Recent developments in agar-based optical devices

Biocompatible optical devices are breakthrough illumination, imaging, and biomedical sensing technologies. Despite the noteworthy developments in silk, cellulose, and hydrogel-based optics, such approaches rely on expensive precursors and intricate fabrication. Therefore, agar extracted from red algae emerges as a promising biodegradable alternative as an edible, low-cost, and renewable material. This paper overviews the state-of-the-art of agar-based optical devices. Firstly, we revisit this phycocolloid’s fundamentals and highlight its appealing mechanical, optical, and electrical characteristics. Subsequently, we summarize the available agar elements, slab waveguides, and optical ﬁbers. Lastly, we discuss their advantages and challenges by envisaging opportunities for future developments and applications.


Introduction
Biodegradable optical materials and devices are emerging technologies to establish biochemical compatibility and safe interaction between light and living tissues.Unlike the widespread glass and plastic structures, bioresorbable elements are typically soft, present no sharp edges, and are absorbable or excretable after use.Besides, some devices can encapsulate cells and simultaneously work as waveguides and growth media [1,2].Promising applications for biocompatible optics encompass intra-body illumination, brain stimulation, optogenetics, imaging systems, light-driven drug delivery, and optical probes for assessing physical and biochemical parameters [3][4][5][6].
Agar arises as an edible and renewable option for biocompatible optics.This gellike substance obtained from red algae (species of genera Gracilaria and Gelidiaceae) presents noteworthy characteristics like transparency, flexibility, moldability, low gelation temperature, and thermo-reversibility [14][15][16].Besides substituting gelatin for culinary purposes, agar also serves as a tissue model, artificial skin, phantom for imaging, and substrate for electrophoresis and culture media [17][18][19].Furthermore, one may attain strengthened and transparent gels by adjusting their chemical composition, making this material suitable for manufacturing optical elements from lenses to structured waveguides.
Therefore, this paper overviews the recent developments in agar-based optical devices.Firstly, we cover general aspects of this gel-like material, like structure, gelation mechanism, and physical/chemical characteristics.Subsequently, we revisit the available biodegradable components, including lenses, slab waveguides, and optical fibers.Lastly, we discuss the advantages and drawbacks of agar-made devices by prospecting opportunities and future applications for this acknowledged, versatile, yet unexplored material.

Agar
Agar is a phycocolloid constituting the polysaccharides agarose and agaropectin, with the former playing a pivotal role in the agar composition as responsible for its thermoreversible gelling behavior [20].The agarose's structure comprises a linear chain of repeating polysaccharides units, namely agarobiose and neoagarobiose [21], as shown in Fig 1 .Agaropectin exhibits a similar arrangement characterized by lower molecular weight and fewer repeating units [16].
The gelation mechanism occurs when a heated agar solution containing random agarose segments cools to the helix point, rearranging the dispersed chains into double helices structures.Further decreasing the temperature leads to a separation point that distributes the agarose molecules into coacervate and dilute phases.If the agarose concentration exceeds a critical value, the helical segments in the dilute phase connect the coacervate particles and form the macro-reticular gel network at the equilibrium point [22].Melting and gel temperatures are typically around 85 and 40 • C, respectively, though these values vary with the phycocolloid source [20].Previous studies Fig. 1 Structure of agarose, the main constituent of agar, comprising repeating 3, 6-anhydro-Lgalactose and D-galactose units, forming agarobiose and neoagarobiose.α-agarase and β-links are hydrolyzed by acids and enzymes, respectively [16,21].also recommend agar concentrations ≥ 0.5 wt% to ensure the formation of chemically and mechanically stable gels [16].
The agarose's thermo-reversibility allows reestablishing the gel condition after cooling a reheated sample.However, this phenomenon causes thermal hysteresis towards lower temperatures for the melting and settling points due to the crystallization of the agarose helices, creating aggregates that disturb the equilibrium between coacervate and dilute phases [22,23].
Furthermore, agar undergoes swelling and syneresis effects depending on its concentration and the environmental conditions.Soaking agar samples with water changes the equilibrium point, expanding their volume as the fluid molecules diffuse into the gel network and exert osmotic stress over the linked helices.Nevertheless, crystallized helical segments impose an elastic restoring force that prevents further water absorption and may generate swelling/deswelling cycles to preserve the equilibrium between gel and solvent-driven forces [22,24].Conversely, syneresis occurs when the gel surface dries, raising the local agarose concentration and reinforcing the gel network.The solvent diffuses to reestablish the equilibrium and eventually escapes through cracks, accumulating over the dried surface.This gradual water expelling explains the agar structure's shrinkage [22,24].

