1 Introduction

In general, electrically conductive adhesives (ECAs) are composite materials containing an insulating adhesive binder resin (that provide adhesion, mechanical strength, and impact strength) and a conductive filler. The widely used binder resins in ECAs are epoxy, silicone, polyamide, and polyurethane (PU). The most popular fillers that conducts electricity are metallic micro- and/or nano-particles such as silver, gold, nickel, and copper [1,2,3]. Among these, epoxy and silver are the most commonly used materials for the fabrication of ECAs. These resin-based ECAs have received a great deal of interest as traditional solder alternative due to their some superior features, including low-temperature processing, finer pitch capability, and environmental friendliness, reducing the weight and cost, more flexible, higher stiffness and load transmission, the possibility to bind diverse materials (not only metals) together, excellent thermo-oxidative aging properties, as well as simple processing [4,5,6,7].

These types of adhesives are divided into two main categories known as isotropic conductive adhesives (ICAs) and an-isotropic conductive adhesives (ACAs) depending on the conductive filler loading level. An ICA is conductive in all x-, y- and z-directions because filler concentration is higher than the percolation threshold in their structure, which causes electrons to move in every direction. In contrast, in ACAs electrons can move only in z-direction due to low filler concentration (0.5–5 wt%). Curing of these adhesives is usually done in high pressure along with heating. Thus, there is no continuous electrical conductivity path in ACAs before joining [1, 8, 9]. Silver-filled ICAs have been widely used in a surface mount technology (SMT), flip chip, chip scale package (CSP), and ball grid array (BGA) applications [1, 5, 10].

In the recent years, so many research project have been conducted toward the improvement of physicochemical properties as well as performance of the ECAs using multi-component fillers based on silver micro-/nano-particles [11,12,13,14]. Furthermore, Paik group obtained some convincing data using epoxy-based Sn–58Bi solders [15,16,17,18]. It should be pointed out that some carbon-based (nano-) materials (e.g., carbon nanotubes (CNTs), graphite, graphene and its derivatives, and carbon black) were also applied as conductive filler toward the fabrication of ECAs. In these types of ECAs, the conductivity arises from the formation of a conduction network with electron tunneling or hopping rather than electron transfer through the contacted carbon-based (nano-) materials [19]. The differences based on the percolation theory (the minimum fraction of fillers needed to establish the conducting network) between an ICA and an ACA is illustrated in Fig. 1.

Fig. 1
figure 1

A typical percolation curve showing the abrupt increase in conductivity at the percolation threshold (NCA non-conductive adhesive) [1]

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Despite the above-mentioned advantages of resin-based ECAs, the main drawback of these adhesives is the higher metallic filler loading to achieve proper conductivity, which may reduce the mechanical properties of the resultant materials. The conductivity of these ECAs decreased, in part due to oxidation and corrosion of metallic filler at the interface of substrate and component over the time as well as localization of charge carriers that originated from the aggregation of the metallic nano- and/or micro-fillers. In addition, these ECAs have relatively lower conductivity and unstable contact resistance in comparison with conventional soldering technology [1, 2, 20, 21].

In this context, the replacement of metallic fillers with carbon-based materials (e.g., carbon nanotubes (CNTs), graphite, graphene and its derivatives, and carbon black) as well as intrinsically conductive polymers (ICPs) such as polyaniline (PANI) and polypyrrole (PPy) is suggested as an efficient strategy [8, 21,22,23]. Among these, ICPs are promising conductive materials mainly due to their superior properties as discussed in the following section. In addition, in comparison with metallic fillers, improved mechanical and electrical properties can be achieved due to a more similar chemical nature of the matrix and the ICPs as the filler.

This review paper deals with a detailed vision of electrically conductive adhesives (ECAs) based on intrinsically conductive polymers (ICPs). The design, fabrication, challenges, future directions, as well as applications of these ECAs have been discussed in detail.

