In our research, we have extensively employed biocompatible semiconductor nanostructures, such as silicon or silicon carbide, to precisely target single cells or specific subcellular components (Table 1). This approach has been instrumental in overcoming the limitations inherent in traditional metal electrode-based devices, which include issues of bulkiness and cell membrane disruption, and it also eliminates the need for genetic modifications. Our work has successfully identified and quantified the various physicochemical outputs, namely photo-thermal, -faradic, and -capacitive effects, of these nanostructured semiconductors at biointerfaces. These findings have demonstrated the potential of semiconductor-based biointerfaces to modulate the electrical activities in a range of cells, including neurons, cardiomyocytes, and bacterial cells (Table 1). Building on this foundation, our current and future focus is on advancing these concepts into translational studies.

 

Table 1: Summary of the non-genetic optically-triggered biological modulation

A) Neuromodulation

Neural stimulation methods remain a cornerstone technique in neuroscience. Besides the traditional electrode-based methods and optogenetics, semiconductor-based biomaterial interfaces have enabled wireless, non-genetic, multiscale, high-resolution, random-access photomodulation of neural activities. We have an active research program that aims to study the mechanisms and to validate the efficacy of recently developed nanostructured semiconductor-based neuromodulation tools.

• Photothermal neuromodulation

My lab set out to demonstrate that silicon’s photothermally induced electric effect could be applied to living cells. We synthesized a deformable and porous type of silicon with molecular-level feature sizes. We introduced the silicon particles over dorsal root ganglia (DRG) neuron cultures and illuminated the cell-membrane-supported particles, eliciting action potentials (signals) in individual neurons (Fig. 1A-B). We also successfully delivered a train of light pulses and repeatedly excited neurons with a one-pulse-one-spike fidelity. This confirmed that the photothermally induced electric effect could indeed be applied to living cells. Silicon’s photothermal effect at the neuron-silicon interface does not require direct physical contact as the heating can be effective for a distance up to one hundred micrometres. This makes it ideal for use in situations such as peripheral nerve stimulations where extracellular matrix or other cellular barriers would usually impede tight biointerfaces.

 

• Photoelectrochemical neuromodulation

For greater efficiency in neuromodulation, a tight interface between the silicon device and the neuron is required. When direct access to the cells is available, the preferred neuromodulation approach would be to use electrons and holes (i.e., the charge carriers) that are generated by light, the way a photoelectrochemical device works.

To investigate the biological applicability of silicon’s photoelectrochemical effect, my lab used coaxial p-type/intrinsic/n-type silicon (PINS) nanowires to wirelessly and photoelectrochemically modulate primary rat DRG neuron excitability (Fig. 1C-D). Our results showed that atomic gold on the nanowires enhances the photoelectrochemical process through which the action potentials in rat DRG neurons were elicited. Essentially, atomic gold reduces the kinetic barrier necessary for the photoelectrochemical current generation, thereby playing the role that a catalyst would play in traditional photoelectrochemical devices.

 

• Formulation of a rational design principle for semiconductor-based modulation tools

Efficient biological modulation requires accurate designs for tight cell-device interfaces. We identified a biology-guided two-step design principle for establishing tight intra-, inter-, and extra-cellular silicon-based interfaces in which silicon and the biological targets have matched mechanical properties and efficient signal transduction. To gain a biophysical understanding of the different biological modulations that silicon could induce, my lab developed a set of matrices to quantify and differentiate the capacitive, faradaic, and thermal outputs from different silicon materials in saline. We confirmed that we could use light to (non-genetically) modulate intracellular calcium dynamics, cytoskeleton-based transport and structures, and cellular excitability, highlighting the diverse utility of these new interfaces. In particular, we showed that flexible and freestanding silicon mesh can modulate brain activities and simple animal behaviors such as induced limb motion from anaesthetized mice (Fig. 1E-F).

 

Figure 1. Neuromodulation. (A-B) Nanoporous silicon-based neuromodulation through an optocapacitance mechanism. (C-D) Coaxial silicon nanowire for photoelectrochemical neuromodulation. (E-F) Multilayered silicon membranes for neuromodulation at the animal level. 