Characteristics of agar samples
Agar is a versatile biomaterial with remarkable mechanical and optical characteristics.One manipulates such properties during the gel preparation by controlling the phycocolloid concentration or incorporating innocuous compounds like glycerol, sugars, and salts.This section overviews the physical properties of agar gels as demanded for potential applications in optical devices.

Mechanical properties
Agarose forms a strong and stable gel due to the prevailing hydroxyl groups and low sulfate content, supporting crystallinity and gelation ability [16,25].Although agarose contributes to a more compact and strengthened network, optical applications usually avoid exceeding concentrations once the gel becomes turbid and opaque.The phycocolloid content reduces deformability since it shortens the chains between agarose rods, decreasing the fracture strain limit [26].Studies also suggest a viscous behavior at low frequencies induced by the network relaxation/rearrangement and the solvents trapped inside the porous structure [26,27].Moreover, the elastic modulus drops with the temperature because the crystallized helices convert into long, flexible chains [28], though the cooling rate and thermal hysteresis also influence the mechanical properties [29].
Alternatively, adding glycerol to the agar solution promotes rigidity and deformability, besides creating more stable and transparent gels even for minor agarose concentrations, although its underlying mechanisms are debatable.Previous analyses suggest that glycerol molecules occupying the agar network's interstices weaken the chains between agarose rods and improve viscous behavior [27].Conversely, the enhanced gel stiffness may be due to the additional hydrogen bonds supported by the glycerol's hydroxyl groups, which facilitates the aggregation of helices [26].Nevertheless, exceeding glycerol concentrations inhibits the gelling ability as it subsides the water content necessary for agarose molecules' hydration [25,27].
Regarding the incorporation of sucrose, results indicate improvements in both elastic and viscous properties as glycerol.The presence of sugars in the network's interstices also homogenizes the gel structure and diminishes the pores' average size.However, concentrations beyond 60 wt% hinder gelation as the viscosity of sucrosewater solution reduces the mobility and promotes nucleation of agarose helices instead of growing the network structure [26,30].

Optical properties
The optical properties typically vary with the algae species.While heated solutions are usually transparent, agar bulk samples become turbid upon gelation due to the helices' aggregation, resulting in more opaque gels for increasing agarose concentrations [29].The gel refractive index approaches the water one, exhibiting subtle increments by augmenting the agar content, ranging from 1.333 to 1.339 for 0 to 4 wt% of agar, respectively [31].Spectral analyses within visible and near-infrared ranges reveal lower attenuation toward longer wavelengths, achieving a peak at ∼ 850 nm followed by a drop at ∼ 950 nm [29,31], i.e., matching the water absorption bands [32].Transmission losses may also be due to the ash content and mineral impurities available in the agar powder [33].Nevertheless, the absence of auto-fluorescent response fulfills the requirements for biomedical imaging and microscopy [19].
Light scattering emerges from the porous structure and impregnated air bubbles.Albeit increasing the agarose concentration hinders the scattering effect by sealing the pores, one recalls its negative impact on turbidity.Furthermore, bubbles appear during the gel preparation by diffusion from heated, degassed water or entrapping air from the environment during the solution mixing.Adding agar also diminishes the bubbles' diameter because the gel network strength impairs diffusion.Fortunately, several approaches are available to circumvent bubble formation, such as optimizing the mixing conditions, centrifugation, and vacuum treatment [34,35].
Ultimately, the optical properties are tailorable by incorporating glycerol or sugars into the agar solution.Previous works show a notable decrease in turbidity for glycerol and sucrose concentrations of 60 wt%, as the agarose affinity to the solvent disfavors the helices aggregation in bundles [26,30].Experimental observations also suggest that fluids confined in the network cages dictate the optical characteristics, attaining refractive indices higher than 1.36 and 1.42 for sucrose and glycerol-enhanced samples, respectively [31,36].Indeed, imaging applications employ sugars, amino acids, and glycerol as innocuous reagents for matching the gel and biological sample's refractive indices, consequently preventing unwanted light diffraction and scattering [19].

