Rapid custom prototyping of soft poroelastic biosensor for simultaneous epicardial recording and imaging

The study addresses limitations of viscoelastic, nanofiller-based printable inks for stretchable biosensors, which exhibit mechanical and electrical hysteresis and potential delamination under cyclic strains typical of cardiac motion. The authors propose poroelastic silicone composites that can be directly ink written (DIW) with high precision, form sponge-like foams with ultralow modulus comparable to cardiac tissue, embed conductive fillers within pores to preserve integrity under deformation, and provide strain-insensitive electrical performance. The goal is to enable custom-fit epicardial biosensors for high-fidelity spatiotemporal ECG mapping and simultaneous ultrasound imaging, improving intraoperative guidance and potentially informing therapeutic interventions.

Prior works established DIW for rapid, mask-free fabrication of functional architectures and soft electronics, using conducting polymer and silicone composite inks; however, viscoelastic systems suffer hysteresis and stability issues with percolation networks under strain. Alternatives like nanowire composites and embedded 3D printing improve stretchability but still face hysteresis and interface challenges. Capillary adhesion and ultrathin devices have been used for conformal biointegration. The present work builds on these by introducing poroelastic, sponge-like silicone composites to reduce hysteresis and enhance tissue coupling while maintaining printability and biocompatibility.

- Custom design and DIW fabrication: 4D ultrasound segmentation (Vevo 3100) of myocardial geometry to custom-design layouts. DIW on Si/PMMA substrates using a nozzle system (Nordson EFD; nozzle ID ~100 µm; repeatability ±3 µm; 20 mm/min). - Ink formulation: Blend base resin (vinyl-terminated diphenylsiloxane-dimethylsiloxane copolymer, methylhydrosiloxane copolymer, siloxane monomer at 6.5:3.3:0.2), dilute resin (Sylgard 184, 10:1), and polysiloxane-treated hydrophobic silica (SiO2-PS) particles. Ratios screened: 5.7:3.3:1.0, 6.0:3.3:0.7, 4.2:5.0:0.8, 4.5:5.0:0.5. Pt catalyst added (0.1 wt% of base resin). Mixed by Thinky mixer. - Pore formation: Steam etching in a pressure rice cooker at 120 °C, 15 psi to form microporous sponge (5–50 µm pores). - Metallization: Immerse porous structures in hexane with Ag flakes (200 nm–5 µm) to infiltrate pores via capillarity. Electroless Cu plating (30 min) followed by Au overplating (2 min) for biocompatibility. - Post-DIW structural layers: Additional DIW layers in open mesh layouts to ensure breathability/stretchability. Temporary handling with 50 µm PVA film; release by dissolving PMMA sacrificial layer and later dissolving PVA on tissue with warm saline. - MD simulations: Material Studio (COMPASS II force field) to compute interfacial interaction energies between SiO2-PS vs SiO2-OH particles and surrounding resins; annealing protocols and NVT dynamics used to assess miscibility. - Rheology: TA DHR-2 rheometer, 25 mm parallel plate; stress sweeps (0.1–65,000 Pa) at 10 rad/s; time sweeps for gel-point/working lifetime; viscosity vs shear rate. - Mechanical/electrical testing: Printed strips (2 cm × 2 mm × ~150 µm) and open mesh coupons tested in uniaxial tension (Mark-10; 20 mm/min). Cyclic loading-unloading up to 30–50% strain for ≥1000 cycles. Electrical resistance (Keithley 2400) under strain; sheet resistance stability in DI water, PBS, ethanol (12 h). - Device integration and in vivo testing: Custom arrays for various animal hearts (piglet, ovine, porcine, bovine) and in vivo murine (n=5) and porcine (n=2) epicardial ECG. Devices ~50 µm thick, 4–6 bipolar channels (mouse), 8 bipolar channels (pig). Electrochemical impedance of 200 µm × 200 µm electrodes measured in PBS at 23 °C. - Myocardial infarction model (mouse): Left thoracotomy, LCA permanent ligation after device placement; simultaneous epicardial ECG (custom array) and global three-lead ECG (control). - Simultaneous ultrasound imaging: Vevo 3100 with MX550D 22–55 MHz probe; short- and long-axis imaging with gating; real-time imaging over implanted patch with ultrasound gel. - Biocompatibility: H9C2 cell viability (MTT) on devices; comparison with bare sponge-like foam and device without Au overcoat. - Anti-biofouling: BSA-FITC (6 mg/ml) incubation 2 h; z-stack fluorescence imaging; intensity quantification and ANOVA. - Histology: Murine device implants up to 14 days; H&E and Masson’s trichrome staining; evaluation of inflammation, granuloma; epicardial thickness measurements. - Cardiac function: LAX ultrasound cine; EDV/PSV via Simpson’s method; EF computed at days 0, 1, 7, 14.