2 Intrinsically conductive polymers (ICPs)

Since the introduction of ICPs by Shirakawa et al. in the mid-1970s [24], more and more research efforts have been devoted on the synthesis, modification, and application of these synthetic polymers mainly on the basis of their superior features, including low cost, ease of synthesis, simple doping/dedoping processes, excellent environmental and thermal stability, good mechanical strength, as well as unique magnetic and optical properties [25,26,27,28].

Among the ICPs, polythiophene (PTh), polypyrrole (PPy), polyaniline (PANI), and their derivatives are the most popular members that are widely used in various practical fields, including organic photovoltaics (OPVs) [29], organic field-effect transistors (OFETs) [30], organic light-emitting diodes (OLEDs) [31], supercapacitors [32], secondary batteries [33], electromagnetic interference (EMI) shielding [34], bio/chemical sensors [35], extraction and chromatography [36], environmental science [37], corrosion protection [38], and biomedical fields [39].

The ICPs can be synthesized by two main approaches known as electrochemical or chemical oxidation of corresponding monomers in various organic solvents and/or in aqueous media [25, 26]. The most important advantages of electrochemical approach can be listed as control over thickness and conductivity of the synthesized film by managing the synthesis parameters, including pH, current density, substrate, as well as nature and concentration of the electrolyte. Moreover, simultaneous doping, entrapment of molecules in the polymer matrix, and ease of synthesis are the other advantages of this approach [25, 26]. In contrast, the most important advantage of the chemical approach is the scale-up synthesis of ICPs that are currently almost impossible with the electrochemical method. Another important advantage of this approach is the possibility for post-functionalization of the polymer backbone [25, 26]. The structure and conductivity of the most important members of the ICPs are illustrated in Table 1.

Table 1 Chemical structures, doping type, and conductivity of the most important members of ICPs
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3 Adhesives

An adhesive defines as a non-metallic compound that applied to bind substrates by surface attachment to avoid their separation. The key function of the adhesive layer in an adhesive joint is transferring load from one substrate to another one. The most important mechanism of adhesion is on the base of the adsorption theory. Other mechanisms of adhesion are interlocking, interdiffusion, adsorption, surface reaction, and electronic or electrostatic attraction theory [2, 9, 40]. The most widely used methods for the fabrication of epoxy/conductive polymers adhesives are as follows:

  1. (1)

    Solution blending

  2. (2)

    Hot-melting

Hot-melt formulation adhesives have a wide range of applications, including automotive and home-appliance fields. However, the most important defective is their lack of high-temperature strength that limited utility fields. Some strategies such as modification with reactive urethanes, moisture-curable urethanes, or silane-modified polyethylene have been suggested for circumventing the issues [41,42,43]. In general, non-curing adhesives (e.g., thermoplastics, latexes) get their permanent adhesion by cooling or solvent casting. These types of ECAs aroused much less attention in the literature [8]. Various approaches, including tensile lap-shear, peel, cleavage, and fatigue are introduced for the investigation of the strength of adhesive bonds [44,45,46,47].

Despite some advantages of adhesives over conventional soldering technology as mentioned in the Introduction section, this technology has some disadvantages, including poor stability at high temperatures, relative weakness especially in bonding large objects with a small bonding surface area, and greater difficulty in separating objects during testing [9, 48].

Depending on the adhesion method, adhesives are divided into reactive and non-reactive adhesives, which refers to whether the adhesive chemically reacts to harden. On the other hand, they can be organized according to the raw stock into natural, synthetic and hybrid, or by their starting physical phase. Another categorization of adhesives is based on their conductivity feature that is known as electrically conductive adhesives (ECAs) and thermally conductive adhesives (TCAs) [9, 49, 50].

It is worth noting that PANI and PPy are the most exploited ICPs that are used for the fabrication of ECAs. The demand for ECAs with proper mechanical and adhesive strengths and suitable resistant to both fatigue, chemical as well as environmental conditions encouraged the researchers for design and development of more efficient adhesives that discussed in the following sections.