Electric properties
The electrical conductivity of agar gels is valuable for imaging and analytical chemistry applications [20,37].This feature arises from the mineral impurities (like Fe, Mg, and Cd) present in the agar composition.Indeed, purified agarose gels exhibit weaker electrical characteristics than agar ones [38].
Apart from the low content of electronegative groups, the phycocolloid can sustain the electric field-driven migration of ions and particles through its porous structure by electroendosmosis [20,29].Experiments found typical conductivity values between 0.05 and 0.2 S/m for assorted agar types with different ash contents [38,39].Moreover, increasing temperatures favor the minerals' solubilization and mobility, thus enhancing the electrical response [38].
Albeit the gel conductivity subtly rises with the agarose concentration, such an approach may be unfeasible for electro-optic modulation due to the turbidity constraints [40].Alternatively, one may dissolve salts in the agar solution to improve ions availability in the dilute phase.The electrical conductivity linearly increases with the NaCl or KCl contents, quadrupling the value for concentrations of ∼ 1 g/mL [39,41,42].Furthermore, the salt's contribution also prevails over the conductivity of glycerol solutions, assisting the creation of electrically-tunable yet strengthened and transparent gels [39,40,43].

Agar-based optical devices and their applications
This section reviews current optical devices made of agar, like lenses and slab waveguides.We emphasize the emerging soft optical fibers and discuss their potential applications in physical and chemical sensing.Lastly, we present examples of non-biodegradable sensors comprising silica fibers coated with agarose gels.

Optical elements
Gels are moldable, cuttable, and demand low-temperature processing, thus easier to fabricate and manipulate than glasses and plastics.Previous works envisaging didactic purposes introduced edible optical elements made of gelatin, like lenses and prisms [44].However, protein-based gels usually exhibit limited chemical and mechanical stability at room temperature, making such devices unreliable for practical biomedical applications.Regarding agar, scientists developed a plano-convex lens molded with a glass lens template, as illustrated in Fig. 2(a).The melted agarose solution contains sugar to enhance transparency and undergoes previous centrifugation to eliminate air bubbles.Subsequently, the authors incorporated the lens into an all-edible aperture made of red bean paste and agar [45].Another approach involves a lenticular lens whose complementary mold comprises a silicone resin stamped by a commercial device.Pouring the agar solution results in an array of lenses imprinted on the gel surface that project distinct images depending on the observation direction [45].
A recent work also proposed agar spheres working as ball lenses, as shown in Fig. 2(b).Drops of heated agar solution deposited on cooled vegetable oil solidify as gel spheres.A magnetic stirrer promotes constant circulation and adjusts the diameters according to the rotation speed.Preliminary experiments confirmed the ball lenses' reliability for imaging systems, besides anticipating prospective applications in environmental monitoring.Moreover, one may add glycerol to the raw solution for adjusting the lens refractive index, granting straightforward control over the focal length [46].Fig. 2(c) illustrates an edible retroreflector obtained by molding the agar with a commercial corner cube array.The device achieves high reflectance due to its smooth surface and refractive index homogeneity, comparable to its glass counterpart.Pursuing computer vision applications in aliments, the authors created a consumable fiducial marker for augmented reality [47].This inventive fabrication method is also compatible with other biodegradable materials such as gelatin, sodium alginate, and sugars [47,48].
Ultimately, researchers developed an agarose-made microfluidic device for cell growth and imaging.A multi-step masking process creates microscopic patterns over the gel surface to stimulate the preferential distribution of bacterial colonies and establish paths for transporting fluids.The authors demonstrated visualization timelapse tracking for single cells and colonies of microorganisms.For instance, agarose overcomes the limitations of traditional PDMS-based chips as a soft and absorbent material, besides owing remarkable transparency and non-auto-fluorescent behavior [49].