- Ink and material properties: The 6.0:3.3:0.7 base:dilute:SiO2-PS ratio provided suitable shear-thinning behavior and extended working lifetime (G′/G″ crossover ~34 h) compared with SE1700 (2 h) and Sylgard 184 (8 h). MD simulations showed lower interaction energy for SiO2-PS versus SiO2-OH (−1.852×10^21 vs −6.969×10^21 kcal mol−1 g−1), indicating better miscibility. - Mechanical softness and poroelasticity: Microporosity (~70%) reduced modulus of printed lines to 0.15 ± 0.02 MPa; open mesh effective modulus 29 ± 12 kPa, comparable to cardiac tissue (29–41 kPa) and >10× softer than SE1700 (>1.11 MPa). Bending stiffness of devices with dissolving PVA support decreased to <8.0×10^7 GPa µm^2, enhancing conformal adhesion. - Hysteresis and durability: Mechanical energy loss at 50% strain 4.3 ± 0.5 kJ m−4 (vs SE1700 23.6 ± 8.7 kJ m−4). Electrical hysteresis between 0.006–0.192 up to 30% strain, at least 10× lower than prior reports. Resistance stable (<5.0× initial) over 1000 cycles at 10–30% strain; stretchability >100% and up to ~150% before fracture with R/R0 < 9.0. - Conductivity: Sheet resistance after metallization <7.72 ± 1.52 Ω/sq; remained nearly unchanged within 0.5–2.5 Ω/sq after 12 h in DI water, PBS, and ethanol. - Electrodes: Electrochemical impedance (200 µm × 200 µm) 2.1 kΩ at 40 Hz, 1.5 kΩ at 150 Hz, 1.0 kΩ at 1000 Hz. - In vivo ECG mapping (healthy): Stable conformal coupling on beating mouse (529.9 ± 9.3 bpm) and pig (85.4 ± 8.5 bpm) hearts. Typical ECG morphology with measured R–R interval, QRS duration, J-point: mouse 116.3 ± 3.8 ms, 2.8 ± 0.5 ms, −0.1 ± 0.9 mV; pig 764.3 ± 65.4 ms, 28.8 ± 16.1 ms, −0.1 ± 0.4 mV. - Myocardial infarction mapping: After LCA ligation, epicardial ST-segment elevations localized near ligation point and propagated apically with velocity ~0.6 mm/s; control three-lead ECG showed reciprocal ST changes. Devices maintained position and coupling over 30 min (≥10,000 beats). - Simultaneous ultrasound imaging: Thin, semi-transparent mesh allowed real-time ultrasound with minimal artifacts, improved with 50 µm vs 200 µm thickness. - Biocompatibility and biofouling: H9C2 viability similar to control for Au-overcoated devices; device without Au overcoat reduced viability (<70%). Moderate chronic inflammation with granuloma observed; epicardial thickening increased from 44.4 ± 8.3 µm (day 1) to 645.9 ± 5.3 µm (day 14). Anti-biofouling performance: fluorescence intensity (a.u.) device 0.7 ± 0.5, SE1700 2.8 ± 1.7, Sylgard 184 10.2 ± 5.5, glass 15.4 ± 6.1; significant reductions vs controls. - Cardiac function: Ejection fraction remained within normal 60–70% over 14 days post-implant, exceeding values typical for ischemia-reperfusion (40–60%) and permanent ligation (20–40%).

By formulating a DIW-compatible poroelastic silicone composite that forms a sponge-like foam, the authors achieve devices that are mechanically matched to cardiac tissue and exhibit minimal mechanical and electrical hysteresis under physiological strains. Embedding conductive fillers within internal pores maintains percolation and prevents delamination, enabling robust tissue coupling and consistent signal quality on beating hearts. The custom-fit arrays, designed from 4D ultrasound-derived geometry, provide precise spatial alignment for epicardial mapping. In a murine infarction model, the sensors localized and tracked ST-segment elevations and propagation, while allowing simultaneous ultrasound imaging for structural-functional correlation. The approach suggests utility for intraoperative guidance (e.g., ablation planning) and supports expansion to higher-density electrode arrays to improve spatial resolution and reduce reliance on post-processing.

This work introduces a rapid, custom DIW process for soft, sponge-like poroelastic silicone biosensors that are strain-insensitive, mechanically compliant, and conformally couple to epicardium. The devices enable high-fidelity epicardial ECG mapping and simultaneous ultrasound imaging, demonstrated in healthy porcine and murine hearts and in a murine acute myocardial infarction model. Key contributions include optimized rheology for printability and pore formation, ultralow modulus comparable to cardiac tissue, minimized hysteresis, and stable electrical performance under cyclic strain. Future directions include increasing electrode density for higher-resolution mapping, long-term/chronic implantation studies with strategies to mitigate inflammation (e.g., antifouling coatings or nano-texturing), and integration with wireless power/data systems for continuous monitoring.

- Biocompatibility: Histology revealed moderate chronic inflammation, granuloma formation, and significant epicardial thickening over 14 days, indicating a foreign body response that may limit chronic implantation without further surface engineering. - Translation: In vivo validation was in mice and acute terminal pig studies; human studies are absent. - Spatial resolution: Electrode count and spacing were limited by nozzle feature size (~100 µm), leaving room to improve spatial resolution. - Material variability: Reported sheet resistance values varied across processes/conditions; long-term stability in vivo beyond short-term/2-week studies not established. - Control comparisons: While three-lead ECG was used as control, broader comparisons with other epicardial mapping technologies were not performed.