A universal packaging substrate for mechanically stable assembly of stretchable electronics

Stretchable electronics integrate disparate modules—metals, ceramics, plastics, and elastomers—on a single packaging substrate. In assembly and operation, large modulus mismatches and diverse surface chemistries cause severe interfacial stress concentrations and strain-induced delamination, particularly for modules that undergo large deformations (e.g., sensors, transistors). While direct monolithic fabrication on one substrate is complex for manufacturing, assembling separately fabricated modules introduces the challenge of maintaining adhesion and mechanical compliance across interfaces. Existing approaches offer chemical bonding or topological entanglement for chemically similar interfaces but struggle to provide universal adhesion across dissimilar materials. Spatial tailoring of substrate stretchability can reduce interface stress, yet maintaining barrier properties while softening elastomers remains challenging. This work targets a universal packaging substrate that simultaneously provides module-specific stretchability to minimize modulus mismatch and strong, universal adhesion to raise interfacial toughness, thereby preventing delamination under extreme strains and enabling robust assembly of heterogeneous modules.

Prior substrates for area-dependent stretchability include PDMS, Ecoflex, SEBS, and polyurethane. Common strategies create rigid islands within a softer elastomer to localize strain; however, softening without compromising barrier properties is difficult. Adhesion strategies based on chemical bonding or topological entanglement work well for chemically similar interfaces (e.g., silicones/glass, carbon elastomers/carbon elastomers), but universal adhesion across metals, ceramics, plastics, and elastomers with distinct surface chemistries remains unmet. The field lacks a standard packaging substrate that can universally assemble varied modules while offering module-specific mechanical compliance. References cited include adhesion enhancements for PDMS/glass, universal interfaces for stretchable devices, and gradient materials for strain isolation, highlighting both progress and outstanding gaps in universal, mechanically stable assembly.

Materials design: The mother matrix is SIBS chosen for high elasticity, chemical stability, and low water permeability. Bulk modulus is regionally tuned by plasticizing SIBS with PIB oligomers to enhance chain mobility and reduce modulus; surface adhesion is made universal by an interposer layer comprising SIBS blended with PP-g-MAH to introduce polar maleic anhydride groups for chemical linking while maintaining topological entanglement with the SIBS substrate.

Module-specific stretchability: SIBS/PIB blends with PIB contents (5–60 wt%) were prepared; AFM and DMA confirmed increased chain mobility (PS domain spacing from 24 nm to 42 nm with 20 wt% PIB; Tg shift from −34 °C to −39 °C). Elastic modulus decreased from 840 kPa (pristine SIBS, 15 wt% PS) to 60 kPa (60 wt% PIB). Elongation at break varied with PIB content. Regional films (hard-soft-hard, HSH; soft-hard-soft, SHS) were fabricated via solvent welding/seamless merging with transition widths down to ~100 µm, enabling strain to concentrate in soft regions.

Barrier property evaluation: WVTR measured at 38 °C/90% RH showed SIBS at 0.35 g·mm·m⁻²·day⁻¹; SIBS/PIB <40 wt% showed marginal WVTR increase; 60 wt% PIB increased to 0.52. HSH/SHS films retained low WVTR and seamless interfaces. SIBS exhibited orders-of-magnitude lower WVTR than SEBS (~5) and PDMS (~70) g·mm·m⁻²·day⁻¹.

Interposer/adhesion process: PP-g-MAH/SIBS solution prepared in toluene. Substrates (PET, Si, Al, PDMS, PEDOT:PSS, chitosan hydrogel) were oxygen plasma treated and functionalized with APTES where applicable. Interposer was blade-cast (~100 µm) and adhesions formed by warm pressing (80 °C, 200 g) or laminating (150 °C). Interfacial toughness measured by 180° peel tests.

Finite element simulation: Abaqus/Standard with neo-Hookean soft regions (400 kPa) and stiff regions (3 MPa); stiff modules at 1 GPa. Simulated nominal strain of 200% to compare stress distributions between homogeneous vs. module-specific substrates.

Device assembly and characterization: A fully stretchable triboelectric generator/sensor assembled by integrating PDMS (dielectric) and SIBS-coated conductive chitosan hydrogel (electrodes) onto hard regions of SHS/HSH substrates; SIBS triboelectric layer formed on hydrogel. HSH and SHS modules hot-pressed together. Outputs (voltage, current) recorded under controlled stretching (1 Hz). Stability tested over 15,000 cycles and immersion in PBS at various pH for 5 weeks.

Biocompatibility and in vivo tests: L929 fibroblast cytotoxicity (CCK-8), morphology (phalloidin/DAPI), and live/dead assays on HSH films. Rat subcutaneous implantation (dorsal, thigh, chest) to record in vivo outputs; histology (H&E) of surrounding tissues and major organs at 2 and 10 weeks; blood/serum analyses.

