Conductive adhesives for soft integrated electronics
A heterogeneous liquid metal based conductive adhesive is developed that reversibly attaches to diverse surfaces with high stretchability, low modulus, and strain-invariant electrical conductivity. This SofT Integrated Composite with TacK through Liquid Metal (STICK-LM) adhesive enables heterogeneous and anisotropic electric conductivity for direct integration into functional systems.
Monodispersed liquid metal droplet size control in elastomer
Liquid metal (LM) droplets have emerged as a replacement of rigid fillers to develop compliant composites. Particularly, gallium based LM alloys such as eutectic gallium-indium (EGaIn) and gallium-indium-tin (Galinstan) which are non-toxic and liquid at room temperature, demonstrate desirable combinations of electrical, thermal, and mechanical properties.
Distinct approaches are needed to control the LM microstructure in composites to enhance the multifunctional properties for flexible electronic systems. We have studied the LM emulsion in liquid phase polymer matrix to obtain micro/nano-scale LM droplets. Specifically, we create four distinct EGaIn particle sizes (D) ranging between 100 nm – 80 µm and disperse these LM droplets into silicone elastomer with four controlled volume loadings (Φ) up to 80 %. This disintegration of LM into uniformly distributed particle sizes is achieved by controlling the mixing speed and duration of dual asymmetric centrifugal mixer and probe sonicator.
The effect of LM particle distribution can be found in different multifunctional properties. Dielectric spectroscopy of the composites shows that relative permittivity varies between 3-60 as function of LM microstructure. Simultaneously, low dissipation factors (tan ∂ <0.5) of the composites even at high frequency indicate a stable dielectric performance. The permittivity values also satisfy the multiple-scattering theory based effective medium solution which would allow predictable multifunctional performance for soft electronics.
Elongated and aligned liquid metal microstructures
In order to enrich the anistorpic properties of composites, elongated and aligned rigid particles (e.g., CNT, graphene, ceramics) are traditionally dispersed in polymeric matrix. In contrast, the liquid fillers (e.g., LM, MR fluids) embedded in composites are spherical in shape and a stable mechanism to control their shape is unavailable.
We have developed a new soft matter processing method to reshape the LM droplets in thermoplastic polymers. These liquid metal programmed elastomers (LMPE) are enabled by a thermo-mechanical fabrication technique in three steps which include (i) stretching, (ii) annealing, and (iii) stress-free recovery. The programmed LM microstructure remains elongated and demonstrate high compliance (elastic modulus <1 MPa) and stretchability (strain limit > 700 %). Furthermore, functional behavior such as thermal conductivity is enhanced due to the elongated microstructure. For an elastomer-like soft material, we have achieved metal-like thermal conductivity of > 10.0 W/(mK). The exceptional combination of high mechanical compliance and high thermal conductivity places LMPE in a unique location in the material property space compared to a broad spectra of stress-free, soft materials.
Lightweight and transferable conductive liquid metal elastomer thin films (LETs)
As a lightweight, conductive, and transferrable medium for flexible electronics, we extend the LM morphology programming concept for micron sized thin films. The thickness tunable (30-70 µm) liquid metal elastomeric thin films (LETs) are developed by controlling the rotational speeds of spin coater. The morphology of pristine thin films are characterized by disk-shaped LM microstructure. Electrical activation of the films are performed by pressure sintering which leads to a Janus microstructure where the same film has conductive and insulated surfaces on opposite sides.
The free-standing LETs demonstrate high compliance (elastic modulus < 500 kPa) and stretchability ( ∈max > 700 %). Simultaneously, these films are highly conductive at both the unstrained state (Conductivity, G > 1 S ), but also maintains electric connection (conductivity, G ≈0.2 S at ∈max ) at the stretching limit. Compared to other transferable, conductive thin-films, LETs display an exceptional combination of maximum conductance Gmax and strain ∈max, appearing in the top right quadrant of a property chart.