4 PANI-based ECAs

It is well established that only a few numbers of ICPs are stable enough under normal processing conditions to be incorporated in practical applications. On the basis of this point, PANI is a promising material that finds many applications, mainly due to its easy synthesis and low cost, high polymerization yield, excellent thermal and environmental stability, and tunable conductivity [51,52,53]. On the basis of these properties, PANI has found many commercial and technological applications, including electromagnetic interference (EMI) shielding, secondary batteries, bio/chemical sensors, solar cells, corrosion protection conductive composite materials, electrical memory devices, biomedical fields, and many more [25, 54, 55].

In general, PANI has three different types known as half oxidized emeraldine (EB) (n = 0.5), fully reduced leucoemeraldine (LEB) (n = 0), and fully oxidized pernigraniline (PAB) (n = 1) (Scheme 1) [25].

Scheme 1
scheme 1

Three different types of basic PANI

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A relatively novel achievement with PANI is ECAs mainly due to its significant advantages compared to the metallic nano- or micro-particles as discussed in the Introduction section. Approximately, all of the fabricated ECAs using PANI is limited to the epoxy-based adhesives. The conductivity of the final adhesive material can be controlled through the filler-to-matrix ratio, the doping level of PANI, as well as the processing conditions (e.g., type of solvent and curing temperature). In general, basic amines (e.g., hexamethylenetetramine (HMTA)) are the most commonly used curing agents for epoxy that tend to reduce the conductivity due to deprotonation of PANI facilitated by the basic nature of the curing agent. This problem can be solved through the curing of epoxy/PANI blends using acidic hardeners, including BF3-amine complex and acidic anhydrides [56, 57]. The following context will discuss some of the recently reported studies regarding PANI-based ECAs.

More recently, Khandelwal and colleagues investigated the effect of CNTs on the properties of electrically conductive epoxy/PANI adhesives [56]. The content of PANI in the final formulations were set as 1, 3, 5, 7, 10, and 15 wt%. According to the results, the association of CNTs and PANI has a synergistic effect on the electrical conductivity of the final material (Scheme 2). In detail, the conductivity of the fabricated epoxy/PANI was obtained to be 10−7 Scm−1 with increasing PANI content. The percolation threshold was observed near 5 wt%. The same conductivity level was achieved by adding only 0.1 wt% of CNTs to epoxy/PANI (1 wt%) binary composite that verifies the synergistic effect between PANI and CNTs to high conductivity. Furthermore, it was found that 0.1 wt% of CNTs enhances the conductivity of the epoxy/PANI composites significantly without sacrificing much on the shear strength. Similar results were obtained in our laboratory as reported in 2015 [21].

Scheme 2
scheme 2

The conductive networks in epoxy composites containing PANI (a), CNTs (b), and PANI/CNTs (c) fillers below their percolation threshold concentration [56]

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Some adhesives based on epoxy resin filled with silver flakes and PANI micro-particles were fabricated and their electromechanical properties were investigated by Gumfekar and co-workers [58]. It was found that the addition of PANI has a complex electromechanical effect on the silver/epoxy conductive adhesive composites. According to their results, the contact resistance of the adhesive was increased significantly by the addition of only a small amount of PANI (2 wt%), mainly due to blocking the contacts among silver flakes and prevent the formation of a continuous path among themselves. In contrast, an increase in the PANI content (6 to 15 wt%) exhibited a moderate increase in contact resistance. They concluded that the incorporation of PANI into the conventional epoxy/silver adhesive has some advantages such as tailor the steady-state contact resistance and its dependence on the compressive strain, reducing the cost, control the conductivity to the adhesive bonds or joints, and development of advanced functional devices.

As mentioned, one of the most important problems regarding the PANI-based ECAs is reduced conductivity of the final material due to deprotonation of PANI by the basic nature of the curing agent. Based on this fact, Desvergne et al. conducted a research project for electrical characterization of PANI or tetra-aniline-based adhesive blends under various curing conditions in terms of the type of the epoxy resin, solvent, temperature, and molecular weight [57]. They were selected BF3-amine complexes (Anchor 1115) as curing agents for a series of epoxy-based pre-polymers known as E 730, H61, and OM 100 (Table 2).

