were connected with an impedance analyzer. The testing, which was performed by Gamry Potetiostat, was conducted in frequencies from 100 kHz to 0,1 Hz in AC voltage amplitude of 0,01 V. Nyquist data were gathered by Gamry Instruments Framework and the conductivity of the samples were determined by the following equation:

σ = 1 R_s ∗ t

a (4)

whereσ stands for the conductivity, Rs is the resistance of environment, t stands for a thickness of the sample and a for its surface.

3.8 Strain resistance study and Gauge factor determination

The strain resistance of the Silk/MXene films was monitored by a Keysight 34461A digital multimeter. Sensors, designed to measure the differences in electrical resistance with extension, were made by attaching two tin plated copper electrodes to the ends of Mxene

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Gaugef actor_i = ∆R/R₀

ϵ (5)

where i stands for different strains: 2, 5 and 10%, R₀for the resistance in the initial gauge length andε stands for strain.

3.9 Recyclability study

Samples of 5 x 5 mm were inserted into a falcon, where they were immersed in 9,3M aqueous solution of LiBr for 24 hours at 60 °C. Images were taken before and after the experiment.

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4 Results and Discussion

4.1 Film preparation

Initial conditions for drying of the films after casting them onto a petri dish was heating for 24 hours at 40 °C. Mostly for Silk/MXene 1% films, these conditions were leading towards breaking of the films into numerous amount of small pieces. That’s why it was assumed to choose drying in lower temperature (30 °C) and for a longer time (36 hours). By this adjustment, an occurrence of problems with breaking the films during drying was reduced.

Also, for several characterization methods, samples were immersed in 2M aqueous solution of LiCl. The exposure times for the immersion were adjusted from the initial plan, that all the films should be immersed for 60 minutes, to 1 minute for 0%, 5 minutes for 1% and original 60 minutes for 5%, as stated in chapter 3.5. However, these methods could be used only for freshly casted films. Samples, that were prepared about 3 months before being immersed in LiCl solution, were degrading and dissolving significantly faster, i. e. 0% films started to degrade after several seconds, 1% films after approximately one minute and the films with the highest filler concentration, 5% of MXene, in around 10 to 15 minutes. This change of behaviour can be addressed to oxidation of films, leading to higher hydrophility of the films. Higher oxidation would also lead to a variety in results in all the studies and that is why all the tests were performed on freshly prepared films.

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4.2 Fourier-transform Infrared Spectroscopy (FTIR)

Results of FTIR are showing, that spectra (and thus the materials structures) differ with both, MXene concentration and methanol treatment (Figure 15). Differences are apparent at the peaks that are referring to amide I, II and III groups (1650 cm^-1, 1510 cm^-1and 1230 cm^-1respectively), as the secondary structures of matrix are changing, leading to a shifting of peaks. For more specific description, amide I peak (1650 cm^-1) was deconvoluted for all three MXene concentrations, both before and after methanol treatment.

Figure 15: General comparison of FTIR spectra for the films with all the MXene concentrations before (a) and after (b) the methanol treatment. The dashed lines are (from left to right) referring

to the absorption peaks for amide I (1650 cm^-1), amide II (1510 cm^-1) and amide III (1230 cm^-1) groups.

Deconvolution serves to distinguish individual peaks, that are together assembling the amide I peak, when each of them refers to a different type of secondary structure. Ac- cording to the position of the deconvoluted peaks and their surface under the curve, yield of secondary structural elements can be determined.

To understand the graphs at Figure 16, several explanations have to be given. In each spectrum, peaks are distinguished by various colors for a better orientation. Black curves refer to the original peaks, that were deconvoluted. Blue curves are representing the bands, whose peak occurs in a region of β turns. Pink curves refer to the bands with a peak occuring in a region of α helices. Green curves are marking the bands with a peak in an area of random coils. Finally, red curves are representing bands with a peak in a region of β sheets.

Range of the regions and their assignments to specific structural elements was determined from literature.^[54]

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Figure 16: Deconvoluted FTIR spectra for the films with all the MXene concentrations before and after methanol treatment. The vertical lines are (from left to right) referring to the borders of the β sheets with β turns region (1696 cm^-1), β turns with α helices (1662 cm^-1), α helices with random

coils (1655 cm^-1) and random coils with β sheets (1637 cm^-1).

