Biology subjects

Jensen, N.

Publications and source records attributed to Jensen, N..

10 recordsLinked to original sources

bioRxiv2026

Biomimetic scaffolds mimic human subretinal debris for preclinical testing of prosthetic vision

Photovoltaic subretinal prostheses replace the lost photoreceptors in patients blinded by atrophic age-related macular degeneration (AMD) and provide central vision by electrically stimulating the inner retinal neurons. Subretinal debris in geographic atrophy separate the inner nuclear layer from the implant by tens of micrometers, increasing the stimulation threshold and reducing the pattern contrast. Absence of this separating layer in animal models diminishes the accuracy of preclinical testing of the next-generation implants, especially with smaller pixels. We developed three-dimensional biomimetic scaffolds nanoprinted onto photovoltaic arrays to provide a precisely controlled separation between the implants and target neurons in animal models of retinal degeneration. Gap sizes of the scaffolds were adjusted to meet the following requirements: they should allow light penetration to the implant, provide unobstructed flow of electric current, but preclude migration of cells into their 3-D structure. Gaps of 3 m were found to be optimal as they restricted cellular migration and maintained the inner nuclear layer above both planar and pillar electrode arrays. Scaffolds did not measurably alter the photovoltaic response and accurately reproduced the distance dependence of stimulation threshold. Following implantation, the initially fluid-filled intra-scaffold space was progressively filled by cellular matter matching the density of the subretinal debris. This transition is expected to increase resistivity of the scaffold to that of tissue, which was evidenced by concurrently improved grating acuity. These results establish a model of the controlled separation between the implant and the target neurons reproducing the features of subretinal debris in geographic atrophy of human patients, thereby providing a platform for accurate preclinical evaluation of the subretinal implants.

bioengineering↗

bioRxiv2026

Eye-tracked Maxwellian illumination system for power-efficient retinal stimulation

Augmented reality glasses typically have poor optical efficiency (<1%), which is detrimental for high power applications, such as prosthetic vision. Maxwellian view optics are a candidate for increasing efficiency, but suffer from a very limited exit pupil and thus field of view. Many approaches to extend the eyebox inherently decrease the optical power. We present a design that uses a high-density VCSEL array and eye tracking to create an extended eyebox without sacrificing the advantages of a Maxwellian view display. Our system achieves a 20 degree field of view, a resolution 48.8 LP/mm (10 m), and an end-to-end optical efficiency of 83%

bioengineering↗

bioRxiv2025

Residual photoreceptors affect the response of a degenerate retina to electrical stimulation

Patients with age-related macular degeneration (AMD) implanted with the PRIMA photovoltaic subretinal prosthesis demonstrated letter acuity closely matching the devices 100 {micro}m pixel size. Improving visual acuity requires smaller pixels, which, in turn, require relaxation of electric field confinement to maintain effective stimulation of bipolar cells. Eliminating local return electrodes broadens the electric field but may inadvertently engage residual photoreceptors adjacent to the implant, thereby altering electrically evoked visual percepts. Here we quantify the contribution of residual photoreceptors to electrically evoked retinal responses across various implant configurations. Monopolar photovoltaic arrays with 20 {micro}m pixels and bipolar arrays with 100 {micro}m pixels were implanted subretinally in Long Evans rats, resulting in local degeneration of photoreceptors directly above the device. Responses were compared with those obtained in RCS rats, which lack functional photoreceptors. Implants were activated by patterns of 880 nm laser at pulse durations varying from 0.5 to 10 ms. Visually evoked potentials were measured in scotopic and photopic conditions, with and without the intravitreal application of mGluR6 agonist L-AP4 to block photoreceptor-driven ON pathways. Experimental thresholds were interpreted using a computational model of retinal network activation in distinct electric field geometries. In locally degenerate retina stimulated with monopolar arrays, blocking photoreceptor input yielded a rheobase (0.06 mW/mm{superscript 2}) and chronaxie ([~]3 ms) of the stimulation threshold, matching that measured in fully degenerate RCS retina, indicating direct activation of bipolar cells. In contrast, when photoreceptor input was intact, stimulation thresholds decreased significantly, and dark adaptation further modulated the threshold in a pulse-duration dependent manner. Bipolar arrays provided identical thresholds in locally degenerate and fully degenerate retina (0.2 mW/mm{superscript 2}) when stimulation was confined to the implant center; however, shifting stimulation to the implants edge lowered the thresholds, revealing a contribution from adjacent photoreceptors. These findings demonstrate that residual photoreceptors can substantially influence responses evoked by subretinal prostheses in a degenerate retina, with direct consequences for perceptual uniformity in patients, such as edge brightening. These results provide guidance for design of the next-generation higher-resolution implants, supporting the use of local return electrodes to maximize resolution while minimizing the effect of photoreceptors in clinical applications.