Table 2 Chemical formula and composition of base pre-polymers E 730, H61, and OM 100 epoxies that are used by Desvergne and co-workers for the fabrication of ECAs [57]
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In the case of the effect of epoxy type, lower percolation thresholds are obtained for resins with higher epoxy network densities. Among the toluene, tetrahydrofuran (THF) and ethyl acetate (EA) solvents, in all epoxies toluene processed samples exhibited lower electrical conductivity. This result may be originated from the higher aggregation level of PANI in toluene than those of the THF and EA. In the case of molecular weight, as expected, the tetra-aniline-based adhesives showed lower electrical conductivity levels. Despite, the most important advantage of tetra-aniline over PANI is the easier processing condition. In addition, it was found that curing in lower temperature values led to adhesives with higher electrical conductivities.

In another study, Ahmad Mir and Kumar developed a series of epoxy (A bisphenol-F (DGEBF) type epoxy resin; Epon 862)-based ICAs using PANI as powder and as nano-fibers [59]. According to the results, PANI nano-fibers exhibited the lower percolation threshold than those of the PANI powder. Despite, the maximum conductivity obtained in both cases remains the same (10–3 Scm−1). This phenomenon confirms the important role of morphology in the percolation threshold. In detail, PANI nano-fibers have a high aspect ratio, the reinforcement is obtained at very low filler content. In contrast, a higher amount of PANI powder required for a similar effect. Another reason for this phenomenon is the uniform dispersion of PANI nano-fibers in the matrix and formation of effective conducting channels than those of the PANI powder. In addition, the fabricated adhesives using PANI nano-fibers exhibited higher humidity and temperature resistance, lap-shear strength, and thermal stability than those of the fabricated adhesives using PANI powder.

Our group also reported the fabrication of epoxy and chloroprene-based ECAs using PANI nanomaterial as conductive filler [21, 60]. In both works, the fabricated ECAs exhibited acceptable thermal, mechanical, as well as electrical conductivity features. More recently, we designed and developed a novel ECA based on novolac-grafted PANI [20]. The fabricated ECA showed some advantages, including high mechanical properties, excellent thermal stability, and low production cost. At the end of this section, some reported results on the PANI-based ECAs are summarized in Table 3.

Table 3 Some results on the PANI-based ECAs
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5 PPy-based ECAs

Among the ICPs, polypyrrole (PPy) is one of the most promising candidates for producing electrically conducting fabrics mainly due to excellent electrical conductivity, good thermal and environmental stability, reversible redox ability, electrochromism, as well as easy synthesis. This conductive polymer can be synthesized through both chemical and electrochemical approaches in various organic solvents and/or in aqueous media [64,65,66,67,68]. The chemical synthesis mainly involve condensation or addition polymerization approaches. This approach allows the scale-up synthesis of PPy, which is currently impossible with the electrochemical method. Another important advantage of the chemical approach is post-modification of PPy backbone [64, 65].

Due to above-mentioned features, PPy has found numerous applications, including bio/chemical sensors, microwave shielding, corrosion protection, electrochromic displays, supercapacitors, organic electronics, and more recently biomedical applications [65, 69,70,71]. It is worth noting that the physicochemical properties of PPy can be improved through various approaches, including copolymerization and monomer modification (e.g., N- or ring-substituted pyrroles). Three different structures of PPy are shown in Scheme 3.

Scheme 3
scheme 3

Three different structures of PPy

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According to superior physicochemical features of PPy as well as the decrease hazards of the traditional joining materials in electronic purposes, this ICP can be considered as promising alternative material for the fabrication of ECAs. Against PANI, in the case of PPy various matrixes applied for the fabrication of ECAs as discussed in the following.