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referring to β sheets. A consequence of this structural difference is well shown at Figure 17, where a large increase of β sheets at the expense of random coils is visible, when comparing pristine silk fibroin with the composites. Similar phenomena can be seen when comparing silk fibroin film with composites after the methanol treatment (spectra b, d and f).

This effect of structural changes from random coils to highly organized β sheets is caused by larger amount of non-covalent interactions, both intramolecular and intermolecular, induced by creation of composites and therefore the interactions between silk fibroin fibres and MXene.

An influence of MeOH immersion on secondary structures, is visible mostly for Silk/MXene 0% film, where the band with the initial peak at 1690 cm^-1 was moved to 1697 cm^-1. For the composite films, yield difference of secondary structural elements caused by the immersion is only very small. This can be attributed to a fact, that the materials got to the highly crystalline state by the fillers themselves and the level of their structural organization cannot be much higher. Anyway, around 5% increase of β sheets yield can be spotted for composite films before and after the MeOH treatment.

Figure 17: Yields of various types of secondary structures for silk materials with different MXene concentrations, before and after methanol treatment.

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4.3 Scanning Electron Microscopy (SEM)

As it is shown at Figures 18 a, b, c; difference between silk and composites cross-sections is apparent at the first sight. After adding two-dimensional MXene, material creates layered structures, that are formed during casting and drying of the films, probably due to sedimentation. These layered structures are surrounded by the polymer matrix, that in combination with high aspect ratio of MXene plates is leading to a big amount of Van der Waals intermolecular forces. This formation of 2-D plates and matrix is called nacre- mimetic structure. This type of structure has an unquestionable effect on materials mechanical properties, especially on its stiffness, due to previously mentioned non-covalent interactions.⁴⁴ Nacre-mimetic layers are getting thicker with increasing yield of MXene, as it is indicated at Figures 18 d and e.

Figure 18: a, b, c: SEM images of 10 000 magnitude for Silk/Mxene films cross-sections with filler concentrations of 0%, 1% and 5% respectively (scale bar: 5 µm). d, e: SEM images of 20

000 magnitude for Silk/Mxene films cross-sections with filler concentrations of 1% and 5%

respectively (scale bar: 2 µm).

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4.4 Energy-dispersive X-ray Spectroscopy (EDX)

Figure 19: EDX spectra for Silk/Mxene 0, 1 and 5% films (a, b, c respectively).

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At the Table 1, elemental composition of each of the examined materials is shown. Pristine silk film consists only from carbon, nitrogen and oxygen, confirming the structure shown at Figure 8. For the composite films, also other elements, such as aluminium, but mostly titanium were detected. The existence of titanium in the samples shows the presence of MXene in the films, whilst aluminium traces refer to not perfectly reduced MAX phases during the synthesis of the material. Increase of carbon yield for Silk/MXene 5% can be attributed to MXene once again, as it consists of titanium carbides. As this analysis is only very basal, it is not showing very precise results and other techniques should be used for more complex elemental analysis. Also, it is important to note, that elemental composition of films will always differ due to differences in batches of both, silk fibroin and MXene. The same results are also depicted at Figure 19.

Table 1: Elemental composition of Silk/Mxene films.

Silk/MXene 0% Silk/MXene 1% Silk/MXene 5%

Element Yield [%] Yield [%] Yield [%]

Carbon 55,39 53,10 57,26

Nitrogen 26,19 21,43 22,05

Oxygen 18,43 22,46 17,72

Aluminium 0 0,99 0,49

Titanium 0 2,02 2,49

Figures 20, 21 and 22 are showing the mapping analysis of specific atoms in the studied samples. As can be observed, pristine silk sample (Figure 20) has evenly distributed its carbon, nitrogen and oxygen atoms and this even distribution is not disrupted even when MXene is incorporated into the biopolymer fibres (Figures 21 and 22). Distribution of titanium in both composite samples is also very uniformed, showing no tendency of MXene to aglomerate or to separate phases.

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Figure 21: EDX spectra for Silk/Mxene 1% film.