neuroscience↗

bioRxiv2025

Amorphous silicon resistors enable smaller pixels in photovoltaic retinal prosthesis

ObjectiveClinical trials of the photovoltaic subretinal prosthesis PRIMA demonstrated feasibility of prosthetic central vision with resolution matching its 100{micro}m pixel size. To improve prosthetic acuity further, pixel size should be decreased. However, there are multiple challenges, one of which is related to accommodating a compact shunt resistor within each pixel that discharges the electrodes between stimulation pulses and helps increase the contrast of the electric field pattern. Unfortunately, standard materials used in integrated circuit resistors do not match the resistivity required for small photovoltaic pixels. Therefore, we used a novel material - doped amorphous silicon (a-Si) and integrated it into photovoltaic arrays with pixel sizes down to 20{micro}m. ApproachTo fit within a few {micro}m2 area of the pixels and provide resistance in the M{Omega} range, the material should have sheet resistance of a few hundred k{Omega}/sq, which translates to resistivity of a few {Omega}*cm. The a-Si layer was deposited by low-pressure chemical vapor deposition (LPCVD) and its resistivity was adjusted by PH3 doping before encapsulating the resistors between SiO2 and SiC for stability in-vivo. Main ResultsHigh-resolution retinal implants with integrated shunt resistors were fabricated with values ranging from 0.75 to 4 M{Omega} on top of the photovoltaic pixels of 55, 40, 30 and 20 {micro}m in size. Photoresponsivity with all pixel sizes was approximately 0.53 A/W, as high as in the arrays with no shunt resistor. The shunts shortened electrodes discharge time, with the average electric potential in electrolyte decreasing by only 21-31% when repetition rate increased from 2 to 30 Hz, as opposed to a 54-55% decrease without a shunt. Similarly, contrast of a Landolt C pattern increased from 16-22% with no shunt to 22-34% with a shunt. Further improvement in contrast is expected with pillar electrodes and local returns within each pixel. SignificanceMiniature shunt resistors in a MOhm range can be fabricated from doped a-Si in a process compatible with manufacturing of photovoltaic arrays. The shunt resistors improved current injection and spatial contrast at video frame rates, without compromising the photoresponsivity. These advances are critical for scaling pixel sizes below 100 {micro}m to improve visual acuity of prosthetic vision.

neuroscience↗

bioRxiv2025

Maximizing the fidelity of a photovoltaic subretinal prosthesis for human patients

