Organic semiconductor materials for neuromorphic and bioelectronic systems design rules and applications

Summary

The ability of organic semiconductors to be mechanically flexible, adjustable in their electronic properties, solution processable, and biocompatible has led to their identification as key materials for Neuromorphic and Bioelectronic systems. This review summarizes the relationship between molecular design, crystallinity and charge transport mechanisms, and device performance and discusses the role of conjugated polymers, small-molecule semiconductors, and mixed ionic-electronic conductors for brain-inspired computing and biointegrated electronics. It also explores key device architectures, including organic field-effect transistors, organic electrochemical transistors, and organic memristors with a focus on their demonstration of synaptic plasticity, learning and neural signal processing. Biosensing, neural interfaces, and implantable bioelectronics are explored in terms of challenges of operational stability, scalability, and long-term biocompatibility. This review seeks to establish a framework that facilitates the development of flexible, energy-efficient, and biologically compatible electronic technologies, by combining materials design with device engineering and novel applications.

Graphical abstract

Applied sciences; Engineering

Introduction

With the rapid development of smart electronic systems, the next-generation technologies aiming to replicate the functionality of the human brain, such as flexibility, parallelism, and energy efficiency, have all received a great deal of interest.¹ Memory and processing units are separate in traditional computing architectures based on the von Neumann model. This requires a significant amount of energy and results in delays when the data are transmitted.² Biological neural systems, conversely, integrate memory and computation in synapses and neurons, so that learning, adaptation, and processing are more efficient.³ These capabilities resulted in developing neuromorphic computing, a discipline aiming to imitate the form and function of a neural network with electronic instruments. Meanwhile, bioelectronics has evolved toward linking electronic systems to life. The first bioelectronic devices were primarily utilized in biosensing and capturing electrophysiological signals.⁴ Yet more recent advances have nudged the field toward interactive systems that can communicate with biological tissue in both directions. For uses such as wearable health tracking systems, neural interfaces as well as implantable medical equipment, there is a need for electronic components that are electrically efficient, yet mechanically compliant and biocompatible.⁵ Conventional semiconductor devices based on crystalline substances such as silicon, gallium arsenide, and metal oxides have been the basic building blocks of modern electronics for decades.⁶ These materials have substantial carrier mobility, plenty of stability, and established techniques to manufacture them, and this renders them suitable for highly efficient digital and analog circuits. Their rigid mechanical structure or requirement for processing over high temperatures makes them unsuitable for use in applications in need of flexible, stretchable, or biocompatible electronic systems.⁷ Such mechanical incompatibility can hinder the long-term reliability and functionality of devices due to the mechanical and biological tissue differences between rigid and flexible inorganic semiconductors present in neuromorphic and bioelectronic applications.⁸ In addition, conventional semiconductor devices utilize mainly electronic charge transport methods, which are not compatible with the ionic signaling processes encountered in the fields of biological communication. Because of this simple difference, conventional semiconductor devices cannot work well in biological environments. Further, the complex and energy-demanding character of classical complementary metal-oxide-semiconductor (CMOS) architectures hinders large-scale, energy-efficient neuromorphic hardware design.⁹

Organic semiconductors have emerged as a good alternative to the first-generation neuromorphic and bioelectronic equipment because of their special integration of electrical, mechanical, and chemical characteristics.¹⁰ The Figure 1 shows the evolutionary progress of semiconductor materials. These properties have π-conjugated molecular structures, which allow them to be tailored in the electronic structure and charge transport characteristics. Organic matter is produced by methods such as spin coating, printing, and roll-to-roll (R2R) fabrication, using low temperature solutions, which enable high volume lightweight and portable electronic devices.¹¹ One of the most remarkable advantages of organic semiconductors is that they can carry out mixed ionic-electronic conduction, a process that is very similar to the ion-mediated signaling mechanisms present in living things.¹² As a function, this enables good integration between electronic devices and biological tissues, making organic materials well-suited to neuromorphic synaptic devices, neural interfaces, and biointegrated sensors. Organic electrochemical transistors (OECTs), organic field-effect transistors (OFETs), and organic memristive devices have demonstrated that they are capable of reproducing important synaptic functions such as short-term plasticity (STP), long-term potentiation, and spike-timing-dependent plasticity.^13,14 These functionalities are essential when attempting to create artificial neural networks that function similar to biological brains that learn and adapt. Organic semiconductors have a highly flexible nature as well, in which they could be shaped to develop molecules and be chemically modified to modify the electronic characteristics and stability and biocompatibility.¹⁵

Figure 1.

Evolution of organic electronic materials as well as devices toward neuromorphic and bio-integrated systems

The emergence of conjugated polymers, small-molecule semiconductors as well as hybrid organic-bioelectronic substances has enabled major advances in the functionality of devices, facilitating adaptable neuromorphic systems and implanted implants in such new-fangled devices for versatile neuromorphic systems and medical innovations.¹⁶ While a number of recent studies have been published on organic semiconductors for either neuromorphic computing or bioelectronic uses, this review offers a wider and more integrated viewpoint through systematically integrating these two emerging fields via typical material platforms, charge transport mechanisms, as well as device engineering approaches. Particularly, this review links molecular design concepts, mixed ionic-electronic conductivity behavior as well as interface engineering to the operational demands of artificial synapses, memristive systems, organic-based electrochemical transistors as well as biointerfacing devices. Also, the recent development of the versatile, stretchable, and self-healing and biologically compatible organic semiconductor elements is systematically summarized in consideration of future-generation intelligent biointegrated systems. This work, compared with previous reports, puts more emphasis on new developments including AI-powered biological electronics, wearable neuromorphic systems, energy-efficient organic hardware and translational issues of longevity, scalability, and practical implementation. Here, we present an integrated interdisciplinary system that combines neuromorphic and bioelectronic approaches and considers potential applications for advanced organic semiconductor systems.

Fundamentals of organic semiconductor materials

Organic semiconductors constitute an attractive electronic substance with electronic structure along with charge transport due to the conjugated organic compounds or polymers.¹⁷ Inorganic semiconductors have recurring crystal lattices, and hence electronic bands can propagate. Organic semiconductors also consist of molecular units bonded by weak van der Waals forces. Grasping such fundamental ideas is necessary to allow materials to enable effective charge transport (Figure 2) as well as stable device functioning in neuromorphic as well as bioelectronic systems.

Figure 2.

