Challenges in Adapting Fiber Optic Sensors for Biomedical Applications

1. Introduction

As sensor technology continues to advance, Fiber Optic Sensors (FOSs) have gained prominence for their accuracy, high sensitivity, and resistance to electromagnetic interference. These characteristics make them particularly attractive for biomedical applications, where accurate and reliable measurements are crucial. From monitoring physiological parameters to aiding in minimally invasive surgeries, fibre optic sensors hold the potential to revolutionize medical diagnostics and treatment [1,2].

Despite their promise, the adoption of fibre optic sensors in the biomedical field requires several obstacles to overcome. The integration of these sensors into medical devices requires overcoming challenges related to biocompatibility, miniaturization, and effective an efficient signal processing. Additionally, ensuring the robustness, real time responsivity and reliability of FOSs in the dynamic and often harsh environments of the human body presents another layer of complexity. Also, their use requires meeting International Standards which can also be challenging in the pathway to commercialisation [3,4,5].

This article investigates the specific challenges encountered in the adoption of fibre optic sensors for biomedical applications. It examines the challenges associated with FOS implementation, with a focus on different medical standards, assessing their limitations and discussing key physical and biochemical measurands relevant to biomedical applications.

1.1. Background

FOSs operate by transmitting light through optical fibers, where changes in properties such as intensity, phase, or wavelength indicate specific physiological conditions. In the biomedical field, FOSs enable real-time monitoring of vital signs, including pressure and temperature, facilitate biochemical detection, and support minimally invasive surgical procedures. Despite their advantages, several challenges hinder their widespread adoption in healthcare. Key issues include ensuring biocompatibility, scaling up manufacturing to meet industry standards, and developing efficient real-time signal processing algorithms. Overcoming these barriers is critical for integrating FOS technology into routine medical diagnostics and treatment, where it holds the potential to enhance accuracy and reliability in patient care [6,7].

Polymer-based (or plasic) optical fibre sensors (POFSs) represent a significant advancement in FOS technology, offering flexible and adaptable sensing solutions. These sensors can operate based on various principles, including waveguide-based [8,9] mechanisms, luminescence [10], surface plasmon resonance (SPR) [11], and optical fiber-based sensing, and they are suitable for both single-point and multi-point applications [12,13,14,15,16]. A particular focus is placed on polymer optical fiber sensors due to their applications in biosensing. The use of polymeric materials, such as biodegradable, biocompatible, hydrophilic, stimuli-responsive, conductive, and molecularly imprinted polymers, further enhances biomedical sensing capabilities. Additionally, various fabrication techniques for polymer optical fibers (POFs), including thermal drawing, extrusion, laser writing, microfabrication, and advanced coatings are included in Figure 1 as a classification on different types of fibers and fabrication techniques [1,17,18].

1.2. Working Principles of FOSs

FOSs vary widely in terms of their physical appearance and characteristics and their operating principles are correspondingly diverse. The primary FOS classification splits into intrinsic and extrinsic sensors [12]. Intrinsic FOSs rely on the light-matter interaction occurring wholly within the fibre itself, where changes in light properties (intensity, phase, polarization, or wavelength) occur due to external influences such as temperature, pressure, or strain. Extrinsic fibre optic sensors, on the other hand, use the optical fibre merely to transmit light to and from an external sensing element. Fiber Bragg Gratings (FBGs) [20,21] have represented a revolution in sensing using optical fibres. They operate by reflecting specific wavelengths of light that shift in response to strain or temperature changes. Interferometric sensors (of which the FBG is an example), generally measure phase changes in light caused by external perturbations [3,22,23]. The FBGs interaction with light as it passes through the grating planes depends on the Bragg condition and the first-order Bragg condition can be stated as below:

$${\lambda }_B=2n_{eff}\mathsf{\Lambda }$$

(1)

where$n_{eff}$denotes the effective refractive index, while $\mathsf{\Lambda }$ represents the grating period, and ${\lambda }_B$stands for the Bragg wavelength.