ObjectivePRIMA subretinal implants provide prosthetic vision to patients blinded by age-related macular degeneration, with acuity closely matching the sampling limit of the pixel pitch: a single 100{micro}m pixel per line of a letter corresponds to 20/420 acuity. Decreasing the pixel size in the same flat geometry is difficult due to the constrained electric field, especially considering a 40{micro}m thick debris layer separating the implant from the target neurons. Here we optimize the electrode design to help overcome such limitations. ApproachAn end-to-end modeling pipeline combines the retinal photovoltaic implant simulator (RPSim) based on the Xyce circuit simulator with an interface to COMSOL Multiphysics for electric field modelling. It was used to generate and characterize implants in an open-loop sampling-based optimization. Implant performance was evaluated with respect to voltage drop across bipolar cells (representing the stimulation strength), pattern contrast, and neural selectivity. Main ResultsThe highest selectivity in stimulation of bipolar cells was achieved with arrays having active electrodes on pillars and return electrodes connected in a mesh surrounding the photovoltaic pixels in the array. Such a design, even with pixels down to 20{micro}m, provides stimulation strength exceeding, and contrast similar to that of flat 100{micro}m PRIMA pixels. SignificanceUsing a novel 3-D electrode design, the pitch of the photovoltaic array can be decreased to 20{micro}m, while providing performance that exceeds the flat 100{micro}m PRIMA pixels. In humans, 20{micro}m resolution on the retina corresponds to a visual acuity of 20/80 - a five times improvement compared to the current clinical device.

bioengineering↗

bioRxiv2024

Accelerated Simulation of Multi-Electrode Arrays Using Sparse and Low-Rank Matrix Techniques

ObjectiveModeling of Multi-Electrode Arrays used in neural stimulation can be computationally challenging since it may involve incredibly dense circuits with millions of intercon-nected resistors, representing current pathways in an electrolyte (resistance matrix), coupled to nonlinear circuits of the stimulating pixels themselves. Here, we present a method for accelerating the modeling of such circuits with minimal error by using a sparse plus low-rank approximation of the resistance matrix. MethodsWe prove that thresholding of the resistance matrix elements enables its sparsification with minimized error. This is accomplished with a sorting algorithm, implying efficient O (N log (N)) complexity. The eigenvalue-based low-rank compensation then helps achieve greater accuracy without significantly increasing the problem size. Results: Utilizing these matrix techniques, we reduced the computation time of the simulation of multi-electrode arrays by about 10-fold, while maintaining an average error of less than 0.3% in the current injected from each electrode. We also show a case where acceleration reaches at least 133 times with additional error in the range of 4%, demonstrating the ability of this algorithm to perform under extreme conditions. ConclusionCritical improvements in the efficiency of simulations of the electric field generated by multi-electrode arrays presented here enable the computational modeling of high-fidelity neural implants with thousands of pixels, previously impossible. Significance: Computational acceleration techniques described in this manuscript were developed for simulation of high-resolution photovoltaic retinal prostheses, but they are also immediately applicable to any circuits involving dense connections between nodes, and, with modifications, more generally to any systems involving non-sparse matrices.

bioengineering↗

bioRxiv2024

Modulation of Neuronal Excitability and Plasticity by BHLHE41 Conveys Lithium Non-Responsiveness

Many bipolar disorder (BD) patients are non-responsive to lithium. The mechanisms underlying lithium (non-)responsiveness are largely unknown. By using gene-set enrichment analysis methods, we found that core clock gene-sets are significantly associated with lithium response. Among the top hits was BHLHE41, a modulator of the molecular clock and homeostatic sleep. Since BHLHE41 and its paralog BHLHE40 are functionally redundant, we assessed chronic lithium response in double-knockout mutant mice (DKO). We demonstrated that DKOs are non-responsive to lithiums effect in various behavioral tasks. Cellular assays and patch clamp recordings revealed lowered excitability and reduced lithium-response in prefrontal cortical layer 2/3 DKO neurons and on hippocampal long-term potentiation. Single-cell RNA sequencing identified that lithium deregulated mitochondrial respiration, cation channel and postsynapse associated gene-sets specifically in upper layer excitatory neurons. Our findings show that lithium acts in a highly cell-specific way on neuronal metabolism and excitability and modulates synaptic plasticity depending on BHLHE40/41.

neuroscience↗

bioRxiv2024

Enhancing Prosthetic Vision by Upgrade of a Subretinal Photovoltaic Implant in situ