Charge transport mechanisms in organic semiconductors: a shift from hopping to band-like regimes

Molecular structure and electronic properties

Organic substances are considered to be semiconductors due to the molecular structures of π-conjugated molecules. Such structures have alternating single and double bonds which allow π-electrons to flow through their molecular backbone.¹⁸ This delocalization forms molecular orbitals through which the charge carriers may pass through the substance. In inorganic semiconductors, the valence band as well as conduction band are, respectively, the HOMO and the LUMO.¹⁹ Extended π-conjugation in conjugated polymeric materials (especially polythiophenes, polyfluorenes, and diketopyrrolopyrrole-based materials) facilitates the movement of electrons and decreases the bandgap, thus facilitating the movement and injection of charges toward the substrate.²⁰ Nevertheless, electron delocalization remains constrained by structural disorder as well as weak intermolecular coupling in nearly all organic semiconductors. This is why charge carriers typically diffuse via localized molecular sites instead of fully delocalized bands. One of the primary issues of organic electronics is to balance the design and spatial organization of molecular frameworks.²¹ The longer length conjugation and enhanced planar structure renders the charges simpler to shift owing to the enhanced intermolecular π-π stacking interactions. However, excessive aggregation causes trap conditions and instability of the shape of the substance. Therefore, molecular engineering strategies such as side-chain modification, donor-acceptor architectures, as well as backbone rigidification are often used to improve electronic properties while still maintaining processability.²² The energy level orientation of organic semiconductors along with device electrodes is essential for the effectiveness of charge injection as well as overall gadget functionality.^23,24,25 Effective charge injection requires a synchronized coordination among the energy levels in the semiconductor and the task activity of the electrode.²⁶ In practical devices, energy level alignment is also influenced by what happens near the interface in terms of dipole formation, chemical reactions at the interface and surface states. This becomes even more challenging for neuromorphic and bioelectronic devices, especially those that are deployed in water or electrolyte environments. These electrolyte ions transform local electrostatic potentials, which, in turn, alter the energy levels and charge carrier densities. As a result, stable interfaces and appropriate electrode materials continue to remain a paramount concern of organic semiconductor device engineering.

Charge motion of organic semiconductors

In a majority of organic semiconductor films, structural disorder gives rise to localized electronic states, that is, hopping transport is observed at temperature rise.²⁷ In this process, the charge carriers move from one molecular site to another by overcoming energetic barriers. Thermal fluctuations usually support them. The energetic distribution of localized states and the distance between molecules greatly affect the hopping process.²⁸ In crystalline inorganic semiconductors, hopping transport generally causes carriers to be less mobile than band transport. It also renders the system extremely sensitive to temperature, disorder, and trap states.²⁹ Better intermolecular coupling between molecules could lead to band-like transfer in highly structured organic crystals or well-structured polymer films, where charge carriers can travel through partially delocalized electronic states.³⁰ This lowers the temperature and therefore increases the movement of the carriers as in the instance of inorganic semiconductors. For band-like transfer, it requires rigors control of the molecular packing, crystallinity as well as the shape of the thin films. Researchers have studied solvent engineering, epitaxial growth, and directional crystallization to improve structural order.³¹ Regardless of all these improvements, it remains difficult to achieve reliable band-like transport in huge-area organic films owing to basic disordered conduct and other differences in the way they are prepared. The molecular ordering in a solid film can be a severe limitation for its capability to couple and transfer charges. The distance between molecules, the orientation of the molecules, and the π-π stacking interactions influence the proximity of nearby molecular orbitals.³² Small changes in the way things are stacked can make a big difference in how easy it is for carriers to move. On the other hand, high crystallinity enhances the charge transport primarily by decreasing the energetic disorder as well as improving the orbital overlap.³³ Nevertheless, high crystallization can render it too inflexible for biointegrated devices as well as flexible electronics. These methods as the incorporation of flexible side chains, the design of semicrystalline polymer structures, are essential to maintain electronic efficiency and mechanical durability.

Key performance outcomes

In organic semiconductors, charge-carrier mobility is one of the most important parameters to assess the electrical performance of materials, as it relates to the speed of charge carriers moving in the material under an applied electric field³⁴ (Table 1). The switching speed, current output, transconductance, and overall efficiency of an OFET is directly determined by the carrier mobility.⁴⁴ The charge carrier mobilities of typical organic semiconductors range from 10⁻⁴ to 10 cm² V⁻¹ s⁻¹.⁴⁵ In particular cases, architectures and materials have been optimized to obtain mobilities >20 cm² V⁻¹ s⁻¹. However, the electron mobility of crystalline silicon is approximately 1,350 cm² V⁻¹ s⁻¹, and the hole mobility is approximately 480 cm² V⁻¹ s⁻¹.⁴⁶ This is because inorganic semiconductors have a highly ordered covalent crystal lattice that allows for efficient charge transport in a 'band' structure, whereas in most organic semiconductors charge transport is by intermolecular hopping between localized molecular orbitals. Although not as mobile, organic semiconductors are typically found to have adequate charge transport properties for use in neuromorphic and bioelectronic devices, where fast switching speeds are not the primary goal of device operation, but rather where the device requires biomedical applications that lower power consumption, analog signal processing, flexibility, and biocompatibility.⁴⁷

Table 1.

Important organic semiconductor materials and electrical activity

Material	Type	Chemical class	Typical charge carrier mobility	Bandgap (eV)	Applications	Reference
Pentacene	p-type	Small-molecule organic semiconductor	∼0.1–3 cm2 V−1 s−1	∼2.2	Very mobile; commonly used in flexible electronics and organic field-effect transistors (OFETs).	He et al.35
P3HT (Poly(3-hexylthiophene))	p-type	Conjugated polymer	∼10−3–0.1 cm2 V−1 s−1	∼1.9–2.0	Can be processed into a solution; often used in organic photovoltaics and neuromorphic devices	Kesornsit et al.36
PEDOT:PSS	p-type (conductive polymer)	Conducting polymer blend	∼10−3–1 cm2 V−1 s−1	∼1.6	It has high conductivity and is clear. It is used in bioelectronics, electrodes, as well as OECTs.	Bhat et al.37
DPP-based polymers	p-type/ambipolar	Donor-acceptor conjugated polymers	∼0.1–10 cm2 V−1 s−1	∼1.2–1.8	Used in OFETs and neuromorphic electronics, it is very stable and mobile.	He et al.38
C60 (Fullerene)	n-type	Carbon allotrope (fullerene)	∼0.01–1 cm2 V−1 s−1	∼1.7	Good at moving electrons; used a lot in organic solar cells and n-type OFETs	Prylutska et al.39
PCBM ([6,6]-Phenyl-C61-butyric acid methyl ester)	n-type	Fullerene derivative	∼10−3–0.1 cm2 V−1 s−1	∼1.8	A common electron acceptor in organic solar cells and hybrid devices.	Nápoles-Duarte et al.40
N2200 (P(NDI2OD-T2))	n-type	Naphthalene diimide polymer	∼0.1–1 cm2 V−1 s−1	∼1.3–1.5	Air-stable n-type polymer; used in circuits that act like brains and electronics that are flexible	Gish et al.41; Sharma et al.42
Rubrene	p-type	Organic semiconductor with small molecules	∼10–20 cm2 V−1 s−1 (single crystal)	∼2.2	Extremely high mobility; utilized in advanced OFET research	Nakayama et al.43

Another significant parameter for the performance of organic electronic devices is its operational stability. Organic semiconductors are prone to degradation from oxygen, water, ultraviolet radiation, and thermal stress.⁴⁸ The presence of such environmental factors can lead to oxidation, molecular degradation, morphological instability, and charge trapping that can affect the working performance and life of the device. The latter challenges are significant for implantable and wearable bioelectronic systems in physiological environments. Several strategies have been developed to improve stability such as molecular engineering of semiconductor backbones, encapsulation technologies, interface optimization, and design of donor-acceptor polymers with improved environmental resistance.⁴⁹ The impressive advancement in device lifetime is an ongoing topic of investigation, but the operational reliability in the actual operating conditions requires further research.