Figure 22: EDX spectra for Silk/Mxene 5% film.

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4.5 Tensile tests

One of the main functions of MXene in the composites and also the reason for its incorporation into silk fibroin is an adjustment of mechanical properties. Specifically, an increase of tensile strength and Young’s modulus with parallel decrease of strain at break. As can be seen in Figure 23, in the small extensions, composite films undergo much higher stress, comparing to the pristine silk samples. Also, strain at break is decreasing dramatically with the yield of fillers. Moreover, 5% MXene films are able to resist higher maximal stress than the other tested materials. All these phenomena are depicted also in Figure 24.

Reason for these differences in mechanical properties are the layered structures in composites, that can be seen at SEM pictures (Figure 18). These structures have a negative effect on mobility of silk fibroin chains and with the increasing yield of reinforcements, this effect is larger and larger, resulting in growth of Young’s modulus and decrease of strain at break.

Also, continuous trend is demonstrated for necking, as the beginning of necking region is shifting with amount of MXene (about 20% strain for pristine silk fibroin film, 10% strain for Silk/MXene 1% and 5% strain for Silk/MXene 5%). Length of the necking region is decreasing with the fillers concentration as well. Nevertheless, there is a clear difference in the shape of the curves in the necking region, as stress for silk fibroin is increasing even during necking, but for the composite films, stress remains constant.

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Figure 24: a, b, c: Values of tensile strength, strain at break and Young’s modulus obtained from tensile testing.

4.6 Water stability studies

Figure 25: Results of water content (a), water degradation (b) and swelling (c) studies.

Water content determination

As can be seen at the Figure 25, moisture content in the composites is going down with increasing of amount of MXene reinforcements. In this study, materials have met our ex- pectations, as silk is a hydrophillic material, whilst MXene is hydrophobic. With increasing yield of a hydrophobic substance, hydrophobicity of a whole composite material is increasing as well. Independently on their hydrophobicity, the occurrence of fillers in the films has also a negative influence on the water content of the materials.

Degradation study

According to the results, that are shown in Figure 25, it is apparent that all the materials are going through the loss of weight caused by a presence of water, mostly in the first 1-2 days, while reaching a plateau region in the longer time period, as at some point the

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material stops degrading, or degrades only very slowly. Also, one more trend can be spotted, when MXene content is increased, a composite material has a lower tendency towards being degraded by water. This result also supports the previous assumptions, due to the same reason as in the previous study, i. e. the hydrophobicity of MXene and hydrophility of silk fibroin.

A note has to be given towards certain measurements difficulties, mostly for samples of 0 and 1% of MXene. Films got dissolved in water during the testing quite intensively, but after drying, the aqueous solution got stuck to a petri dish, used for the experiment. By this, certain amount of samples was lost and was not taken into a consideration when weighing. Generally, it can be concluded, that the weight loss for 0% and 1% of MXene films was not that high, as it is depicted at Figure 25.

Swelling study

In the first minutes of the experiment, samples showed a large water uptake, while reaching the maximum in approximately 20 minutes after the start of the exposure, as shown in Figure 25. Subsequently, the results got stabilized at a plateau with submaximal values.

However, the reduction of weight from the maximum to the plateau level was not caused by lower water uptake. This difference in mass of the samples between maximum and plateau is probably caused by their degradation.

Just as in the previous cases of water interaction studies, similar trends can be seen for the swelling experiment, i.e. the interaction with water is lower and lower with increasing content of MXene in the samples. The interactions are not dependent only on the MXene content itself, but also by the changes in the protein secondary structures caused by the fillers incorporation, as it is described in chapter 4.2. However, one exception can be detected. Results for pristine silk fibroin film have lower values, comparing to the films with 1% of MXene throughout the entire experiment. This trend has to, once again, be taken into a context with the previous study, the water degradation testing. The silk fibroin film degraded so quickly, that a significant part of the samples got dissolved even in the very first minutes of the experiment and thus the whole curve for Silk/MXene 0% is shifted down.