 

B) Cardiac modulation

The electrical conduction system of the heart allows for the coordinated contraction of cardiomyocytes to produce heartbeats. Abnormalities in this system can lead to delayed mechanical activation of specific regions of the heart or pathologically slow heart rates (bradyarrhythmias). Thus, therapies that can either resynchronize the heart or increase the overall beating frequency of the heart are necessary for the treatment of these disorders.

• Extracellular cardiac modulation

The Tian lab recently developed a photoelectrochemical method to optically modulate cardiac beating to a specified target frequency in primary cultured cardiomyocytes and adult rat hearts ex vivo (Fig. 2). To achieve this modulation, we used a low irradiance moving laser stimulus and a biocompatible polymer-silicon nanowire composite material (Fig. 2A). We integrated the PINS nanowires, initially developed for neuromodulation, with polymeric mesh, creating a composite sheet for conformal cardiac interfaces both in vitro (Fig. 2B) and ex vivo (Fig. 2C-D). Upon light stimulation, the PINS nanowires produce photoelectrochemical output, which depolarizes cardiomyocyte plasma membrane and eventually leads to the training or pacing effect. This work has implications for future bioelectrical studies of the cardiac conduction system as well as therapeutics for cardiac conduction disorders in the clinic.

 

Figure 2. Extracellular cardiac optical modulation with polymer/Si nanowire composite mesh. (A) Optical microscopy image of the composite. (B) Optical training in vitro, showing gradual increase in the cardiac beating frequency. (C) Schematic diagram for optical training and pacing in vivo. (D) ECG recording, showing the optical pacing with the composite mesh.

 

Recently, we presented a significant innovation in optoelectronic devices for biological modulation (Fig. 3). We introduced a porosity-based heterojunction, created in p-type silicon. This novel approach marks a departure from conventional methods that rely on materials with varying doping or composition, which are often expensive and complex to fabricate. Our methodology involves a blend of stain etching and oxygen plasma treatment to craft optoelectronic devices from bulk silicon. This approach is markedly simpler and more direct than traditional fabrication methods, such as chemical vapor deposition. It enables us to produce devices capable of modulating biological tissues using low optical power densities and near-infrared light. Our findings show that these stain-etched heterojunctions can generate robust photocurrents upon light illumination, significantly boosted by oxygen plasma treatment. This enhancement is attributed to increased hydrophilicity and reduced surface recombinations. Electrochemical tests demonstrated improved capacitance and reduced impedance of the heterojunctions, signaling efficient charge injection and ion transfer capabilities, distinct from traditional gold-decorated p–i–n junctions. We also found that the photoelectrochemical currents depend on the power and wavelength of the illumination light, responding to visible and near-infrared light but not to ultraviolet light. Finally, we demonstrated the application of these devices in modulating isolated cardiac tissue and in vivo sciatic nerve biomodulation. Our devices can interface with biological tissues without adhesives or physical modifications, responding to light-induced stimuli with varying intensity-dependent effects. The simplicity and efficiency of this fabrication approach, coupled with the use of biocompatible p-type silicon, make our porosity-based heterojunction a promising candidate for future bioelectronic therapies, with significant potential for diverse therapeutic applications.

Figure 3. We introduce porosity-based heterojunctions in p-type silicon for bioelectronics, offering a simpler, cost-effective alternative to traditional methods and enabling efficient, leadless optoelectronic stimulation of biological tissues. Our innovative approach, combining stain etching and oxygen plasma treatment, facilitates the development of flexible bioelectronic devices with significant potential in cardiac and nerve modulation, and broad applications in bioelectronic therapies.

 

• Intracellular cardiac modulation

The current material tool kit for cardiac modulation is primarily based on synthetic or nonliving components. When interfacing with live and dynamically changing tissues, seamless integration of the device is limited by the remaining mechanical invasiveness of the materials and non-natural biological signal transduction at the biointerface. The Tian lab recently proposed that a living bioelectrical system with dynamic and developing behaviors can advance bioelectrical interfaces due to the adaptability and motility of the cellular components and the diverse physical properties of the materials components. This approach is fundamentally different from biological pacemakers, which are generated by transferring genes that encode transcription factors to transform working myocardium into a surrogate sinoatrial node.