There exist several different types of FOSs and Polymer-based optical fibre (POF) sensors based on different working principles which cannot be included in this article owing to page length restrictions. e.g., fluorescence-based sensors [24,25] detect variations in fluorescence emitted by certain materials subject to external light excitation, and are often used for biochemical measurements. Other types of FOSs include interferometric-based devices such as Fabry-Perot [22,26], Surface Plasmon Resonance (SPR) [27,28], Optical Coherence Tomography (OCT) [29,30] utilized in imaging applications and often incorporate specialist fibre e.g., Photonic Crystal Fiber (PCF) [24,31]. They all leverage the interaction between light and the external environment to provide sensitive and accurate measurements, in a wide range applications [23,32,33,34,35]. They can be categorized in various ways, including fibre type, application, and sensing mechanisms, as outlined in Table 1.

Also, fiber optic sensors (FOSs) can be classified based on their measurands into physical and biochemical types. Physical FOSs detect parameters including pressure, temperature, and strain by analyzing optical signal changes. Biochemical FOSs identify specific analytes, such as glucose or pH, e.g., using functionalized coatings. This classification highlights their versatility in biomedical and industrial applications. This review focuses on their applications in biomedical sensing.

1.3. Physical Measurands in Healthcare

FOSs are well suited for providing measurements of various physical measurands and are becoming increasingly accepted for patient monitoring and diagnostics. These measurands include temperature, pressure, strain, flow, liquid level, displacement, vibration, rotation, radiation, and biochemical markers. For example body temperature measurements are vital for tracking temperature fluctuations during surgeries, post-operative care, and critical care settings, ensuring patient stability and the early detection of infections [53,54]. Pressure sensors are extensively used to monitor several medical pressure parameters including blood pressure, Intracranial Pressure (ICP) [55,56], and Intraocular Pressure (IOP) [57,58], which are critical for managing conditions including hypertension, Traumatic Brain Injuries (TBI) [59], and glaucoma [58] respectively. Strain sensors are employed to monitor respiration by measuring chest wall movements, offering valuable data for respiratory therapy [3], sleep studies, and managing conditions including asthma or chronic obstructive pulmonary disease (COPD) [4,60]. By providing real-time data, fibre optic sensors offer enhanced clinical decision-making, improved patient outcomes, and contribute to the advancement of personalized medicine [61,62].

Biomechanical measurands encompass the physical parameters of the human body where the focus is on physical structure and movement, and accurate measurement is crucial for various successful monitoring. FOSs can measure strain and deformation in tissues and organs, providing critical data for orthopaedic and rehabilitation applications [1,2]. For instance, it is possible to monitor the stress and strain on bones and joints during physical activities, aiding in the assessment and treatment of musculoskeletal disorders. Additionally, they have been used for posture monitoring and ulcer formation detection in patients who are required to use a wheelchair [63]. Furthermore, these sensors are employed in the development of prosthetics and wearable devices, providing real-time feedback on the mechanical performance and interaction with the body. By measuring these biomechanical parameters, FOSs support the diagnosis, treatment, and rehabilitation of various conditions and advancing the field of biomechanics in healthcare [4,5,64,65,66,67].

1.4. Biochemical Measurands in Healthcare

All biochemical measurands could be considered vital parameters in healthcare, providing essential information about the whole physiological and metabolic states of patients. FOSs are increasingly used to measure these biochemical markers with high sensitivity and specificity. One significant application of FOSs is in continuous glucose monitoring (CGM), particularly for diabetes management. These sensors can measure glucose levels in blood or interstitial fluid, providing real-time data that aids in maintaining optimal glycemic control [68,69,70,71,72]. pH monitoring is used in assessing metabolic conditions and the body’s acid-base balance, which is vital in some critical care setting and surgical procedures [73,74,75,76]. Additionally, fibre optic sensors are used for multiple blood component detection [69], to detect specific proteins [77,78], enzymes [71,79,80], and hormones [31,81,82], aiding in the diagnosis and monitoring of various diseases, such as cancer [83,84,85] and hormonal imbalances [32,86]. These sensors can be functionalized to detect biomarkers at the molecular level, enabling early disease detection and the tracking of treatment efficacy. By measuring these biochemical parameters, fibre optic sensors provide data that present real time monitoring opportunities, enhancing diagnostic accuracy and treatment monitoring which are directly related to biochemical measurands [32,87,88].