Overall, films with pure silk are more prone to interactions with water, compared to the composite, which was an expected result. Spotted exception for 0% and 1% films at a swelling study is not caused by the intensity of interactions with aqueous environment in general, but by a specific type of the interaction, when the pristine matrix material rather tends to degrade than to swell.

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4.7 Ionic conductivity study

Figure 26: Ionic conductivity values of measured samples.

First of all, an impedance spectroscopy was measured in a frequency range 100 kHz to 0,1 Hz, as depicted in Figure 26 a. From the initial point of the Nyquist plot, R_s, as the resistance of environment was measured and used for the calculation of ionic conductivity according to the equation (4).

The expected effect of mixing highly electrically conductive MXene with silk fibroin was a significant increase of ionic conductivity, which is crucial in the area of wearable electronics, as it was described in chapter 2.1. Difference in conductivity of pristine silk fibroin film (5,92 ± 4,82 e^-6S/m) and Silk/MXene 1% film (2,00 ± 2,13 e^-5S/m) is only approximately triple in behalf of the composite sample. However, when adding 5% of reinforcements, the conductivity values for the composite film (2,02 ± 1,17 e^-3S/m) are 87 times higher, when comparing with silk fibroin only. Disproportion of an influence over different concentrations is caused by an existence of percolation threshold, when around this point, a slope of conductivity as a function of MXene yield, changes dramtically. Apparently, this threshold lays somewhere in between 1 and 5% and a specification of this point is a further challenge in the electrical characterization of Silk/Mxene systems.

Pristine silk fibroin films are conductive in the hydrated state, as the protein end groups, COOH and NH₂are creating conjugated systems in the aqueous environment, where the hydrogen protones move around the polymer chains. That is why the films are immersed in an aqueous soloution of LiCl before the measurements. Another effect of this treatment is also to increase of mobile ions (Li protones), that are having positive influence on the ionic conductivity. Their conductivity is then increased by the incorporation of electrically conductive fillers.

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4.8 Strain resistance study and Gauge factor determination

Strain resistance studies were performed in order to describe electrical conductive behaviour of the studied films regarding to stretching. In Figure 27, the materials are showing that their sensitivity, defined by the change of electrical resistance with strain, is stable over the increasing deformation rate. This property is desirable for applications in wearable electronics, as the material is able to work as a sensor for various types of extensions.

Moreover, electrical resistance is not stable only with changing deformation rate, but also with cyclings, ensuring materials stability for repeated usage.

Figure 27: Dependence of electrical resistance on increasing extension rate. Applied maximal strain for all extension rates and samples: 10%.

Figure 28 indicates growing electrical resistance with increasing maximal strain. An important trend can be detected, as the resistance is increasing almost linearly with the linear increase of strain. This fact leads to stable values of Gauge factor over various extensions, as it is going to be shown more specifically in the following paragraphs. Also, it is illustrated, that the materials are able to distinguish between different extensions, even in a small difference (3%), showing that films display high electrical sensitivity.

Figure 28: Dependence of electrical resistance on increasing strain. Applied extension rate for all maximal strains: 10 mm/min.

Similar phenomena is shown in Figure 29, where the change of electrical resistance is

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Figure 29: Dependence of resistance on repeated extensions.

Significant (approximately double) increase of Gauge factor can be observed when adding MXene to the silk fibroin films, from very small concentrations (1%), as shown in Figure 30.

Also, it is important to note, that the Gauge factor is not increasing further with increasing amount of reinforcements. In fact, its difference between for 1% and 5% films is negligible.

Moreover, this trend is detected for various strains.

Therefore, not only conductivity was improved by MXene, but also an electrical sensitivity of the films.

Figure 30: Dependence of Gauge factor on different concentrations of MXene for various extensions.

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As can be seen in Figure 32, the Gauge factor is not dramatically changing over the growing extensions. This ensures stable electrical sensitivity of the materials when being stretched. Keeping the sensitivity over deformations is a very important characteristic for materials used in wearable electronics.

Figure 31: Dependence of Gauge factor on increasing extension for various concentrations of MXene.