Organic semiconductors are also mechanically flexible. Unlike the naturally strong and brittle inorganic semiconductor wafers, a significant number of organic semiconductors can be deposited on flexible polymeric substrates without degrading the electrical properties of the material when it is bent, stretched, or otherwise mechanically deformed.⁵⁰ The property is created by the molecular structure of organic materials as well as the ability of solution processing methods to be compatible with light-weight polymer substrates. The properties render organic semiconductors more appealing for electronics that can be worn, implanted in the human body, attached to skin, and flexible neuromorphic electronics. Bioelectronic applications are also interesting for their biocompatibility.⁵¹ Biological adverse responses of the organic semiconductor can be modified, and stable interactions with living tissues can be obtained. Furthermore, for certain materials ionic and electronic conductivities are available which allows for efficient signal transfer between the biological system, which communicate primarily by means of ions, and the electronic device, which operates by means of electronic charge carriers.⁵² A mixed conduction is especially attractive for neural interfaces, biosensors, and neuromorphic devices that emulate biological signal processing. Although considerable progress has been made, there is still a significant materials design challenge that needs to be overcome to achieve high charge-carrier mobility, long-term stability, mechanical compliance, and biocompatibility, all at the same time. In the future, interdisciplinary advances in molecular engineering, thin film processing, interface science and device physics and biomedical materials research are needed to advance the field.⁵³ Low noise and reliable performance of future organic neuromorphic and bioelectronic systems will likely be achieved by further development of design of semiconductors, device architecture, and fabrication technologies.⁵⁴

Classes of organic semiconductor materials

Organic semiconductor materials for neuromorphic and bioelectronic systems can have a variety of molecular architectures, electronic structures, and interface properties. The materials examined in the present review are primarily classified based on their intrinsic material architecture, not device function, transport mechanism, etc., to obtain a consistent, non-overlapping classification scheme. Organic semiconductors can be classified into three classes: (1) conjugated polymer semiconductors, (2) small-molecule organic semiconductors, and (3) organic composite and hybrid semiconductors.⁵⁵ Within these structural classes, the functional characteristics such as the mixed ionic-electronic conduction are discussed as specialized subclasses because these properties can be exhibited by different material families. The classification, which is based on architecture, eliminates the ambiguities caused by category overlaps and offers a systematic way to compare structure-property relationships and device performance in the context of neuromorphic and bioelectronic applications.

Polymer semiconductors

Conjugated polymer semiconductors are among the most well-studied classes of organic electronic materials because of their mechanical flexibility, solution processability, and tuneable electronic properties.⁵⁶ The behavior of these materials is a semiconductor because of efficient charge transport through delocalized electronic states along the polymer backbone, as a result of the extended π-conjugation. The materials are particularly appealing to flexible and large-area electronics. One of the most studied conjugated polymers is polythiophene derivatives, particularly regioregular poly(3-hexylthiophene) (P3HT), which has been adopted as a reference polymer for organic electronics.⁵⁷ Ordering of molecules, crystallinity, and \pi-\pi stacking between molecules is strongly affecting the charge transport properties of P3HT. Greater structural regularity leads to a higher overlap of the orbitals and better mobility of the carriers. The traditional polythiophenes, however, are not always environmentally stable and have moderate electron transport activity.⁵⁸ For these reasons, donor-acceptor (D-A) conjugated polymers have been created.

Small-molecule organic semiconductors

Organic electronic materials are a class of materials represented by small-molecule organic semiconductors, which are made up of discrete π-conjugated molecules with well-defined molecular structures and high chemical purity.^59,60 Unlike conjugated polymers, these materials have well defined molecular architectures that allow for reproducible electronic properties, efficient molecular packing, and relatively high charge-carrier mobility. The molecular order and crystallinity of these materials are responsible for the efficient charge transport through π-π stacking interactions and thus are desirable for high-performance organic electronic devices.⁶¹ Representative small-molecule semiconductors are pentacene, rubrene, diketopyrrolopyrrole (DPP)-based molecules, perylene diimide (PDI) derivatives, naphthalene diimides (NDIs), and various D-A molecular systems.⁶² High molecular engineering capabilities through side-chain modification, heteroatom incorporation, and energy levels can offer the opportunity to tailor the charge mobility, environmental stability, and the interfacial properties for specific device applications.⁶³

For mimicking synaptic functions such as STP, long-term potentiation, paired-pulse facilitation, and learning behavior, small-molecule semiconductors have been applied in OFETs and memristive devices used for neuromorphic electronics.⁶⁴ They possess well-defined molecular structures that enable controlled charge trapping and detrapping processes, which are critical for the programmable conductance states and low-power information processing. These materials have also been investigated in the field of bioelectronic materials for use in soft substrates and flexible sensors, in wearables and implantable electronic interfaces, where the solution processability, bioelectronic compatibility, and mechanical flexibility of these materials allow effective communication with biological tissues.⁶⁵ Nevertheless, small-molecule semiconductors have a number of drawbacks, such as film non-uniformity, mechanical strength, and large-area fabrication that cannot be overcome by polymeric semiconductors. Advances in molecular design, crystal engineering, and integration of hybrid materials are making them more stable, flexible, and scalable, expanding their possible applications in next-generation neuromorphic computing and bioelectronic devices.⁶⁶

Organic composite and hybrid semiconductors

Organic composite and hybrid semiconductors are another group of structures that consist of combining organic semiconducting matrices with a functional element (such as organic molecules or biological molecules) to improve certain properties of the material.⁶⁷ These materials are advantageously composed of synergistic combinations of multiple components which are unlike polymer and small-molecule semiconductors. Hybrid semiconductors can have enhanced biocompatibility, biodegradability, mechanical strength, environmental stability, or charge transport depending on the type of component used.⁶⁸ Bio-derived semiconductors based on proteins, peptides, nucleic acids, or natural pigments, and organic-inorganic composites based on metal oxides, carbon nanomaterials, and two-dimensional materials are typical examples.⁶⁹ The hybrid architectures are promising for applications such as biosensing, neural interfaces, implantable bioelectronics, and neuromorphic electronics. The molecular structure and the low molecular weight of small-molecule organic semiconductors can be well defined and can be synthesized in a reproducible way, and systematic structure-property studies can be carried out.⁷⁰ These materials often have better crystallinity and packing of the molecules compared with polymeric semiconductors, resulting in better charge carrier mobility. Small-molecule semiconductors include the most studied of the acenes, such as pentacene. They have planar aromatic structures that enable intermolecular π-π interactions and efficient charge transport in crystalline films. The mobilities of the carriers in devices fabricated from pentacene can rival those in amorphous silicon.⁷¹ But a practical application of pentacene devices is often restricted by the sensitivity to oxidation and photo-induced degradation. The DPP class of small-molecule materials is another important class. The planar structures and electron-withdrawing group are rigid enough and are strong enough to facilitate the extended conjugation and efficient intermolecular interactions, which lead to its excellent charge transport and optical properties.⁷² Moreover, n-type small-molecule semiconductors including NDIs and PDIs have high electron affinity, chemical stability, and efficient electron transport.⁷³ These properties make them desirable additions to the predominantly p-type character of much of the rest of conjugated polymers and make them particularly well suited for use in ambient conditions in neuromorphic and bioelectronic systems.