FOSs are adept at detecting various substances in both gas and liquid phases. In the gas phase, FOSs can detect a wide range of gases including oxygen, carbon dioxide, and Volatile Organic Compounds (VOCs) [89,90,91,92] with high sensitivity. In respiratory monitoring [60,93] and detecting gases in environmental health studies. In the liquid phase, FOSs are widely used for measuring biochemical substances, such as glucose, electrolytes, and pH levels in bodily fluids like blood, urine, and saliva [86,94,95,96]. This capability is essential for continuous glucose monitoring in diabetic patients and assessing kidney function or urinary protein [95,97,98]. Accurate, real-time measurements can be achieved using different sensing mechanisms, based on different responsive materials. These sensors also provide access to a vast range of use cases as described in Table 2 [24,99,100]. Table 2 provides an overview of recently developed FOSs designed for pH measurement and glucose detection, highlighting their fibre type and detection range with the responsive material.

FOSs have also demonstrated potential in the monitoring of glucagon-like peptide-1 (GLP-1), a critical biomarker in glucose metabolism and insulin regulation. The ability to continuously monitor GLP-1 levels offers significant advantages in the management of diabetes and other metabolic disorders. A genetically encoded sensor (GLPLight1) has been developed by engineering a circularly permuted green fluorescent protein into the human GLP-1 receptor (GLP1R), Figure 2. This sensor accurately detects receptor conformational activation in response to pharmacological ligands, as indicated by its fluorescence signal. [111,112].

Ensuring accurate parameter values is paramount in sensor technology, particularly in the case of fiber optic sensors for biomedical applications. A comprehensive approach to achieving high accuracy must span the entire lifecycle of sensor development and deployment as shown in the example step graph of Figure3. This entails fundamental materials research during the design phase to comprehend material properties, transduction mechanisms, and device physics, ultimately leading to optimized materials and device structures. Iteration between scientific inquiry and engineering optimization enhances sensor accuracy, alongside considering factors such as the 3S’s (size, speed, and sensitivity).

Fabricated device accuracy and consistency is pivotal in moving towards manufacturability and deployment. Additionally, large-scale validation employing standardized procedures and benchmarking against gold-standard measurements are indispensable for obtaining reliable calibration curves [114].

2. Challenges for FOSs in Biomedical Applications

The development of biodegradable and biocompatible optical fibres for biomedical applications presents several key challenges. Firstly, material selection is critical, as the fibres must degrade safely in the body without releasing toxic byproducts. Natural or synthetic polymers, such as polylactic acid (PLA) or phosphate-based glasses, must be carefully evaluated for their mechanical properties and degradation rates to match specific biomedical requirements. Secondly, fabrication techniques such as thermal drawing, extrusion, or 3D printing must be optimized to ensure that the fibres maintain structural integrity during use while also being scalable for clinical applications. Thirdly, fibre design plays a significant role; features like microstructured or step-index profiles must balance light-guiding efficiency with mechanical flexibility and controlled biodegradability. Finally, application-specific considerations must be addressed, such as ensuring effective light delivery in photodynamic therapy or accurate biosensing under dynamic physiological conditions. PLA-based fibres have shown promise due to their adjustable degradation rates and biocompatibility. However, challenges such as mechanical fragility and inconsistent degradation in physiological environments necessitate further research. Similarly, phosphate-based glass fibres are attractive for their complete dissolution in biological fluids, but optimizing their mechanical properties without compromising their optical performance remains a significant challenge. These challenges highlight the need for continued advancements in material science and fibre design to achieve reliable, biodegradable, and biocompatible optical fibres for diverse biomedical applications [115]. Figure 4 illustrates a schematic representation of the four key challenges encountered in the development of biodegradable and biocompatible optical fibres.