Slab waveguides
Slab waveguides are appealing for optical communications and sensing, presenting versatile design, large contact area, and ability to support evanescent or leaky modes [50].Several types of planar devices comprising hydrogels sandwiched between polymer or glass layers are available in the literature.Nevertheless, researchers pursue allbiodegradable, soft waveguides for prospective uses as implantable probes [2,5].
Despite the agar's noteworthy optical and mechanical characteristics, few works employ this material as a light-guiding medium in planar structures.Indeed, most setups comprise external agarose layers interacting with evanescent modes to detect humidity levels while the core modes travel through conventional substrates [51,52].
An example of a flat device comprises a gelatin core surrounded by agarose layers.Fabrication proceeds through sequential spin coating to achieve an assembly thickness ranging from millimeters to micrometers.The authors successfully coupled light from a 633 nm laser into the core and measured the fluorescence spectrum after doping the material with a marker.Apart from its potential to conceive implantable probes for biomedical applications [53], gelatin concentrations of ≥ 10 wt% are mandatory to ensure mechanically stable gels, which may impact the optical losses due to turbidity and light scattering [54].
Afterward, another group designed an all-agarose waveguide containing 130 µm × 130 µm square-core multimode structures integrated into a chip.PDMS molds manufactured by soft lithography stamp the melted agarose to define its geometry, followed by vacuum treatment to eliminate possible bubbles in the solidified gel.The authors adjusted the refractive indices of core and cladding regions by changing the agarose concentration, achieving an optical loss of ∼ 13 dB/cm at 633 nm.This device also exhibits extraordinary features like microfluidic channel integration and cell encapsulation capability, as validated by fluorescence microscopy analyses [55].

