In the implantable electronics, non‑planar electrodes create multiple irregular boundaries between the electrode contacts, external interconnects, and the encapsulation layer. Elastic encapsulation layers fail to conformally follow the three-dimensional structures of non-planar electrodes, leading to microscopic gaps at the interface. Such gaps can further develop into interconnected defect channels. The non‑conformal interface defects allow biofluids to penetrate the electrode interior, causing encapsulation failure and subsequent issues, including material delamination, wire detachment, electrode damage, and corrosion, all of which severely compromise the long‑term stability and operational lifetime of the electrodes.
This paper presents a viscoplastic interlayer composed of polyisobutylene (PIB) and maleic anhydride-grafted polypropylene (MAPP). Chain slippage and permanent disentanglement enable conformal contact between the viscoplastic interlayer and various non-planar electrodes. When combined with covalent bonding, this synergy yields defect-free sealing. The viscoplastic interlayer provides fruitful implications for improving the long-term stability of implantable bioelectronics (Figure 1). The related work is published in the National Science Review (NSR) under the title “Intimate encapsulation of non‑planar electrodes via a viscoplastic interlayer.”
To induce bulk viscoplasticity, the polymer network needs to minimize elastic energy storage and maximize energy dissipation by irreversible mechanisms, such as chain slippage, permanent disentanglement, and breakage of covalent bonds. Guided by this principle, our viscoplastic interlayer (VPI) was assembled using a physically entangled, lowly permeable polyisobutylene as the polymer matrix, PIB oligomers as the plasticizer, and maleic anhydride-grafted polypropylenes as the foreign plastic domains. The physically entangled PIB matrix had a high tendency to slip its chains under stress due to its non-crosslinking nature. The PIB oligomers further promoted the chain slippage by increasing free volume and lubricating the long-chain PIB. The MAPP domains ensured the permanent disentanglements through spatially confining the slipped long-chain PIB. These energy dissipation processes synergistically suppressed the elastic recovery and endowed a large residual strain for the VPI.
The bulk viscoplasticity, interface bonding, and low water vapor transmission rate (WVTR) jointly allow VPI to hermetically seal non-planar electrodes. The peel tests show that the steady‑state peel force of the viscoplastic encapsulation is 30 times higher than that of conventional elastic encapsulation. The calcium corrosion tests reveal that when a sealed calcium film is immersed in water, the encapsulation lifetime of the viscoplastic encapsulation is 14 times longer than that of the elastic encapsulation. The conformal contact and tight binding effectively block water permeation at the electrode–encapsulation interface.
The viscoplastic encapsulation strategy demonstrates outstanding sealing protection for non‑planar electrodes. In simulated physiological environments, elastically encapsulated electrodes fail after 2 weeks due to interfacial leakage, whereas viscoplastically encapsulated electrodes maintain a stable signal‑to‑noise ratio (34.5 dB) for up to 50 weeks. Moreover, the viscoplastic encapsulation strategy remained steady under varying pH conditions and cyclic mechanical deformation. In long‑term implantation studies using rat models, elastic encapsulation shows delamination and blood penetration after 2 weeks, leading to a significant drop in the signal‑to‑noise ratio. In contrast, the viscoplastic encapsulation strategy extends the signal fidelity time of the bioelectronic devices to 45 weeks.
National Science Review
Experimental study