Emerging conductive inks based on 2D materials and bio-based polymers are transforming the field of printed electronics by offering enhanced conductivity, printability, and novel functionalities. These materials enable the fabrication of flexible, stretchable, and environmentally sustainable electronic devices, opening new possibilities for applications in wearables, sensors, and energy harvesting systems.

### 2D Material-Based Inks

Graphene and transition metal dichalcogenides (TMDCs) are among the most widely studied 2D materials for printable conductive inks. Graphene inks exhibit high electrical conductivity, often reaching values exceeding 10,000 S/m when properly formulated. The high carrier mobility and mechanical flexibility of graphene make it suitable for printed flexible circuits and transparent conductive films. However, achieving stable dispersions without compromising conductivity remains a challenge.

TMDCs such as molybdenum disulfide (MoS₂) and tungsten diselenide (WSe₂) offer semiconducting properties, making them ideal for printed transistors and photodetectors. These materials can be exfoliated into nanosheets and dispersed in solvents to form stable inks. Printed MoS₂ thin-film transistors have demonstrated field-effect mobilities in the range of 10–30 cm²/V·s, suitable for low-power logic circuits.

MXenes, a class of 2D carbides and nitrides, have also gained attention due to their metallic conductivity and hydrophilicity, which facilitate ink formulation without additional surfactants. Ti₃C₂Tₓ MXene inks have achieved conductivities above 8,000 S/m when printed, with applications in supercapacitors and electromagnetic shielding.

### Bio-Based Polymer Inks

Bio-based polymers are emerging as sustainable alternatives to conventional synthetic materials in printable electronics. These polymers are derived from renewable sources such as cellulose, chitosan, and proteins, offering biodegradability and reduced environmental impact.

Conductive polymers like poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) are widely used due to their high conductivity (up to 1,000 S/m) and compatibility with printing techniques. Recent advances have improved their stretchability and adhesion to flexible substrates, making them suitable for wearable electronics.

Cellulose nanofibrils (CNFs) and nanocrystals (CNCs) serve as biodegradable binders and substrates for printed electronics. When combined with conductive fillers like silver nanoparticles or carbon nanotubes, these composites achieve conductivities in the range of 100–1,000 S/m while maintaining mechanical flexibility.

Protein-based inks, such as silk fibroin, have been explored for their biocompatibility and ability to host functional materials. Silk fibroin mixed with conductive polymers or nanoparticles can be printed into flexible biosensors and transient electronics that dissolve after use.

### Printability and Processing

The printability of conductive inks depends on rheological properties such as viscosity, surface tension, and particle dispersion. Inkjet printing requires low-viscosity inks (1–20 mPa·s) with fine particle sizes to prevent nozzle clogging, while screen printing tolerates higher viscosities (1,000–50,000 mPa·s).

Graphene and MXene inks are compatible with inkjet and aerosol jet printing due to their colloidal stability. However, post-printing treatments such as thermal annealing or laser sintering are often necessary to enhance conductivity.

Bio-based polymer inks typically require mild processing conditions to avoid degradation. Water-based formulations are preferred for environmental safety, though additives like glycerol or ethylene glycol may be incorporated to improve film formation and flexibility.

### Novel Functionalities

Printed electronics benefit from the multifunctionality of these emerging inks. For instance, 2D material inks can be engineered for optoelectronic applications. Graphene quantum dot inks exhibit tunable photoluminescence, enabling printed light-emitting devices. Similarly, TMDC-based inks can detect light across visible to near-infrared spectra, useful for printed photodetectors.

Bio-based polymer inks enable biocompatible and transient electronics. Silk-based circuits can dissolve in physiological fluids, making them suitable for implantable medical devices. Cellulose composites with conductive nanoparticles have been used in printed pressure sensors for health monitoring.

Thermoelectric inks based on polymer-carbon nanotube composites demonstrate Seebeck coefficients of 50–100 μV/K, paving the way for printed energy harvesters. These materials convert waste heat into electricity, with potential applications in self-powered wearable devices.

### Challenges and Future Directions

Despite progress, challenges remain in scaling up production and ensuring long-term stability. 2D material inks require cost-effective synthesis methods to compete with conventional materials like silver nanoparticles. Bio-based polymers must improve their environmental stability without sacrificing biodegradability.

Future research will focus on hybrid inks combining 2D materials with bio-based polymers to achieve synergistic properties. Advances in AI-driven ink formulation could accelerate the discovery of optimal compositions for specific applications.

The development of these emerging inks is driving innovation in printed electronics, offering sustainable and high-performance alternatives to traditional materials. As processing techniques mature, these inks will enable next-generation devices with unprecedented functionality and environmental compatibility.