Optical fibers
Several groups reported biocompatible optical fibers comprising distinct materials, including silk fibroin, cellulose, and hydrogels [2,5].Despite the severe optical losses compared to silica waveguides, applications as resorbable and implantable probes for light therapy and intra-body surveillance motivate further developments in fiber structures and materials.
The no-core optical fiber of Fig. 3(a) presents a solid cylindrical structure enclosed in a fluid medium of lower refractive index to confine light.The procedure depicted in Fig. 4(a) involves aspiring melted agar solution inside the silicone tubing using a syringe.Once cooled in the refrigerator, the experimentalist releases the solidified gel from the mold by pushing the plunger, then cleaves the fiber end faces with a razor blade to improve the coupling efficiency [36].
Alternatively, researchers proposed a structured optical fiber owing a solid agar core surrounded by periodic air holes, creating an integrated core/cladding structure, as shown in Fig. 3(b).The fabrication involves supplying a glass tube with heated agar solution and inserting solid rods along the waveguide extent to establish air gaps.Subsequently, the experimentalist releases the fiber from the glass mold, removes the rods, Fig. 3 Agar-based optical fibers: (a) 2 wt% agar, 60 wt% glycerol no-core fiber with 2.5 mm diameter; (b) 2 wt% agar, 0 wt% glycerol structured fiber with core, cladding, and holes diameters of 0.6 mm, 2.5 mm, and 0.5 mm, respectively (reprinted from [31]).
Fig. 4 Fabrication of agar-based optical fibers: (a) silicone tubing filled with melted agar produces no-core fibers; (b) glass tube mold with stacked rods produces structured fibers with air holes surrounding the solid core (adapted from [31,40]).and cleaves the end faces according to Fig. 4(b) [31].Differently from standard cylindrical waveguides, light confinement in the structured fiber's core region results from the average refractive index difference between the solid agar core and the neighboring air holes.
The mold geometry defines the agar fiber diameter and length.Commercial silicone tubings are available to manufacture no-core waveguides with diameters below 1 mm [36].Regarding structured fibers, current work assumed a 0.6 mm core, 2.5 mm cladding, and 60 mm-long multimode design containing six holes of 0.5 mm [31].
Furthermore, one may adjust the refractive index according to the agar solution composition.Whilst setting the agarose content between 0.5 to 2 wt% is recommendable for mechanical stability management, adding glycerol or sugar improves transparency and enables fine refractive index control [31,36].
Cutback tests at 633 nm revealed optical losses ≤ 3.2 dB/cm for 2 wt% agar fibers [31], lower than those reported for square waveguides [55] yet not negligible.Transmission possibly degenerates due to scattering by air bubbles and inhomogeneities in the agar structure.Additional losses arise from coupling efficiency as the large agar core may mismatch the optical system.Nevertheless, incorporating 60 wt% of glycerol improved the optical loss to 0.81 dB/cm, corroborating the improved gel transparency [36].
The agar's singular properties encourage applications in optical fiber sensing.Apart from the widespread intensity and wavelength-based interrogation schemes, the multimode nature of agar fibers makes them appropriate for speckle pattern analyses.Fiber speckles are granular patterns produced by the interference of several guided modes, established in multimode waveguides illuminated by coherent light.Despite their apparent chaotic and random behavior, specklegrams encode detailed and consistent information about the fiber state [56].Since disturbances from physical or chemical stimuli modulate the speckle field by mode coupling and phase deviation effects [57], one may acquire the output specklegram and evaluate its spatiotemporal changes to predict the input variable.Notably, this approach demands a visible laser and a camera instead of expensive broadband sources and spectrometers, besides achieving distributed response and high sensitivity, being comparable to interferometric systems [58].
Mechanical measurements tested the response of a 2 wt% agar, 60 wt% glycerol fiber to the transverse static forces exerted by a load cell.A computer routine captures sequential images and evaluates the correlation coefficient to quantify the specklegram deviations [59], yielding an average sensitivity of 37.4 N −1 [60].Additional dynamic characterization via step response attained a time constant of 0.72 s [36].The glycerol content influences the fiber's stationary and transient behaviors because the gel becomes more deformable to increasing stresses.Indeed, a pure agar-water fiber is less sensitive to force and exhibits a delayed recovery time [36].Besides, one may optimize the agar viscosity to enhance the softness sensation desired for tactile sensing, for example.
Recalling syneresis, the agar fiber may release water droplets under ambient conditions, causing continuous speckle pattern changes viewable as reductions in the correlation coefficient even at undisturbed states.A chemical sensor exploits this phenomenon by dripping an analyte over the fiber surface and assessing the subsequent specklegram evolution [31].Unlike the observed for air and water, the correlation curve drastically decreases after wetting the fiber with acetone once this substance absorbs solvent molecules in the gel network [22].A more elaborate chemical sensing approach completes the structured fiber's holes with the fluid of interest.Depending on the refractive index differences between the core/cladding, hole, and the surrounding medium, guided light may escape from the core and illuminate its vicinities.Consequently, one may estimate the sample concentration by measuring the light intensity profile in each fiber region [31].As the sensitivity varies with the gel refractive index, experimentalists can choose the glycerol concentration to cover specific measurement ranges.
Ultimately, a recent paper demonstrated electric current sensing by perforating the agar fiber with pin header connectors and then applying a voltage drop in the illuminated waveguide.Mineral impurities in the gel composition establish the current flow, promoting localized heating and modulating the agar's refractive index by thermooptic effect.Such an aspect culminates in specklegram fluctuations that become more vigorous as the electric current magnitude increases [40].Based on the correlation curve's decrease rate, this sensor attained a practical resolution of 0.4 µA for stimuli ≤ 100 µA, matching bioelectrical signals standards [61].Moreover, results reveal an agreement between the direction of current flow and speckles displacement in the detection plane, i.e., the agar fiber retrieves both the magnitude and direction of the applied electrical stimulus.
Scientists also proposed agarose-based cylinders doped with hybrid silica-carbon fluorescent nanoparticles.The authors excite the device with UV light and acquire the output spectrum to evaluate the fluorescence peak shift as a function of the pH, which motivates possible applications as a growth medium with integrated chemical sensing capability [62].Another concept involves doping a poly(methyl methacrylate) (PMMA) microfiber with dissolved agarose powder and functionalizing the polymer waveguide as a humidity probe.The system employs evanescent coupling to detect the transmission spectrum and identify the measurand-driven wavelength shifts, though PMMA is not biodegradable like agar [63].