Recently, Wen and colleagues [72] reported the application of PPy NPs in the fabrication of ECAs. For this purpose, PPy NPs were synthesized through a chemical oxidation polymerization in the presence of poly(vinyl pyrrolidone) (PVP) as the surfactants. These NPs were added in different mass ratios into a mixture of epoxy resin and Ag powder (70 wt%) to obtain PPy NPs-doped ECAs. In this study, the percolation threshold of silver flakes was obtained to be 65 wt%. In detail, when only a small amount of PPy NPs (1 wt%) was added into the 60 wt.% silver-filled ECAs, the electrical conductivity was increased sharply from 125 to 1639 Scm−1. A similar result was observed for other ECAs that filled with various amounts of silver filler. This phenomenon confirms the critical roles of PPy NPs in the improvement of electrical conductivity.

Furthermore, they investigated the effect of curing temperature on the electrical conductivity of the samples. It was found that for the sample with 65 wt% silver flakes doped with 0.3 wt% of PPy NPs increasing the curing temperature from 120 to 180 °C led to increase in the electrical conductivity of the fabricated ECAs. It should be pointed out that in the absence of PPy NPs the curing temperature has a substantial effect on the electrical conductivity of the samples. Finally, incorporation of PPy NPs (2.5 wt%) into 70 wt% silver-filled ECAs and curing at 160 °C for 60 min was found as an optimum condition to afford highest electrical conductivity. They concluded that printed flexible electrical patterns on paper and polyimide substrates can be used as conducting circuits to light LED devices.

Pomposo et al. [73] fabricated a series of intrinsically conducting hot melt adhesives (ICHMAs) composed of PPy and poly(ethylene-co-vinyl acetate) (EVA) copolymer for application in electromagnetic interference (EMI) shielding. The electrical conductivities of the samples were investigated through the four-point probe method, and the percolation threshold and the critical exponent values were found to be 0.07 and 1.21, respectively. Adhesion strength investigation by T-peel test exhibited that the T-peel strength was increased by increasing of PPy volume fraction up to 0.203 and above it’s the value was decreased. In detail, samples containing 20 to 25% volume fraction of PPy exhibited an average T-peel strength about 15 to 20% higher than those of the insulating EVA hot melt adhesive. The higher T-peel strength was rationalized by intermolecular hydrogen bonding between NH groups of PPy and carbonyl groups of EVA copolymer that resulted in the establishment of a reinforcing network of contacts.

The EMI shielding effectiveness (EMI SE) of the samples were calculated using Collaneri–Shacklette expressions and it was found that the fabricated samples have good potential as commercial lightweight quick-bonding EMI shielding adhesive or sealant. In addition, the conductivities were preserved for six months under laboratory environmental conditions post-fabrication.

Ahmad Mir and Kumar developed PPy/epoxy composites as ICAs [74]. The effect of various content of PPy on the curing behavior and thermal degradation properties of the final materials were studied using DSC and TGA, respectively. It was found that the incorporation of PPy has no significant influence on the curing behavior of the epoxy. However, the increasing of PPy content leads to an increase in the curing time and temperature. TGA results indicated that the incorporation of PPy decreases the thermal stability of the samples especially in the temperature range of 100 to 150 °C.

Lap-shear strength analysis revealed that incorporation of PPy in the epoxy matrix decreased the adhesion strength especially in the case of 5 wt% PPy concentration. Despite, there is only a slight decrease observed in lap-shear, thereafter up to 15 wt%. The electrical conductivity of the sample was increased continually with the increasing of PPy content up to 15 wt%. In detail, the conductivity of the samples showed a sharp increase even with the incorporation of only 3 wt% of PPy. Even the threshold percolation taken at 10–6 Scm−1 comes at a lower PPy concentration, i.e., between 5–10 wt%. According to the results, they concluded that the composite with 15 wt% of PPy had the best properties and can be considered as a potential material in the field of ECAs.

At the end of this section, some reported results on the PPy-based ECAs are summarized in Table 4.