We achieved the living bioelectrical system by intracellular integration of silicon nanowires with myofibroblasts through phagocytosis (Fig. 4A). We then demonstrated that this living hybrid tool can be used to investigate intercellular electrical coupling in vitro and in vivo. Using the myofibroblast-nanowire hybrid tool to compare myofibroblast-myofibroblast electrical coupling with myofibroblast-cardiomyocyte coupling in vitro, we detected two different calcium flux propagation mechanisms – one for amplified cardiomyocytes propagation and the other for passive myofibroblasts propagation. We found that, unlike bare silicon nanowires, the myofibroblasts-silicon nanowire hybrids can be seamlessly integrated into contractile cardiac tissue (Fig. 4).

Figure 4. Myofibroblast/Si nanowire composites for intracellular modulation of cardiac activities. (A) The cellular composites are prepared and then used for bioelectrical studies in vitro and in vivo. (B) Procedure for bioelectrical studies ex vivo.

 

C) Microbial modulation

Bacterial ensembles can behave like multicellular and adaptable organisms through intercellular communication and cooperation. Our knowledge of how bacteria respond or adapt to transient physical insults (e.g., thermal and mechanical shocks) is very limited, although such cellular responses are likely crucial to their survival in the highly dynamic natural environment.

Existing interrogation tools lack the desired spatiotemporal precision to study fast microbial signal transduction in response to transient perturbations. The stimuli are typically delivered by injection and subsequent diffusion, taking seconds to equilibrate. Therefore, only slow or global changes in metabolic activities, gene expression levels, medium compositions, and thermal or mechanical environments can be induced. Optical modulation techniques using various non-genetic transducers hold great promise for introducing transient (down to millisecond level), localized (with an arbitrarily-aimed diffraction-limited spot), multiplexed (photothermal, photocapacitive, photofaradaic, and photoacoustic), and controllable (in duration and intensity) perturbations to the microbial community.

Recently, the Tian lab extended the utility of silicon-based materials to the study of transient intercellular bioelectrical signalling in bacteria (Fig. 5). We designed various structured silicon materials which fit different bacterial systems and allow delivery of optically-induced transient and local stresses. Upon optical stimulation, we captured a new form of rapid, photothermal gradient-dependent, intercellular calcium signalling within the biofilm. We also discovered an unexpected coupling between calcium dynamics and biofilm mechanics, which could be of importance for biofilm resistance. Our results suggest that functional integration of silicon materials and bacteria, and associated control of signal transduction, may lead to hybrid living matter for future synthetic biology and adaptable materials.

 

Figure 5. Si-based optical modulation microbial activities. (A) Our approach uses local, transient and physical stimulations. (B) We discovered that thermal gradient can elicit calcium wave in biofilms, which leads to a range of applications such as biofilm dispersal.

 

Selected Publications

• Y. W. Jiang, J. L. Carvalho-de-Souza, R. C. S. Wong, Z. Q. Luo, D. Isheim, X. B. Zuo, A. W. Nicholls, I. W. Jung, J. P. Yue, D.-J. Liu, Y. C. Wang, V. De Andrade, X. H. Xiao, L. Navrazhnykh, D. E. Weiss, X. Y. Wu, D. N. Seidman, F. Bezanilla, B. Z. Tian, Heterogeneous silicon mesostructures for lipid-supported bioelectric interfaces, Nature Materials, 2016, 15, 1023–1030. Link

 

• J. F. Zimmerman, R. Parameswaran, G. Murray, Y. C. Wang, M. Burke, B. Z. Tian, Cellular uptake and dynamics of unlabeled free standing silicon nanowires, Science Advances, 2016, 2, e16010139. Link

 