An emerging class of materials is organic-bio and hybrid composites, semiconductors, which are an organic semiconducting matrix with a biological or inorganic functional part.⁷⁴ These systems are designed to incorporate electronic function and enhanced biocompatibility, biodegradability, environmental stability, or mechanical strength. Bio-derived semiconductors are semiconductors that contain biomolecules such as proteins, peptides, nucleic acids, and natural pigments. They typically have poorer charge transport properties than fully synthetic semiconductors, but being intrinsically compatible with biological environments, they are promising for use in biosensing, neural interfacing, and implantable bioelectronic applications. The organic-inorganic hybrid semiconductors are composite materials that have the structural flexibility of organic materials and the excellent electrical, optical, or mechanical properties of inorganic materials.⁷⁵ Hybrid nanocomposites of metal oxides, carbon nanomaterials, and 2D materials exhibited improved charge transport and improved environmental stability and reliability of devices.⁷⁶ These hybrid devices have demonstrated significant potential for the development of future generations of neuromorphic electronic devices that demand high performance and durability.

Device architectures for organic neuromorphic electronics

Based on working mechanism and the device architecture, organic neuromorphic systems can be classified further. While material classification is done based on the composition of molecules and charge transport properties, device classification is done based on the way an electrical signal is generated, modulated, stored, and processed in a neuromorphic circuit.⁷⁷ In the context of this review, the most prominent device architectures mentioned are the OFETs, OECTs, electrolyte-gated organic transistors (EGOTs), and organic memristive devices (Figure 3).

Figure 3.

A look at the differences between organic electronic devices like OFETs, OECTs, and organic memristors

OFETs

In OFET-based neuromorphic devices, trapping and de-trapping of charges at the semiconductor-dielectric interface or at specific dedicated charge storage layers is the main control on the synaptic response.⁷⁸ With the application of the gate voltage pulse, for example, the channel conductance is slowly changed by trapping of charge carriers in dielectric defects, ferroelectric domains or interfacial states.⁷⁹ As a result of the repeated stimuli, the trapped charges add up to create the same conductance enhancement as is observed in synapses (potentiation) (Table 2); the loss of the trapped charges or recombination of them creates the same conductance reduction as is observed in synapses (depression) (Figure 4). Depending on the trapping dynamics and retention time, OFETs are able to simulate both STP and long-term plasticity (LTP).⁹⁶ Furthermore, the degree of conductance modulation is dependent upon pulse frequency, amplitude and duration, which can lead to implementing learning behaviors like paired-pulse facilitation, spike-rate-dependent plasticity, and memory retention.⁹⁷ Therefore, OFETs provide a controllable platform for emulating the charge storage-induced weight updates in biological synapses.

Table 2.

Efficiency comparison of organic neuromorphic devices documented in literature

Device type	Active material	Device structure	Operating voltage	Response time	Synaptic functions demonstrated	Typical applications	Reference
Organic electrochemical transistor (OECT)	PEDOT:PSS	Electrolyte-gated transistor	∼0.1–1 V	ms – s	Short-term plasticity (STP), and long-term plasticity (LTP)	Neural interfaces and biosignal processing	Ling et al.80; Mariani et al.81; Yamamoto et al.82
Organic field-effect transistor (OFET)	Pentacene, DPP-based polymers	Thin-film transistor	∼5–40 V	μs – ms	Spike-timing dependent plasticity (STDP) is a type of learning behavior.	Circuits for neuromorphic computing	Torricelli et al.83; Wang et al.84; Yu et al.85
Organic memristor	P3HT, PEDOT:PSS	Metal-organic-metal structure	∼0.5–5 V	ns – μs	Resistive switching and memory retention	Artificial synapses, and memory devices	Wang et al.86; Lee et al.87; Luo et al.88
Electrolyte-gated organic synaptic transistor	P3HT/Ion-gel	Electrolyte-gated device	∼0.1–2 V	ms	STP, LTP, and paired-pulse facilitation (PPF)	Brain-inspired computing	Bao et al.89
Hybrid organic-inorganic memristor	Organic polymer + metal oxide	Crossbar array structure	∼1–3 V	ns – μs	STDP, changing the weight of synapses	AI hardware accelerators	Khichar and Hazra90; Min et al.91
Organic ferroelectric transistor	P(VDF-TrFE) with organic semiconductor	Ferroelectric gate transistor	∼2–20 V	μs – ms	Memory that doesn’t change, synaptic plasticity	Neuromorphic memory devices	Cai et al.92; Sun et al.93
Organic ionic transistor	PEDOT-based materials	Ion-conducting channel	∼0.1–1 V	ms – s	Analog modulation of conductance	Bioelectronic interfaces	Clua Estivill et al.94; Inal et al.95

Figure 4.

Imitating biological synapses in organic neuromorphic electronics: mechanisms and device strategies

OECTs

Ion-driven electrochemical modulation of the semiconductor channel causes the neuromorphic behavior of the OECTs.⁹⁸ Under a gate voltage, ions enter from the electrolyte into the bulk of the organic semiconductor and cause reversible electrochemical doping/de-doping. This process results in a change of the carrier concentration within the entire channel volume, thus leading to gradual and analog conductance changes similar to the biological synaptic weight adaptation.⁹⁹ The ionic migration kinetics lead to a time-dependent response which enables OECTs to reproduce STP through transient ionic redistribution and LTP through stable electrochemical states (Figure 5). A series of voltage pulses can cause a gradual increase in the conductance of the channel (synaptic potentiation), and vice versa, negative polarity pulses will cause the conductance of the channel to gradually decrease (synaptic depression).¹⁰⁰ Ionic transport and relaxation happen on time scales relevant to biology, therefore OECTs are particularly well suited to reproduce synaptic behaviors in time scales relevant to biology, such as paired-pulse facilitation, spike-timing-dependent plasticity, and memory retention phenomena.

Figure 5.

Hybrid organic bioelectronic interfaces for neural sensing and neuromorphic applications

EGOTs

The dynamic modulation of the electric double layer charge accumulation at the electrolyte-semiconductor interface is responsible for the synaptic functions in EGOTs. Under the influence of voltage pulses, a fast redistribution of the ions around the semiconductor interface causes reversible changes in the number of channel carriers and the channel conductance.¹⁰¹ Unlike OECTs, where the ions penetrate into the bulk of the semiconductor, the operation of EGOTs is mainly based on interfacial electrostatic effects. The fast response of the conductance to charging and uncharging the electric double layer is akin to biological STP.¹⁰¹ Repeated stimulation can lead to the accumulation of residual ionic distributions, which in turn can lead to longer-lasting conductance states that can allow memory effects and LTP.^102,103 This allows EGOTs to generate a variety of key synaptic properties, such as the response to excitatory postsynaptic current, paired-pulse facilitation, potentiation, depression, and frequency-dependent learning behaviors, at low voltages and at minimal electrochemical degradation.

