Interfacing Thin-film Bioelectronics with Bone Regenerative Cryogel Scaffolds for Transosseous Cortical Access

Introduction: : Chronic brain-machine interfaces often require direct cortical access, posing long-term challenges for cranial defect closure and implant integration [1,2]. Conventional methods such as fixation hardware or bone cement introduce risks like infection, hardware exposure, and pressure offset. Tissue engineering approaches for cranial repair—particularly cryogel scaffolds—offer a solution with bioresorbability, tunable mechanical properties, and a microporous structure that supports native bone regrowth [3-5]. Meanwhile, thin-film bioelectronic devices are increasingly favored for brain interfaces in research due to their ultra-flexibility, biocompatibility, and high-fidelity signal acquisition [6-8]. However, efforts to directly integrate electronic implants into bone tissue are limited. Here, we show a combination implant system of embedding a thin-film recording array within a cryogel scaffold to simultaneously enable cortical recording and cranial defect regeneration. The cryogel structure supports bone ingrowth while preserving electrode function, offering a stable, regenerative approach to long-term cortical access. This study evaluates the osseointegration and recording performance of the combined system in a murine calvarial defect model, laying a foundation for future clinical neuroprosthetic and bone-electronic interface applications.

Materials and

Methods: : Thin-film microelectrode arrays were constructed using microfabrication techniques, forming 32-channel Parylene-C (PaC) encapsulated devices with PEDOT:PSS-coated electrodes (Figure 1a-b). Devices were bonded to flexible cables for backend connection to a recording system. Cryogels were synthesized from a gelatin-chitosan blend, crosslinked via glutaraldehyde at subzero temperatures to create porous, mechanically robust scaffolds (Figure 1c-d). The recording implant and scaffold were combined either during cryogel solution crosslinking to create an intimate interface or in a modular fashion during surgery to allow for a more flexible intraoperative placement (Figure 1e-f). To examine long-term integration capabilities, C57BL/6 mice underwent bilateral calvarial defect surgeries. Each mouse received a cryogel-only scaffold and a contralateral scaffold embedded with a non-functional PaC device. Bone regeneration was assessed by microcomputed tomography (microCT) and hematoxylin and eosin (H&E) staining at 4 weeks (n = 5 mice) and 8 weeks (n = 6 mice). MicroCT imaging quantified bone ingrowth volume and mineral density, while histological analysis assessed early-stage bone formation and tissue integration within the scaffold pores. To confirm recording fidelity of the combination device, acute electrophysiology studies were performed on anesthetized mice, and sensorimotor cortex activity was recorded during hind paw stimulation. Signal-to-noise ratios (SNR) were calculated against baseline activity to validate neural responsiveness. A chronic pilot study further confirmed device functionality in freely-moving animals. All data were statistically analyzed using two-way ANOVA or t-tests where appropriate.

Results, Conclusions, and Discussions:: Integration of a thin-film recording device with a cryogel scaffold enabled both bone regeneration and cortical signal acquisition without significant compromise to either function. MicroCT imaging demonstrated comparable bone volume and mineralization between device-present and cryogel-only groups at both 4 and 8 weeks, indicating that the embedded implant does not impede osseointegration (Figure 2a). The terminal 8-week timepoint was selected with the expectation of complete defect closure [9,10]. MicroCT analysis showed that full defect remineralization was not achieved, however bone infiltration was consistent between groups and widespread, indicating the scaffold’s maintained osteoconductive potential when combined with a thin-film electronic device (Figure 2b-c). Additionally, histological staining revealed active new bone formation throughout the scaffold pores, even where mineralization remained incomplete, suggesting ongoing woven bone development not fully captured by imaging (Figure di-iv). This delay in full cortical regrowth, we hypothesize this is due to the removal of the dural layer which has been shown to be an integral structure for optimal bone regeneration. The resection of the dura is a necessary addition to the surgical procedure, however, to achieve direct cortical access [11]. Acute electrophysiology recordings validated high-fidelity neural signal acquisition, with a significantly higher SNR during paw stimulation contralateral to the analyzed sensorimotor cortex compared to baseline or ipsilateral paw stimulation (Figure 2e-g). A chronic pilot study confirmed continued device performance in freely-moving mice. Importantly, cryogel presence did not impede electrical recording quality, thus confirming the scaffold’s compatibility with bioelectronic function. Future work could accelerate healing by incorporating osteoinductive factors like BMP2 and using larger animal models with thicker dura, enabling a dural incision rather than removal, for translational relevance. Overall, this study presents a novel implantable system for stable, long-term cortical interfacing through regenerating cranial bone. The combination platform holds promise for advancing brain-machine interfaces, neuroprosthetics, and regenerative bioelectronics by offering a method to fuse electronics and hard tissue in a stable, biocompatible, and functional manner.