In patients with atrophic age-related macular degeneration, subretinal photovoltaic implant (PRIMA) provided visual acuity up to 20/440, matching its 100m pixels size. Next-generation implants with smaller pixels should significantly improve the acuity. This study in rats evaluates removal of a subretinal implant, replacement with a newer device, and the resulting grating acuity in-vivo. Six weeks after the initial implantation with planar and 3-dimensional devices, the retina was re-detached, and the devices were successfully removed. Histology demonstrated a preserved inner nuclear layer. Re-implantation of new devices into the same location demonstrated retinal re-attachment to a new implant. New devices with 22m pixels increased the grating acuity from the 100m capability of PRIMA implants to 28m, reaching the limit of natural resolution in rats. Reimplanted devices exhibited the same stimulation threshold as for the first implantation of the same implants in a control group. This study demonstrates the feasibility of safely upgrading the subretinal photovoltaic implants to improve prosthetic visual acuity.

neuroscience↗

bioRxiv2021

Rational control of structural off-state heterogeneity in a photoswitchable fluorescent protein provides switching contrast enhancement

Reversibly photoswitchable fluorescent proteins are essential markers for advanced biological imaging, and optimization of their photophysical properties underlies improved performance and novel applications. Here we establish a link between photoswitching contrast, a key parameter that largely dictates the achievable resolution in nanoscopy applications, and chromophore conformation in the non-fluorescent state of rsEGFP2, a widely employed label in REversible Saturable OpticaL Fluorescence Transitions (RESOLFT) microscopy. Upon illumination, the cis chromophore of rsEGFP2 isomerizes to two distinct off-state conformations, trans1 and trans2, located on either side of the V151 side chain. Reducing or enlarging the side chain at this position (V151A and V151L variants) leads to single off-state conformations that exhibit higher and lower switching contrast, respectively, compared to the rsEGFP2 parent. The combination of structural information obtained by serial femtosecond crystallography with high-level quantum chemical calculations and with spectroscopic and photophysical data determined in vitro suggests that the changes in switching contrast arise from blue- and red-shifts of the absorption bands associated to trans1 and trans2, respectively. Thus, due to elimination of trans2, the V151A variants of rsEGFP2 and its superfolding variant rsFolder2 display a more than two-fold higher switching contrast than their respective parent proteins, both in vitro and in E. coli cells. The application of the rsFolder2-V151A variant is demonstrated in RESOLFT nanoscopy. Our study rationalizes the connection between structural and photophysical chromophore properties and suggests a means to rationally improve fluorescent proteins for nanoscopy applications.

biophysics↗

bioRxiv2020

The Positive Switching RSFP Padron2 Enables Live-Cell RESOLFT Nanoscopy Without Sequential Irradiation Steps

Reversibly switchable fluorescent proteins (RSFPs) can be repeatedly transferred between a fluorescent on- and a non-fluorescent off-state in response to irradiation with light of different wavelengths. Negative switching RSFPs are switched from the on- to the off-state with the same wavelength which also excites fluorescence. Positive switching RSFPs have a reversed light response where the fluorescence excitation wavelength induces the transition from the off- to the on-state. Reversible saturable optical linear (fluorescence) transitions (RESOLFT) nanoscopy utilizes these switching states to achieve diffraction-unlimited resolution, but so far has primarily relied on negative switching RSFPs by using time sequential switching schemes. Based on the green fluorescent RSFP Padron, we engineered the positive switching RSFP Padron2. Compared to its predecessor, it can undergo 50-fold more switching cycles while displaying a contrast ratio between the on- and the off-state of more than 100:1. Because of its robust switching behavior, Padron2 supports a RESOLFT imaging scheme that entirely refrains from sequential switching as it only requires beam scanning of two spatially overlaid light distributions. Using Padron2, we demonstrate live-cell RESOLFT nanoscopy without sequential irradiation steps.

molecular biology↗