Organic memristors and devices that switch resistance

The history-dependent resistance switching that occurs in organic memristors is attributed to the trapping of charge in the organic layer, migration of ions, redox reactions or the formation of conductive filaments within the organic layer.¹⁰⁴ Internal resistance state of the device is modulated by external voltage stimuli. This results in non-volatile conductance modulation which directly imitates the weight storage of synapses. Gradual increases of conductance correspond to synaptic potentiation and gradual decreases to synaptic depression.¹⁰⁵ Memristors’ resistance state is reliant on the electrical signals that have been applied before, making the device have memory properties that are similar to those of biological synapses.¹⁰⁶ Moreover, the analog resistance modulation enables the incorporation of learning functions and mechanisms like long-term potentiation and depression, spike-timing-dependent plasticity, and associative learning.¹⁰⁷ These properties give organic memristors significant promise for high-density applications in neuromorphic computing systems and hardware neural networks.

Performance optimization strategies

But despite a huge increase of novel research in organic semiconductor materials and device architectures, the quest for high-performance and long-term reliability in neuromorphic and bioelectronic applications remains extremely challenging.¹⁰⁸ Organic semiconductor devices have extreme sensitivity to any structure, interfacial impact, and environmental changes, having a significant influence on their functionality and stability.

Designing molecules and adding chemical functions

The molecular structure is of crucial importance for the investigation of the electronic, structural as well as mechanical behavior of organic semiconductors. Chemical synthesis alters organic semiconductors, unlike inorganic solids, which have stable crystalline shapes.¹⁰⁹ The bandgap, the charge carrier mobility, solubility, and the intermolecular interactions may be altered by altering the molecular backbone or by adding functional side groups. One important strategy is to promote π-conjugation and molecular planarity. It makes moving charges among molecules simpler since the orbitals of the two molecules overlap more.¹¹⁰ Rigid and flat molecular backbones are especially favorable for π-π stacking interactions, which enable better mobility of charge carriers. However, too much molecular aggregation usually causes defects in crystals and unevenness of the film. Therefore, a balance in the molecular design is required. Side-chain engineering is an effective approach to improve organic semiconductors. The solubility of the material can be increased by flexible alkyl or polar side chains, which improves the capability of packing the films and consequently allows the application of solution-based processing processes.⁵⁴ Side chains, too, impact how molecules pack, and how they pair electronically, changing the routes charge can traverse. Chemical functionalization can also be applied in neuromorphic and bioelectronic applications to introduce ionic or hydrophilic groups that facilitate the occurrence of mixed ionic-electronic conduction.¹¹¹ These changes enable the device to interface more readily with biological electrolytes and more sensitized to ionic signals. However, incorporation of the ionic functions could also further degrade electronic mobility or make things unstable, so the molecular structure has to be carefully optimized.

Engineering interfaces and changing surfaces

In organic semiconductor devices, the interfaces are very important because the charge transport usually occurs at the boundary of two different materials, e.g., semiconductor-dielectric interfaces and semiconductor-electrode interfaces.¹¹² Poor interfaces may lead to trap states that disrupt passage, further complicate charge flows, and make devices more difficult to work. The ultimate goal of interface engineering is to make electronic coupling better and make these edges of electronics tighter. A popular way has been to change the electrode surfaces by making them self-assembled monolayers or a few interfacial layers, changing the work function of the electrode.¹¹³ Plasma treatment, polymer buffer layers, and dielectric surface functionalization are among the most widely used methods that facilitate better interfacial properties.¹¹⁴ Similarly, if bioelectronic gadgets are to interface with living organisms, interface engineering should take into account how bioelectronic devices interface with living organisms. Surface modification with biocompatible coatings or functional biomolecules can improve the stability of the devices and allow them to sense biological signals more selectively. Alternatively, adding interfacial layers can complicate the gadget and could impact the path charge takes if not done carefully.¹¹⁵

Doping and charge injection engineering

Doping is a successful method of modifying the electrically conductive attributes of organic semiconductors through alteration of charge carriers’ values in organic semiconductors.¹¹⁶ Doping increases the conductivity and decreases the contact resistance, so that the charge can enter electronic devices efficiently. Depending upon the required charge properties, p- or n-type doping techniques are used. Charge transfer interactions can transmit electrons or holes from the molecular dopants to the semiconductor, thereby adding more charge carriers. This gives a greater carrier density and better electrical conductivity. Design for the charge injection process at electrode and semiconductor interfaces is another important component of improving performance.¹¹⁷ Poor charge injection is also detrimental to the device functioning, even if the semiconductor has a high intrinsic mobility. Implementing injection layers, altering the work functions of the electrodes, or implementing high-capacitance electrolytes can all drastically enhance charge injection results.¹¹⁸ In neuromorphic devices, the controlled doping can modulate how the synapses behave by altering the memory and conductivity of the devices as well. But if doing doping too much, leakage currents can get worse and devices become less stable, or introduce new trap states. So to optimize a device’s performance, we need to be able to precisely control the concentration and distribution of doping.

Improving stability and placing things in capsules

Nevertheless, among the features of organic semiconductor devices is sustained stable operation. O₂, moisture, UV radiation, temperature changes, etc., often affect organic materials.¹¹⁹ These things can lead to chemical breakdown, oxidation, and even a change in its shape, all of which make the device work even worse. To resolve this problem, researchers have provided a range of methods to make more stable materials and devices. One way to do so is by producing molecular architectures that by themselves are stable and not at the touch of light or oxygen degrade. Donor-acceptor polymer systems and fused-ring aromatic structures have been consistently superior to older organic semiconductor materials.¹²⁰ The simplest approach of protecting organic devices from the elements is to put them in a box. Thin obstacle layers of inorganic oxides, polymers, or hybrid materials—that keep oxygen and moisture out but still enable the device to bend. Multilayer encapsulation structures are common for elevated barrier performance in flexible electronics. For bioelectronics, encapsulation methodologies must be safe for life forms and stable for chemical operation.¹²¹ Encapsulation layers must not only prevent the semiconductor from breaking down but also allow for its interaction with biological signals as needed. But encapsulation complicates a process and can alter the flexibility of a device or its ability to transmit a signal. Light and durable encapsulation materials remain one of the top research directions for advancing organic semiconductor technology.¹²² In other words, you have to use several fields of molecular engineering, interfacial control, doping strategies, stability, to optimize this performance at the organic scale to the fullest from the point of view of the chip you are producing. The optimization methods here are necessary to improve charge transport and reduce energy waste that means better life in a device.^123,124 These fields must continue to advance for high-performance organic semiconductor devices to thrive in neuromorphic computing and bioelectronic devices.