Optical fibers sensors based on agar transducers
Several setups employ agar coatings and substrates as transducers for glass and plastic optical fibers rather than conceiving all-gel-constituted waveguides.Humidity sensors rely on the agarose's volume enlargement due to the moisture amount as the gel network gradually swells water to restore the equilibrium between the coacervate and dilute phase [22].One also predicts subtle refractive index changes due to deviations in the water concentration, which suggests potential applications in chemical and temperature measurements.
Fiber Bragg gratings coated with agar solutions undergo bending due to the gel expansion, leading to variations in the grating periodicity that modulate the reflection spectrum [64].A similar approach considers long-period gratings wherein displacements in the inscribed patterns promote coupling between core and cladding modes, affecting the transmission spectrum [65].Moreover, researchers developed sensing elements that expose the core modes to the surrounding agar layer, including U-bent fibers [66], tapers [67], and multimodal interferometers with photonic crystal fiber segments [68].Differences in the gel's refractive index emerge as intensity modulation or wavelength shift.Alternatively, intercepting the single-mode/holey fiber assembly with an agarose layer generates spectral features once the coupling rate depends on the mode field diameter tuned by the gel's refractive index [69].
Recent papers proposed in-fiber Fabry-Pérot interferometers by coating the end face of a single-mode optical fiber with a few-micrometers agarose film.The phycocolloid acts as the resonance cavity, inducing phase shifts as its length or refractive index change.Then, the interrogator acquires the reflected light to quantify the ambient humidity or temperature [70,71].Furthermore, we attained improved sensitivity splicing a capillary fiber segment to the single-mode fiber probe, then supplying an agar film to create an air gap for resonance.This approach also extends its application to temperature and pressure detection besides humidity.We will detail such results in a forthcoming paper.
Another group created a micro-knot resonator covered with agarose film by exploring the Vernier effect to achieve enhanced relative humidity sensitivity.The knotted microfiber forms a compact Mach-Zehnder interferometer with traceable spectral signatures [72].Ultimately, a whispering gallery modes resonator comprising tapered fiber and glass sphere attains humidity response by coating the round surface with agarose, increasing its refractive index by water absorption.The authors also characterized the resonator's sensitivity and quality factor as a function of the gel layer's thickness and concentration [73].
Lastly, the agar's viscoelastic characteristics are valuable for conceiving tactile sensors.Glycerol-enhanced structures exhibit improved resistance to fracture stress besides soft touching feedback comparable to living tissues, as required for developing phantoms and artificial skins [14,74].The tactile device comprises a 50 mm × 50 mm × 10 mm agar block embedding a hetero-core optical fiber structure.The single-mode fiber probe section intercepts the multimode segments, exposing the core-guided modes to boost the sensitivity to mechanical perturbations.In this case, the soft agar pad supports incoming forces on its surface and spreads the resulting load over the fiber probe.Consequently, part of the exposed modes recouples to the output multimode fiber, generating a mechanically-modulated speckle pattern.Experiments with punctual loads actuating on a 2 wt% agar, 60 wt% glycerol transducer yielded a 6 × 10 −3 N resolution regarding forces ≤ 0.5 N, while dynamic calibration indicated a time constant of ≤ 45 ms.Moreover, this sensor can identify localized or distributed stimuli once specklegrams natively encode spatial information, providing a maximum resolution of 8 × 10 −3 mm [75].The experimentalist may change the glycerol concentration to tailor the gel substrate's elastic and viscous behaviors, endorsing potential applications in medical and virtual-reality setups as a versatile platform for emulating different types of tissues.