Table 4 Some results on the PPy-based ECAs
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6 ICPs-based adhesives for biomedical applications

In the recent years, so many research efforts have been focused on the design and development of bioadhesives. These biomaterials can be used in various biomedical fields, including drug delivery [74], tissue engineering [78], wound hilling [79], and biosensors [80]. In this context, poly(pyrrole-co-dopamine) [P(Py-co-DA)] or poly(aniline-co-dopamine) [P(ANI-co-DA)] or their composites attracted a great deal of interest due to their numerous applications in both industrial as well as biomedical fields [81,82,83,84,85,86]. It is worth noting that the copolymerization of Py or ANI with DA led to some superior physicochemical as well as biological features that are not available when homo-polymer was used alone. In detail, the copolymerization of DA with Py or ANI can improve solubility, biocompatibility, and adhesive properties [85, 86].

More recently, Tan et al. investigated the physicochemical and biological properties of a series of P(ANI-co-DA) copolymers with different monomer ratios [86]. The size of the P(ANI-co-DA) particles was decreased by the increase of DA content in the copolymer. In addition, the dispersibility and stability of the samples in water were improved through the increasing of DA content in the copolymer. This phenomenon may be originated from the hydrophilic catechol groups of DA. The adhesion properties of the fabricated copolymers were investigated using Scotch tape peel-off test, and it was found that the P(ANI-co-DA) has much higher adhesion strength than those of the pure PANI. In detail, almost 100% adhesion of the films was achieved for the copolymer film resulting from 0.95 of DA/ANI mole ratio. Further investigation was conducted using Posi-Test adhesion tester, and the adhesion strengths for PANI and DAPANI-0.19 were obtained almost zero. In contrast, the adhesion forces for DA-PANI-0.48 and DA-PANI-0.95 samples were obtained to be 0.2 ± 0.06 and 0.8 ± 0.03 MPa, respectively. The electrical conductivity of the samples revealed that the incorporation of poor conductive PDA did not enormously weaken the electrical conductivity of PANI, and it still showed good electrical conductivity. In detail, when the DA/ANI mole ratio was below 0.48, the electrical conductivity of copolymer was higher than 0.322 S cm−1 which was good enough for numerous industrial or biomedical applications.

Despite the good biocompatibility of PANI in purified form, it still has been questioned in many cases. In contrast, PDA is a biological neurotransmitter, the excellent biocompatibility and bioactivity of DA have been confirmed by many researchers. The in vitro cytotoxicity of the fabricated PDA, PANI, and DA-PANI-0.48 samples were investigated using MTT assay against HeLa cells. The results showed that the DA-PANI-0.48 has excellent biocompatibility and the cells viability is ~ 84% even at higher concentration (200 µg mL−1). Based on the obtained physicochemical and biological properties, they concluded that the fabricated P(ANI-co-DA) can be used in the various biomedical application, including serving as a surface coating of implant materials or conductive platforms in tissue engineering.

In 2016, Kim and his co-workers reported electrochemical co-deposition of pyrrole and DA (as a bio-inspired adhesive molecule) monomers onto indium tin oxide (ITO) electrode as a conductive and adhesive material [87]. It was found that the addition of DA facilitates the polymerization of pyrrole that indicates a catalytic role of DA. Chronoamperometry (0.5 V) polymerization of various monomer ratios of DA and Py revealed that with increasing DA concentration the oxidation currents increased accordingly, indicating greater and faster copolymerization of DA and Py monomers. Investigation of the deposited copolymeric mass using electrochemical quartz crystal microbalance (EQCM) exhibited that a monomer ratio of DA/Py = 0.20 is an optimum monomer concentration in term of mass deposited and electrical conductivity. In addition, the adhesive strengths of the samples are increased continually with increasing DA concentration. According to biological activity as well as the electrical conductivity of the deposited PDA/PPy copolymer, they concluded that such coated electrode can be employed as bioelectrodes.