Technologies of organic devices fabrication of neuromorphic devices

Fabrication technologies are categorized according to the physical state of the semiconductor precursor material when the film is formed to offer a uniform classification system.¹²⁵ The manufacturing techniques can then be assigned to solution-based processing technique and to vapor phase deposition techniques, where the semiconductor materials are deposited from a solution and where the thin film is formed by evaporation and condensation processes under controlled atmosphere, respectively.¹²⁶ This classification excludes the overlaps of fabrication techniques and gives a clearer idea about the advantages and disadvantages of each technique and its suitability for the fabrication of an organic neuromorphic device.

Solution-based processing techniques

One of the most popular manufacturing techniques for organic neuromorphic and bioelectronic devices is solution-based fabrication techniques because of their low cost, flexibility, and large area fabrication.¹²⁷ In these methods, an appropriate solvent is used to dissolve or disperse the organic semiconducting materials which are finally coated or printed onto the substrates. Solution based approaches are very attractive for scalability in electronics and for wearable electronics, as they involve less complex equipment, less material usage and lower processing temperatures to produce electronics in comparison with traditional microfabrication techniques.¹²⁸

In laboratory-scale research, one of the most widely used solution-processing methods to fabricate uniform organic semiconductor thin films is spin coating. The process used consists of spreading a thin layer of the semiconductor solution on a substrate and spinning it at a high speed, this will spread the solution evenly by centrifugal force.¹²⁵ Parameters such as spin speed, solution viscosity, concentration, and rate of solvent evaporation, all influence the final thickness of the film. The spin coating method, which can be used for the preparation of highly uniform and reproducible films, is widely used for the preparation of OFETs, photovoltaic devices, and neuromorphic components.¹²⁹ The size of the material waste from spinning, however, and the challenge of coating large area substrates in the industry preclude a wider application of the technique.

Inkjet printing is one of the additive manufacturing technologies for organic electronics. This involves placing the semiconductor ink in a very precise manner on a substrate using a computer-controlled nozzle, in the form of droplets of the order of picoliter. The inkjet printing process results in material savings and can print electronic structures directly without the complex lithographic processes needed.¹³⁰ A technique has been extensively applied for the fabrication of flexible organic transistors, sensors and neuromorphic device arrays (Table 3). In addition, several functional layers such as semiconductors, dielectric materials, and conductive electrodes can be deposited one after another. Potential effects such as droplet spreading, coffee-ring effects, solvent-substrate interactions, etc., however, could impact the uniformity of the film and the reproducibility of devices and necessitate careful optimization of the ink compositions and printing parameters.¹³⁰

Table 3.

Functions and structures of organic neuromorphic devices

Device architecture	Typical materials used	Working principle	Neuromorphic function emulated	Key advantages	Typical applications	Reference
Organic field-effect transistor (OFET)	Pentacene, spike-timing-dependent plasticity (STDP)-based polymers, and P3HT	The electric field controls how charges move through the semiconductor channel.	STDP and synaptic weight modulation	High carrier mobility and well-known fabrication	Artificial synapses and neuromorphic circuits	Wang et al.84; Wu et al.131
Organic electrochemical transistor (OECT)	PEDOT:PSS, polymers that let ions pass through	Ionic charges from an electrolyte change how well a channel conducts electricity.	Short-term plasticity (STP) and long-term plasticity (LTP)	Low voltage for operation and strong coupling with biological signals	Neural interfaces and biosignal amplification	Yamamoto and Malliaras82; Zhao et al.132; Gao et al.133
Organic memristor	P3HT, PEDOT:PSS, and small-molecule semiconductors	Resistive switching via charge trapping or filament creation	Memory that doesn’t change, synaptic learning	Simple design, and low energy use	Artificial synapses, and memory devices	Wang et al.,86 Kousseff et al.134
Electrolyte-gated organic transistor	P3HT, ion gels, and polymer electrolytes	Ion migration in the electrolyte changes how well the channel conducts electricity.	Paired-pulse facilitation (PPF), and STP	Low-voltage operation and the ability to make devices in different ways	Computers that work like brains	Wu et al.135; Zhao et al.136; Scullin et al.137
Organic ferroelectric transistor	P(VDF-TrFE) with organic semiconductors	Ferroelectric polarization regulates channel conductivity.	Memory that lasts a long time, and synaptic plasticity	Stable memory states and operation that doesn’t change	Neuromorphic memory storage	Shen et al.138; Sun et al.139; Shang et al.140
Organic ionic transistor	PEDOT-based materials that are both ionic and electronic conductors	Ionic and electronic charge transport change the flow of current.	Processing analog signals and learning how to adapt	Very sensitive to biological signals	Bioelectronics, processing neural signals	Kim et al.141; Del Olmo et al.142
Organic photonic synapse	Polymers that can be conjugated and photoactive organic semiconductors	Charge generation caused by light changes the conductance of a device.	Optical synaptic plasticity	Allows for optoelectronic neuromorphic systems	Optoelectronic computing and artificial vision systems	Ding et al.143; Li et al.144; Lan et al.,145; Li et al.146

Another commonly used solution-processing method for large-area organic electronic fabrication is screen printing. Under mechanical pressure, conductive or semiconductive inks are pushed through a screen mesh that has been patterned in a specific design onto a substrate.¹⁴⁷ Screen printing is well suited for the production of thick conductive layers like electrodes and interconnects and has a good compatibility to industrial scale production. The technique can be used in various materials such as plastics, textiles and paper, which are ideal for wearable bioelectronics and disposable sensing platforms.¹⁴⁸ The relatively low printing resolution, however, does limit the use of this method where there are very small features in the device. Large-scale production of organic electronic devices is one of the most promising methods, R2R manufacturing.¹⁴⁹ This is an ongoing process; the functional layers are repeatedly deposited, patterned, and dried as the flexible substrates are being rolled to rotate. The R2R processing is an integrated process of several fabrication steps which significantly decreases the fabrication cost and boosts production rates. This is especially appealing for flexible displays, wearable, and large area sensor arrays. The uniformity of the film quality, registration of the layers and the consistency of the device performance over long production runs is a major challenge.¹⁵⁰ Advanced process control strategies are necessary to control device reliability as substrate tension, drying, and ink rheology vary.

Vapor-phase deposition techniques

An alternative method for fabricating organic semiconductor devices is through the use of vapor-phase deposition techniques and is especially applicable to high-performance thin-film electronics.¹⁵¹ These techniques are different from solution-based techniques, in which semiconductor films were formed by the evaporation and the subsequent condensation of molecular species under carefully controlled vacuum conditions. Vapor-phase deposition, in general, offers greater control over film thickness, organization of molecules in the film, its purity, and quality of the interfaces and is therefore crucial for the reproducible performance of a device.¹⁵²

The most frequently employed vapor-phase deposition processes for small-molecule organic semiconductors are thermal evaporation processes. This process involves the evaporation of organic materials, under vacuum, to a cooler substrate, and subsequent condensation of the material on the substrate to form a thin film. The thickness and rate of deposit of the film can be controlled with accuracy in thermal evaporation. The layers appear to be highly uniform without contamination. The technique is especially well suited to the manufacture of organic transistors, optoelectronic devices and neuromorphic architectures, which require a well-defined multilayer structure and sharp interfaces.¹⁵³ But vacuum systems are not compatible with large-area and low-cost flexible electronics because of the complexity and cost of the manufacturing.