• R. Parameswaran, J. L. Carvalho-de-Souza, Y. W. Jiang, M. Burke, J. F. Zimmerman, K. Koehler, A. Phillips, J. Yi, E. Adams, F. Bezanilla, B. Z. Tian, Photoelectrochemical modulation of neuronal activity with free-standing coaxial silicon nanowires, Nature Nanotechnology, 2018, 13, 260-266. Link

 

• Y. W. Jiang, X. J. Li, B. Liu, J. Yi, Y. Fang, F. Y. Shi, X. Gao, E. Sudzilovsky, R. Parameswaran, K. Koehler, V. Nair, J. P. Yue, K. H. Guo, Y. Fang, H.-M. Tsai, G. Freyermuth, R. C. S. Wong, C.-M. Kao, C.-T. Chen, A. W. Nicholls, X. Y. Wu, G. M. G. Shepherd, B. Z. Tian, Rational design of silicon structures for optically-controlled multiscale biointerfaces, Nature Biomedical Engineering, 2018, 2, 508-521. doi:10.1038/s41551-018-0230-1. Link

 

• R. Parameswaran, B. Z. Tian, Rational design of semiconductor nanowires for functional subcellular interfaces. Accounts of Chemical Research, doi: 10.1021/acs.accounts.7b00555. 2018. Link

 

• Y. W. Jiang, B. Z. Tian, Inorganic semiconductor-enabled bioelectronic and biophotonic interfaces. Nature Reviews Materials, 2018, 3, 473-490. Link

 

• P. Parameswaran, K. Koehler, M. Rotenberg, M. Burke, J. Kim, K.-Y. Jeong, B. Hissa, M. Paul, K. Moreno, N. Sarma, T. Hayes, E. Sudzilovsky, H.-G. Park, B. Z. Tian, Optical stimulation of cardiac cells with a polymer-supported silicon nanowire matrix. Proc. Natl. Acad. Sci. USA, 2019, 116, 413-421. Link

 

• A. Prominski, J. Y. Shi, P. J. Li, J. P. Yue, Y. L. Lin, J. Park, B. Z. Tian, M. Y. Rotenberg. Porosity-based heterojunctions enable leadless optoelectronic modulation of tissues. Nature Materials, 2022, DOI: 10.1038/s41563-022-01249-7. Link

 

• Y. W. Jiang, R. Parameswaran, X. J. Li, J. L. Carvalho-de-Souza, X. Gao, L. Y. Meng, F. Bezanilla, G. M. G. Shepherd, B. Z. Tian, Non-genetic optical neuromodulation with silicon-based materials. Nature Protocols, 2019, 14, 1339–1376. Link

 

• B. Z. Tian, C. M. Lieber, Nanowired bioelectric interfaces. Chemical Reviews, 2019, 119, 9136-9152. Link

 

• H. Acaron Ledesma, X. Li, J. L. Carvalho-de-Souza, W. Wei, F. Bezanilla, B. Z. Tian, An atlas of nano-enabled neural interfaces. Nature Nanotechnology, 2019, 14, 645-657. Link

 

• B. Z. Tian, Nongenetic neural control with light, Science, 2019, 365, 457, DOI: 10.1126/science.aay4351. Link

 

• M. Y. Rotenberg, N. Yamamoto, E. N. Schaumann, L. Matino, F. Santoro, B. Z. Tian, Living myofibroblast-silicon composites for probing electrical coupling in cardiac systems. Proc. Natl. Acad. Sci. USA, 2019, 116, 22531-22539. Link

 

• X. Gao, Y. W. Jiang, Y. L. Lin, K.-H. Kim, Y. Fang, J. Yi, L. Y. Meng, H.-C. Lee, Z. Y. Lu, O. Leddy, R. Zhang, Q. Tu, W. Feng, V. Nair, P. J. Griffin, F. Y. Shi, G. S. Shekhawat, A. R. Dinner, H.-G. Park, B. Z. Tian, Structured silicon for revealing transient and integrated signal transductions in microbial systems. Science Advances, 2020, 6, eaay2760. Link

 

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