In another interesting project, Han and co-workers fabricated a transparent, conductive, stretchable, and self-adhesive hydrogel by in situ formations of PDA-doped PPy nanofibrils, which were interwoven in an elastic and transparent polyacrylamide (PAm) network [77]. The PDA-PPy NPs were synthesized by a dispersion oxidative polymerization method in the presence of poly(vinyl alcohol) (PVA) as a surfactant, and then incorporated into a PAm hydrogel network through the free radical polymerization of acrylamide monomers in the presence of a crosslinker (N,N′-methylene bisacrylamide; BIS-Am), and an accelerator tetramethylethylenediamine (TMEDA). It should be pointed out that dopamine monomer would be not oxidized to PDA with a higher molecular weight in a FeCl3 solution with low pH value. Thus, there is no covalent bonding between PPy and PDA, and they are have π–π stacking and hydrogen bonding interactions. Therefore, under this condition, only DA oligomers with low molecular weights were formed.

They hypothesized an interesting mechanism for the in situ formation of PPy-PDA nanofibrils with the aid of APS under the templating effect of polymer chains. At first, the APS-generated radicals continuously broke down the PDA-PPy NPs into nanodots through reduction of π–π stacking interaction between the oligomeric units in PDA-PPy complex. In the second step, guided by the PAm chains as a template in the hydrogel, the PDA-PPy nanodots were self-assembled to form nanofibrils (Scheme 4).

Scheme 4
scheme 4

The formation of PDA-PPy nanodots and an opaque pre-gel according to Han and co-workers hypothesis [77]

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Electrical conductivity measurements revealed that the aging of hydrogels leads to higher conductivity, mainly due to the structural change of PDA-PPy NPs to nanofibrils in the hydrogel. In detail, the electrical conductivity of the hydrogel containing 0.6 wt% of the PDA-PPy NPs was increased from 7 Sm−1 for the pre-gel to 12 Sm−1 after 3 days of aging. Examination of mechanical properties revealed that the fabricated PDA-PPy-PAm hydrogel could be stretched up to six times to its original length. In addition, it could be compressed and recovered immediately (during 1 s after stretched to six times of its initial length). In addition, the increasing of PDA-PPy content in the hydrogels increased the adhesion strength of the samples. In the case of aging time, one day was found as the optimum to reach the highest adhesion strength. They evaluated the biocompatibility of the fabricated PDA-PPy-PAm hydrogels through MTT assay against bone marrow stem cells (BMSCs) to approve its applications in biomedical fields. It was found that the fabricated PDA-PPy-PAm hydrogels can promote the proliferation of BMSCs more than pure PAm hydrogel. This phenomenon may be originated from high cell affinity of catechol groups in the PDA as well as a conductive substrate that improve intercellular communication [88, 89]. According to various physicochemical as well as biological analysis results, they concluded that the fabricated conductive and self-adhesive PDA-PPy-PAm can be applied in biomedical fields such as soft bioelectrodes and optoelectronic devices.

Another interesting electrically conductive adhesive was fabricated through the conjugation of RGD-based peptide and poly(3,4-ethylene dioxythiophene) (PEDOT) by Alemán group using an electrochemical process (chronoamperometry; CA) as shown in Scheme 5 [90]. SEM and AFM studies revealed that the incorporation of the peptide at the ends of PEDOT chains are not influenced by the morphology and topology features. In contrast, the wettability as well as electroactivity of the conjugates are higher than those of the pure PEDOT. The bioactivity of the fabricated electroactive surface was investigated in terms of cell adhesion using epithelial (Vero and Saos-2) and fibroblast (Cos-7 and MRC-5) cell lines. It was found that the number of cells adhered to the surface of the fabricated RGED-conjugated PEDOT is higher than those of the pure PEDOT or negative control (tissue culture polystyrene plate; TCPS) for all cell lines. These results confirmed the cell-specific adhesion promoted via the RGED ligands conjugated to the PEDOT chains.

Scheme 5
scheme 5

The overall strategy for the synthesis of RGED-conjugated PEDOT [90]

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As the results, they concluded that the fabricated RGED-conjugated PEDOT can be used in various biomedical fields, including the fabrication of multifunctional biomedical platforms for regenerative medicine, and the development of artificial skin based on ICPs, which in addition, promote the regeneration of natural skin.