Challenges and opportunities for agar-based optical fibers
Fig. 5 summarizes the characteristics of selected biodegradable optical fibers.One observes improved transmission for agar-glycerol fibers (0.81 dB/cm at 663 nm), attaining losses comparable to PEG/alginate waveguides (≤ 1 dB/cm at 532 nm) [77].Despite the promising results, the overall performance of biodegradable devices cannot overcome standard silica single-mode fibers.The optical losses are probably due to light scattering in the gels' porous structure besides intrinsic material attenuation.Such losses become impeditive for agar devices exceeding tenths of centimeters, i.e., practical applications demand a launching fiber cable to illuminate the biocompatible probe.Nevertheless, further improvements in agar transmission are achievable by eliminating air bubbles during the gel formation via vacuum treatment or centrifugation [45].
Regarding the fiber dimensions, the mold characteristics constrain the diameter and length.Unlike the continuous fabrication of glass and plastic waveguides, the agar's fracture strain limit is incompatible with fiber drawing towers.This aspect may be troublesome in structured waveguide processing as the experimentalist must insert rods with the desired size instead of reducing their diameter by preform pulling.Disregarding the low cost and practicability of available agar molding setups, one envisages developments toward scaleup and fiber diameter reduction.Fortunately, multimode fibers with large cores are still suitable for illumination besides physical and biochemical sensing, as attained by the emerging specklegram analyses.
Interrogation setups for intra-body applications presume a fiber termination for light output rather than transmission probes linking source and detector.For instance, implantable sensors should operate in reflection mode to isolate the interrogator electronics from the assessed medium.Optical circulators may route the input/output signals, though improvements in the light coupling strategy are necessary to circumvent the core size mismatch between silica fiber and agar devices.Alternatively, light delivery setups are realizable by simply perforating the agar core with the launching fiber once it disregards optical readings.
Since agar releases water droplets and progressively shrinks at room conditions, the experimentalist must conserve the optical devices in water or use plastic films to avoid degradation, preferably in a cooled environment.Adding glycerol also delays agarose degeneration, extending the fiber lifetime to several weeks even under exposure to the atmosphere at ambient temperature.Yet, clinical studies focusing on the agar device digestion and absorption by the organism are still pending.
Apart from the remaining challenges, we prospect several opportunities for designing novel optical fibers and devices that explore the physical and chemical characteristics of this versatile material.For example, scientists may incorporate nanoparticles or other substances than sugar and glycerol to control the agar's properties or incorporate additional features regarding its mechanical, chemical, and optical behavior.Experiments on metallic coating and surface functionalization could endorse new applications for agarose gels in plasmonics and biochemical analyses.Exploiting the agar's moldability and flexibility also establishes paths for conceiving unconventional fiber structures, like holey, asymmetrical, and multi-core designs, to pursue sensitive and robust sensors.Alternatively, combining agar with glasses, polymers, and hydrogels may alleviate the optical loss constraints.Moreover, one may implement microfluidic channels in the fiber holes or use the agar core as a growth medium for microorganisms, providing real-time optical surveillance of its biochemical parameters.Ultimately, we anticipate active optical devices based on the mechanical or electrical modulation of the transmitted light for beam shaping and mode control.

Conclusion
Agar is a biocompatible, edible, and renewable substance with noteworthy features like thermo-reversibility, chemical and mechanical stability, and transparency.Current works encompass several examples of optical devices manufactured with this promising material, including lenses, slab waveguides, and optical fibers.The latter exhibits overall performance comparable to other gel-based waveguides besides exceptional physical and biochemical sensing capabilities.Furthermore, modifying agar properties like rigidity, viscosity, transparency, refractive index, and electrical conductivity is straightforward once such characteristics depend on the gel composition.
Apart from the successful and established uses in culinary, growth media, biochemical extraction and analyses, textural studies, artificial skins, and tissue engineering, we emphasize the agar's capabilities to conceive innovative optical technologies to fulfill the imminent demand for biocompatible devices in imaging, phototherapy, optogenetics, and sensing.Indeed, several studies on agar properties and creative applications arose since the classical papers detailing the agarose's helical structure.Such new perspectives for this popular material anticipate forthcoming developments toward soft, bioresorbable optical fibers and implantable devices devoted to biomedical and environmental surveillance.
Authors contributions.Study conception and design were performed by all authors.Eric Fujiwara conducted the material preparation, data collection, and analysis.The first draft of the manuscript was written by Eric Fujiwara and all authors commented on previous versions of the manuscript.All authors read and approved the final manuscript.