A more sophisticated vapor phase fabrication method is organic molecular beam deposition (OMBD) that offers better control over molecular assembly and the growth of thin films.¹⁵⁴ Highly controlled molecular beams impinge on the substrate after evaporation of the organic molecules under ultra-high vacuum conditions. This technique can precisely control film morphology, crystallinity, molecular orientation, and interface engineering. In this respect, OMBD is particularly applicable to basic research of the charge transport and to the optimization of the molecular ordering in high-performance organic electronic devices.¹⁵⁵ It is, however, a technique that uses special equipment, is quite expensive and is used at relatively low deposition rates, so it is only really for research laboratories and specialty device fabrication. Overall, the quality and structural control of the film by vapor-phase deposition techniques are superior to those of solution-based techniques. But their high cost, vacuum, and low scalability make them impractical for general application in industry. Therefore, the choice of fabrication method is a compromise between the application of interest, the requirements on the device performance, manufacturability, and cost.

Pattern creation and design with flexible substrates

In real-world neuromorphic and bioelectronic systems, there are many organics that must be connected all into complicated arrays and circuits. Patterning methods are a very influential technique for determining device configuration, electrode shape, and connection properties.¹⁰⁸ Organic semiconductors and traditional photolithography, which are commonly used for electronic circuits in silicon, can sometimes struggle to cooperate. This is due to the fact that solvents, UV exposure, and processing conditions will damage organic materials. New ways of making patterns as a result of this are soft lithography, laser-based patterning, and printing-based patterning.¹⁵⁶ Such approaches allow deposition of or stripping off of the organic semiconductor layers without harming the fabric of the materials. For another important part of manufacturing organic electronics, it is necessary to be able to attach the electronic materials onto flexible substrates. Electronic devices can be formed into bendable and flexible devices using polymer substrates such as polythene terephthalate (PET), polythene naphthalate (PEN), and elastomeric substances.¹⁵⁷ Flexible substrates, however, have problems of their own, like thermal limitations, rough surfaces, as well as mechanical distortion during fabrication. To tackle these issues, several investigations were performed regarding the surface treatment system, barrier layer deposition, and device architecture that were in the same order that the mechanical loading mechanism must be sustained. For future wearable devices, implantable devices, and neuromorphic electronic systems to be successful, consistent integration of organic semiconductor devices with flexible substrates is needed. More generally, progress in fabrication and processing of materials are very important for the development of technologies in organic semiconductors.¹⁵⁸ Solution-based printing methods are good for making things in large quantities and are cheap. On the other hand, thin-film deposition techniques enable very precise control of the film shape and device structure. These techniques of design based on emerging materials and devices design for high-quality, large-area organic electronic systems with new materials to be employed for neuromorphic computing and bioelectronics uses will clarify the development paths of these systems.¹⁵⁹

Organic semiconductor devices for bioelectronics

Bioelectronics is the branch of science developing biotechnology, which combines biological systems with electronic technologies to provide monitoring, modulative and therapeutic effect in living tissue.¹⁶⁰ However, inorganic semiconductors are poor in terms of mechanical rigidity, processing temperatures, and compatibility with biological systems’ ionic signaling mechanics, which are the main drawbacks of conventional electronic materials for bioelectronic tasks. Organic semiconductors have been receiving increasing attention because of some unique features such as processability in solution at low temperature or dual charge carrying (like electrically and ionically).¹⁵⁸

Neural/brain-computer interfaces

Neural interfaces allow electronic devices to communicate with the nervous system by studying or exposing neural tissue to electrical signals.¹⁶¹ Such technologies are pivotal in the future of brain-machine interfaces (BMIs) to help patients with neurological disorders or injuries regain their sensory and motor activities. Organic semiconductor materials have excellent applications in neural interfaces as their mechanical properties closely mimic those obtained with biological tissues. Generally, metallic electrodes and silicon-based devices often have stiffness, which can lead to mechanical mismatch and long-term inflammation in the tissue-device interface.¹⁶² On the contrary, infusing organic material into flexible, stretchable structures can reduce mechanical stress and ultimately allow for increased biocompatibility over time. Studies of OECTs and conductive polymers, for the recording and stimulating of neural signals, have generated much work.¹⁶³ As a result of their high transconductance, they are able to very sensitively amplify weak neural signals. And, since they operate at low voltages, they’re less susceptible to damaging tissue. At the same time, you can create organic materials with greater interfacial capacitance and thus, that charge will be more easily able to travel between the device and neural tissue. Despite this, it is also hard to maintain such high-speed physiological processes over long periods. Organic materials can break down in contact with oxygen, water, or electrolytes.¹⁶⁴ And to ensure that the device has a stable electrical connection to neural tissue without getting it into a dangerous condition for the body remains a problem in research.

Biosensors and bio-signals detection

Organic semiconductor devices have shown great potential for detecting biochemical and electrophysiological signals as biosensors. They are readily responsive to changes in the chemical environment surrounding them due to sensitivity to surface interactions.¹⁰ The reason why they are effective in detecting biomolecules including proteins, nucleic acids, enzymes, and neurotransmitters. OFETs and electrolyte-gated transistors are frequently employed in biosensing platforms, as the uptake of target biomolecules by the system will change a semiconductor surface’s response to electrical signals.¹⁶⁵ Using selective recognition elements such as antibodies, aptamers, or enzymes on the surfaces of devices allows the detection of biological analytes in very small amounts. The other strong point of organic semiconductor biosensors is that they have the advantage of being compatible with flexible and wearable electronics. Based on solution-based fabrication techniques, lightweight sensor arrays that can be incorporated on uneven materials such as skin can be developed.¹⁶⁶ Nonetheless, there are a couple of issues that need to be improved to ensure biosensors function adequately. Alterations in temperature, humidity, and chemicals can all damage organic materials. Biofouling, or the accumulation of biological material on sensor surfaces, can result in a loss of sensitivity and accuracy over time with biofouling, as well.¹⁶⁷

Bioelectronics with worn or implanted implants

The presence of organic semiconductor devices in an implantable and wearable electronic system is one of the signs of a rapidly emerging biomedical research field.¹⁶⁸ These technologies are developed to help monitor and treat it without causing discomfort to a person/organ, or limiting the natural range of motion. Scientists have produced organic electronic devices, too, to wear, that can track things such as your heart rate, muscle movement or brain waves—all on a display that requires no extra equipment.¹⁶⁹ This can also add flexible organic transistor arrays and sensor platforms to materials, so you can use them as you would fabric, patches, or soft skin (to match the body). They are also suited for watching your health over time because they are lightweight and don’t consume much power. Implanted organic electronic gadgets, which can be implanted have also found prospects in neural prosthetics, retinal implants, and targeted drug delivery systems.¹⁷⁰ Organic semiconductors can safely stimulate tissues or modify the function of cells, for example, because they work at low voltages and interact with ionic, biological environments. But it is extremely difficult to stick something in for long periods of time. Organic materials must preserve their mechanical and electrical properties in complex physiological settings characterized by enzymes, salts, and reactive species.¹⁷¹