• Universal, mechanically stable assembly: Combining regional modulus tuning and an adhesive interposer increased the strain tolerance of stiff and soft module/substrate interfaces up to 600% without delamination (case 4), exceeding prior reports.
• Modulus tunability: Regional Young’s modulus adjustable from ~60 kPa (SIBS + 60 wt% PIB) to ~3 MPa (hard SIBS); pristine SIBS (15 wt% PS) modulus ~840 kPa; elongation at break up to ~850% at moderate PIB.
• Stress reduction: Finite element analysis at 200% strain showed maximum Mises stress reduced from 12.46 MPa (homogeneous with interposer) to 4.65 MPa (module-specific with interposer), mitigating debonding risk.
• Universal adhesion: Interfacial toughness with interposer reached ~390 J·m⁻² (PET), 220 (Al), 200 (PDMS), 120 (Si); PEDOT:PSS and hydrogel exhibited cohesive failure (interface stronger than material). Toughness >900 J·m⁻² achievable at 150 °C lamination.
• Adhesion stability under strain/cycling: Al adhered to hard region of HSH maintained toughness nearly independent of substrate strain up to 600%; survived 20,000 cycles at 100% strain with stable adhesion.
• Barrier performance: WVTR of SIBS 0.35 g·mm·m⁻²·day⁻¹ at 38 °C/90% RH; regional HSH/SHS films retained low WVTR; substantially lower than SEBS (~5) and PDMS (~70).
• Device-level performance: Fully stretchable triboelectric device produced Vpp ≈ −7 V at 150% strain; voltage and current increased monotonically with strain (0.5 V/4 nA at 10% to 7 V/16 nA at 150%). Distinct signal signatures under bending, twisting, crumpling, compressing.
• Durability and chemical stability: Stable output after 15,000 cycles at 70% strain (≈ −3 V) and after 5 weeks immersion in PBS at pH 3.6, 7, and 10.
• In vivo performance and biocompatibility: No inflammation or tissue/organ pathology at 2 and 10 weeks; blood parameters unchanged. Implanted devices generated stable signals: respiration sensing at chest (~0.5 V), ~0.7–1.5 V under gentle stretching at thigh/dorsal; dorsal implantation maintained ~1.5 V for 10 weeks (long in vivo lifetime among stretchable implantable biomechanical sensors).

The study addresses interfacial delamination in heterogeneous stretchable electronics by simultaneously reducing the crack driving force (G) and increasing interfacial toughness (Γ). Regional modulus tuning via PIB plasticization minimizes modulus mismatch to lower G, while the PP-g-MAH/SIBS interposer enhances surface polarity for chemical bonding (e.g., with APTES-treated surfaces) and provides topological entanglement with the SIBS matrix to raise Γ. This combined bulk/surface strategy yields robust adhesion across metals, ceramics, plastics, and elastomers and maintains interface integrity up to 600% strain. Finite element results confirm significant stress mitigation at interfaces. The SIBS-based substrate’s intrinsically low WVTR adds chemical stability, enabling long-term operation in aqueous and physiological environments. Device demonstrations show large, strain-dependent electrical outputs and resilience to diverse deformations, along with extended in vivo functionality, highlighting relevance as a versatile packaging platform for stretchable bioelectronics operating under complex mechanical and physicochemical conditions.

A SIBS-based packaging substrate with regionally programmable stretchability (60 kPa to 3 MPa) and universal adhesiveness via a PP-g-MAH/SIBS interposer enables mechanically stable assembly of heterogeneous modules that withstand up to 600% tensile strain. The substrate combines low water permeability with biocompatibility to support durable device operation: robust triboelectric performance under varied deformations, stable outputs over 15,000 cycles and 5 weeks in PBS, and 10-week in vivo function in rats. These results establish a versatile, near-hermetic platform for assembling stretchable electronics requiring high mechanical and chemical stability.

• Increased permeability at high plasticizer loading: SIBS with 60 wt% PIB showed elevated WVTR (0.52 g·mm·m⁻²·day⁻¹) due to additional free volume from PIB oligomers, indicating a trade-off between extreme softening and barrier performance.
• Surface conditioning requirement: Universal adhesion relied on APTES surface functionalization and warm pressing/lamination, which may add processing steps and constraints for certain modules.
• Conductive interconnect durability: In demonstration circuits, failure under extreme stretch (>400%) was linked to deformation-induced electrical breakdown of stretchable conductors, not the substrate, highlighting system-level limits outside the packaging layer.
• Biosafety note: Although the interposer (with MAH groups) was internal and isolated by SIBS, the biosafety of MAH groups is noted as under debate; biocompatibility tests here showed no adverse effects in the evaluated timeframes.