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4.9 Recyclability study

One of the main goals in materials science of wearable electronics is to use materials, that are either recyclable or easily degradable, both ideally in a way corresponding with principles of a circular chemistry. Mostly, it is because of the increasing problems connected with landfills and degradation of materials to toxic compounds. Other reason for simple recyclability is also responsible usage of natural sources and ideally, if the recycling process is cheap, also the price for the input materials. For these reason, an initial recyclability study was conducted. Results are showing that even in the room temperature, strong ionic solution (9,3 M LiBr aqueous solution) is rapidly separating the composite components to their initial state. On the other hand, solution is that strong that it degrades MXene into non-toxic titanium oxides or oxides of other metals, that sediment at the bottom of the vials. Nevertheless, this study has indicated, that the two compounds are easily separable, whilst silk fibroin can be used again and therefore to be fully regenerated and MXene can be easily degraded to non-toxic compounds.

Figure 32: Before (left) and after (right) photos depicting the degradation of Silk/Mxene films in LiBr solution.

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5 Conclusion

Silk fibroin is an interesting biobased material, whose properties, such as biocompatibility, flexibility, transparency and ionic conductivity are suggesting it to have a potential in wearable electronics.^[2,25-27]This material was enhanced by MXene, 2-D layered material, that is highly conductive and dispose a large toughness and strength.^[45,46]Created composite films of 1% and 5% yield of MXene in silk fibroin, along with pristine silk fibroin film used as a reference, were created by simple mixing and casting of solutions. After successful preparation of these reinforced hydrogels, materials were tested and characterized from several perspectives.

First of all, casted films were immersed in methanol, in order to increase their stability by changes in secondary structures. Outcome of these efforts were studied by FTIR, as can be seen at Figure 17, where the beneficial influence of MXene and also of methanol immersion on the secondary structures, a crystallinity and therefore the materials stability is apparent.

Changes in composition caused by incorporation of MXene were shown and described also by SEM, when cross-section pictures of studied materials with reinforcements were showing a development of a layered structures (Figure 18). Other support for detecting the changes in structures and material compositions was investigated by EDX analysis.

As can be seen at Table 1 and Figures 20, 21 and 22, titanium is present in the composite films, showing also good dispersity with no aglomeration.

The morphological changes were subsequently reflected in different material characteristics, i. e. mechanical properties, namely in the increase in tensile strength and Young’s modulus and parallel decrease of strain at break with increasing amount of MXene (Figure 24).

Other perspective, from which the influence of incorporated MXene fillers was studied, was the materials behaviour towards aqueous environment. This behaviour was examined by determination of water content in the films, by their inclination towards water degradability and also by their tendency to swell (Figure 25). As it was expected, interactions with water were lower with increasing amounts of inorganic, hydrophobic reinforcements.

An intended purpose of MXene in the composite films was also an improvement of their electrical properties. This expectation was fulfilled remarkably, as Silk/MXene 5% films

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extensions, strain resistance studies were performed. Different types of measurements were executed as shown in Figures 28, 27 and 29, leading to the results, that even small amount of MXene in a composite doubles its Gauge factor, comparing to pristine silk fibroin films (Figure 30). Also, in the region of strain from 0-10%, Gauge factor remains relatively stable for all three materials.

Biodegradability and partial recyclability was initially studied by immersion of the films into highly concentrated LiBr solution. Phases were separated easily, enabling reuse of silk fibroin while degrading MXene.

Considering the mentioned results, MXene reinforced silk fibroin hydrogels, dispose a promising potential for motion sensing, due to their high conductivity, decent Gauge factor and good durability in aqueous environment. Nevertheless, areas of its usage are limited, mostly because of their flexibility, that does not reach to values over 80%. Because of this and also due to relatively high sensitivity, sensors based on this materials could be used in less motion demanding sensing, such as heart rate and blood pressure monitoring, potentially also for following the activity of respiratory system.

MarekBukovský(s192844)Supervisor:AlirezaDolatshahi-Pirouz(DTU)Co-supervisor:FirozBabuKadumudi(DTU)Examiner:LarsWågberg(KTH) Flexibleandrecyclableelectronicsmadefromnanoreinforcedsilk TechnicalUniversityofDenmarkKTHRoyalInstituteofTechnology

Technical University of Denmark KTH Royal Institute of Technology

Flexible and recyclable electronics made from nanoreinforced silk