Organic electronics and their application in stimulating and engineering tissue

Increasingly, scientists focus on these organic semiconductor materials for use in tissue engineering: electricity can affect cellular behavior and promote the healing of tissues.¹⁷² Electrodynamics, the electrical impulses that regulate how cells communicate, grow, and change, depending on the tissue’s physiology are crucial to many biological processes, such as in neural tissue, heart tissue, and cardiac tissue.¹⁷³ It is possible to choose organic materials which conduct electricity through scaffolds or through substrates to stimulate cells that are growing or regenerating in a certain way. This method has been shown to facilitate cells’ growth, align and transform into different types of tissues. For instance, conductive polymers that conduct electricity, scaffolds can develop neurons instead of tissue and repair damaged nerve tissue more effectively.¹⁷⁴ Organic electrochemical materials lend themselves best to this kind of use, which can directly interact with ionic biological environments. They are suitable for long-term biomedical use since they can alter electrical signals without generating damaging side-products. While these are strong strides forward, bringing in electronic functionality to biomaterial scaffolds is complex. Materials must not only be able to move electric current, work beautifully with the living tissue and be safe to operate for long periods of time, but they also need to be good for the environment.¹⁶⁹ It is also important to regulate the degradation of organic materials so that they are able to interact reasonably effectively with living organisms. Most recently, the organic semiconductor device has opened up new avenues for bioelectronics by integrating electronic soft, flexible, and biocompatible systems that can directly interface with biological systems.¹⁶⁰ The application of these to biosensors, neural interfaces, wearables, and health monitoring devices, as well as in the process of tissue engineering, shows how these technologies can transform the field of biomedical technology.

Challenges and limitations

Despite the emergence of organic semiconductor materials for neuromorphic along with bioelectronic applications, significant challenges remain that limit their widespread use.¹⁰⁸ Environmental factors like moisture, oxygen, or thermal stress induce long-term harm to organic substances over time. This degradation results in inferior electrical performance. In addition, neuromorphic devices repeatedly send electric signals, which speeds up the breakdown process. Recent approaches such as encapsulation and stable molecular designs have demonstrated some progress, but long-term stability such as silicon remains a struggle. Then there’s the problem of manufacturing needing to expand. There are some techniques of potential for large-scale production: printing and R2R processing. But it is difficult to bring laboratory techniques into industrial environments.⁵⁴ Moreover, combining several functional layers demands tight alignment and compatibility, and a device density that resembles that in conventional technologies still has not been realized. The instability in different devices complicates the construction of efficient neuromorphic systems since molecular disorder and the manufacturing approach of devices will cause performance issues.¹⁷⁵ This affects the vital electrical properties in organic thin films and thus there is a difference in charge transport. Also the stochastic nature of the resistive devices makes the design of hardware for neural networks more difficult. To address these issues, there is a need to improve the purity of materials and the design of device architectures. In the case of bioelectronics, we need to consider how long organic semiconductors can be in contact with living tissues, and without triggering any reaction which damages the material in terms of immune response or damage.¹⁷⁶ In the long run, the mechanical mismatch between the soft biological tissues and the electronic devices may also render these devices less useful. Biocompatible polymers and encapsulation approaches are being investigated, and stable and reliable sustained long-term implantation is still challenging.¹⁰⁰ Cross-disciplinary research that includes materials chemistry and device fabrication is crucial to achieve their viability.

Future perspectives and emerging trends

Organic semiconductor gadgets are expected to be crucial to the development of bioelectronics as well as neuromorphic computation. Mechanical adaptability, light weight, simple use, as well as biological efficiency make them optimal for future intelligent systems that can work in a living system.¹⁶⁰ With the progress in materials research, device creation, and integrating systems, a variety of such patterns are currently emerging in this area. One beneficial direction is to make electronics that can heal themselves and break down naturally. Self-healing substances can repair damage to structure or electrical wiring by itself, so devices endure better and work longer.¹⁷⁷ This is especially useful for adaptable and wearable gadgets that are under severe mechanical stress. Biodegradable electronics are more suitable for short-term biomedical devices and environmental detectors. Organic semiconductors as well as polymer substrates can be broken down to non-toxic byproducts after they’ve done their job. That is, they don’t have to be surgically cut out and saved—that is preferable for the environment. Biodegradable conductive substances and transient substrates are being used to build electronic devices that last.¹⁷⁸ Future neuromorphic uses are anticipated to combine processing and sensing in a single platform. Organic semiconductor substances can be fabricated on elastic substrates and integrated into massive sensor arrays. This renders them ideal for neuromorphic sensing uses. Neuromorphic sensors are not only able to sense temperature, pressure, light, chemical, and various other signals but also process natural signals. Theories according to human skin, including tactile sensing systems, employ a variety of organic transistors as well as memristive apparatus to convert pressure trends into neural-like signals.¹⁷⁹ Such systems could be utilized for prosthetics, health-tracking wearables, and human-machine communication interfaces. These organic-based semiconductor devices (OECTs and memristive elements) can imitate synaptic plasticity, where the circuits can acquire information and respond to external stimuli. This adaptive electronics could propel neuromorphic computation with its minimal energy use and smart edge devices. Furthermore, organic electronics, soft robotics, as well as healthcare systems are able to be integrated to construct flexible prosthetics, rehabilitation services, and live physiology-based systems which can also have tracking functionality in environments.¹⁸⁰ This way, these gadgets may contribute to the realization of biocompatible intelligent as well as adaptable electronics and are likely to be upcoming integrated approaches in the areas of medicine as well as robotics.

Conclusions

Organic semiconductor components are anticipated to have an important role in new generation neuromorphic as well as bioelectronic systems due to their mechanical versatility, tuneable electronic attributes, low-temperature solution processability, as well as biocompatibility. Recent improvements in conjugated polymers, small-molecule semiconductors as well as blended ionic-electronic conductors have enhanced charge movement as well as synaptic function imitation, resulting in key devices such as OFETs and organic memristive gadgets with biomimetic attributes such as plasticity and adaptive learning. In 3–5 years, blended ionic-electronic conducting polymers as well as bio-inspired hybrid systems will be the nucleus of energy-efficient neuromorphic computing and bio-electronic sensing. The significance of stretchable as well as self-healing organic semiconductors is developing for incorporation into wearable and implantable gadgets as well as medical devices. Nevertheless, challenges still exist such as longevity, environmental sensitivity, device heterogeneity, lack of standardized testing procedures, etc. Future research ought to focus on building strong material architectures, developing more effective interfacial strategies, harnessing AI in material design, and developing uniform performance standards. For the further development and use of organic neuromorphic and bioelectronic-based materials and systems, the interdisciplinary collaboration such as materials science, electronics, and neuroscience remains necessary for various future applications.

Declaration of interests

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests.

Declaration of generative AI and AI-assisted technologies in the writing process

During the preparation of this manuscript, the authors used generative AI tools to assist with improving the language, readability, and clarity of the text originally written by the authors. The use of AI was strictly confined to language enhancement tasks such as translation and grammar checking. After using this tool, the authors reviewed and edited the content as needed and take full responsibility for the content of the publication.