7 Conclusions and future remarks

Recent progress in the design, fabrication, and application of electrically conductive adhesives (ECAs) based on intrinsically conductive polymers (ICPs) have been described. The most important advantages of these adhesives over conventional tin/lead solders are higher electrical, mechanical, and thermal features, excellent pitch capability, low-temperature processing, and strong adhesion/reliability requirements, low weight and cost, environmental friendliness, the possibility to bind diverse materials (e.g., ceramics, metals, glass, plastics, and paper products) together, as well as simple processing condition. However, these types of adhesives have some disadvantages such as high metallic filler loading to achieve suitable conductivity that reduces the mechanical properties of the final material, and decreases the electrical conductivity due to oxidation and corrosion of metallic filler over the time as well as localization of charge carriers due to aggregation of the metallic filler. Furthermore, metal-filled ECAs have relatively lower conductivity and unstable contact resistance in comparison with conventional soldering technology. Therefore, some improvements are suggested as follows toward the fabrication of more efficient ECAs based on ICPs for both biomedical as well as biological applications.

The simultaneous use of ICPs and carbon-based materials (e.g., CNTs and graphene) as fillers can be led to ECAs with higher performances in terms of electrical conductivity and mechanical properties mainly due to synergic effect of ICPs and mentioned electrically conductive nanomaterials. It is expected that these ECAs exhibit low electrical percolation threshold values due to high electrical conductivity, nanoscopic size, and high aspect ratio of the nanomaterials that facilitate the formation of a conductive network in the association with ICPs. In addition, designing and development of novel adhesives using nanostructured ICPs in association with carbon-based nanomaterials such as graphene and CNTs can improve the physicochemical properties of the final ECAs mainly due to higher ratios of surface area to volume in the nanomaterials that dramatically reduce the percolation threshold as well as facilitate the formation of a conductive network. More interestingly, the graphene quantum dots (GQD) and carbon quantum dots (CQD) can be considered as novel nanomaterials for the fabrication of ECAs mainly due to their superior physicochemical features. In the case of metal-filled ECAs, the simultaneous filling of metallic micro- and/or nano-particles with ICPs can be led to higher performance in term of electrical conductivity. However, some problems can occur regarding the usage of metallic materials as mentioned above.

The design and application of novel curing agents as well as dopants to achieve proper mechanical properties and conductivity are necessary for coming in the case of the ICPs-based ECAs. Considering environmental issues, the design as development of water-processable adhesive systems is necessary for coming due to the hazardous effect of organic solvents or volatile organic compounds. The chemical grafting of ICPs onto the binder resin is neglected defiantly. It seems that this strategy leads to more efficient ECAs due to well-known advantages of copolymers over the corresponding blends.

At the current time, the knowledge of the effect of conductive particles (in the case of this paper ICPs) on the curing process is still on progress. Thus, further studies must be conducted to achieve desired outcomes to control the composition and properties of these materials. The most portion of ECAs is based on PANI and PPy. The use of other ICPs members are neglected defiantly. In this context, the design and development of PTh-based ECAs can be considered as a new research line on the topic.

If the electrically conductive bioadhesives based on ICPs could be developed in such a way that the efficiency and safety improved, while manufacturing costs were kept down, it would be expected to translate into clinical applications in the coming. In this context, animal or vegetable origin materials (e.g., mammalian collagen, casein, blood albumen, and gums) can be considered as promising materials for the development of electrically conductive bioadhesives through the combination (physical blending or chemical bonding) with ICPs. However, due to some problems, including poor mechanical properties and low stability in the physiological environment, the modification of most numbers of natural polymers seems to be essential before the fabrication of bioadhesives. Thus, the design and development of new synthetic or semi-synthetic methodologies for chemical modification of natural polymers toward the efficient bioadhesives seem to be necessary.