Acknowledgements and/or References (Optional):: Acknowledgments:
The authors acknowledge support from the National Institutes of Health (NIH) National Institute for Biomedical Imaging and Bioengineering (NIBIB) T32 Training in Surgical Innovation Program (5T32EB021966-07). The authors also acknowledge the Dartmouth Pathology Shared Resource (RRID: SCR_023479) and the US National Science Foundation (NSF) EPSCoR RII program (Grant OIA-1757371). K.R.H. acknowledges the Dartmouth Cancer Center Support Grant (P30CA023108).

References:
[1] N. S. Card et al., “An accurate and rapidly calibrating speech neuroprosthesis,” medRxiv, p. 2023.12.26.23300110, Apr. 2024, doi: 10.1101/2023.12.26.23300110.
[2] G. Herrera-Arcos et al., “Closed-loop optogenetic neuromodulation enables high-fidelity fatigue-resistant muscle control,” Science Robotics, vol. 9, no. 90, p. eadi8995, May 2024, doi: 10.1126/scirobotics.adi8995.
[3] H. Omidian, S. Dey Chowdhury, and N. Babanejad, “Cryogels: Advancing Biomaterials for Transformative Biomedical Applications,” Pharmaceutics, vol. 15, no. 7, p. 1836, Jun. 2023, doi: 10.3390/pharmaceutics15071836.
[4] M. P. Nikolova and M. S. Chavali, “Recent advances in biomaterials for 3D scaffolds: A review,” Bioactive Materials, vol. 4, pp. 271–292, Dec. 2019, doi: 10.1016/j.bioactmat.2019.10.005.
[5] K. R. Hixon, T. Lu, and S. A. Sell, “A comprehensive review of cryogels and their roles in tissue engineering applications,” Acta Biomaterialia, vol. 62, pp. 29–41, Oct. 2017, doi: 10.1016/j.actbio.2017.08.033.
[6] T. Someya, Z. Bao, and G. G. Malliaras, “The rise of plastic bioelectronics,” Nature, vol. 540, no. 7633, pp. 379–385, Dec. 2016, doi: 10.1038/nature21004.
[7] A. J. Boys and S. T. Keene, “Bioelectronic interfacial matching for superior implant design,” CR-PHYS-SC, vol. 5, no. 8, Aug. 2024, doi: 10.1016/j.xcrp.2024.101877.
[8] A. J. Boys, “There and Back Again: Building Systems That Integrate, Interface, and Interact with the Human Body,” Advanced Biology, vol. 8, no. 5, p. 2300366, 2024, doi: 10.1002/adbi.202300366.
[9] E. M. Robertson, K. R. Hixon, S. H. McBride-Gagyi, and S. A. Sell, “Bioactive impact of manuka honey and bone char incorporated into gelatin and chitosan cryogels in a rat calvarial fracture model,” Journal of Biomedical Materials Research Part B: Applied Biomaterials, vol. 111, no. 10, pp. 1763–1774, 2023, doi: 10.1002/jbm.b.35283.
[10] R. M. Samsonraj, A. Dudakovic, P. Zan, O. Pichurin, S. M. Cool, and A. J. van Wijnen, “A Versatile Protocol for Studying Calvarial Bone Defect Healing in a Mouse Model,” Tissue Eng Part C Methods, vol. 23, no. 11, pp. 686–693, Nov. 2017, doi: 10.1089/ten.tec.2017.0205.
[11] Ö. R. Özerdem et al., “Roles of Periosteum, Dura, and Adjacent Bone on Healing of Cranial Osteonecrosis,” Journal of Craniofacial Surgery, vol. 14, no. 3, p. 371, May 2003.