2.1. Biocompatibility

Ensuring non-toxicity for any material to be used in fabricating sensors is one of the primary challenges to achieving biocompatibility. The selection of materials needs to ensure that the FOSs are mechanically safe, non-toxic and do not provoke an immune response when implanted in or used on the human body. This requires extensive testing to ensure that the materials do not cause inflammation, allergic reactions, or any other adverse effects [5,116]. This is generally conducted by regulating bodies such as the Food and Drug Administration (FDA) in the USA (section 2.5 of this article).

FOSs often require coatings to enhance biocompatibility. These coatings must be able to withstand the harsh biological environment without degrading. Ensuring long-term stability and functionality of these coatings is crucial for the reliable performance and regulatory approval of the sensors. Furthermore, sensors often need to endure various sterilization methods such as autoclaving, gamma irradiation, or chemical sterilization. A major challenge remains to design sensors that maintain their performance characteristics post-sterilization, as these processes can sometimes compromise sensor integrity and functionality [20,117]. Table 3 presents an overview of biomaterials used in the fabrication of optical fibres, highlighting their base material, advantages and disadvantages based on literature.

2.2. Miniaturization, Durability and Longevity

Reducing the size of fibre optic sensors without sacrificing sensitivity or accuracy is an ongoing significant challenge. Miniaturized sensors must still be capable of delivering accurate and reliable measurements. Also, small-sized sensors need to be seamlessly integrated into medical devices and systems, often requiring custom design solutions. This integration must ensure that the sensors do not interfere with the overall device performance and that they are easy to incorporate into existing medical infrastructure [87,131].

Producing miniaturized sensors at scale while maintaining high quality and consistency presents additional manufacturing challenges. Advanced fabrication techniques and stringent quality control measures are necessary to address these issues. FOSs often need to maintain their functionality over extended periods, particularly in chronic disease management and long-term monitoring applications. This requires materials and designs that are resistant to degradation over time [6,98,132].

Sensors must be able to withstand the dynamic and often harsh conditions encountered within the human body, such as movement, pressure changes, and exposure to various bodily fluids. Ensuring mechanical robustness while maintaining sensor sensitivity is a critical challenge [18,133].

Ideally, fibre optic sensors used in medical applications should require minimal maintenance. Developing sensors that can operate reliably over long periods without the need for frequent recalibration or replacement is essential for their practical use [134,135]. Also, it is highly advantageous if the sensor can be delivered inside a standard medical catheter as this often overcomes the problem of mechanical robustness [136,137,138].

2.3. Signal Processing, Data Integration, and Interoperability

Biological environments are often inherently noisy e.g., external electromagnetic interference from scanning equipment (MRI and CT), which can interfere with the signals detected by fibre optic sensors. If it is not possible to make the sensor immune to these sources of interference, developing advanced algorithms and signal processing techniques to filter out this noise becomes crucial for accurate measurements. The signals from FOSs often require sophisticated interpretation, especially when monitoring dynamic biological processes. This necessitates the development of advanced computational models and machine learning algorithms to accurately analyse and interpret the data [139,140,141].

Many types of medical applications require real-time data processing and feedback. Ensuring that the sensor systems can handle the computational load and providing timely, accurate information is a significant and ongoing technical challenge. FOSs must be compatible with existing healthcare IT systems and electronic health records (EHRs). Ensuring seamless integration and data interoperability is therefore essential for effective use in clinical settings [142,143].

Developing standardized data protocols to ensure that data from fibre optic sensors can be easily shared and interpreted across different platforms and systems is crucial. This includes ensuring sound data security and maintaining patient privacy. Finally, at this stage providing user-friendly interfaces that allow healthcare professionals to easily interact with and interpret data from fibre optic sensors is important for their adoption. This involves developing intuitive software and visualization tools [87,132,144].

2.4. Production Cost and Manufacturing

High production costs can be a barrier to the widespread adoption of fibre optic sensors. Developing cost-effective manufacturing processes without compromising quality and performance is crucial for making the sensors affordable [37,88]. Additionally, it may be necessary to accommodate further scaling up of production while maintaining consistency and reliability. The latter requires advanced manufacturing techniques and stringent quality control measures to ensure that each sensor meets the required standards. Finally, achieving economies of scale to achieve lower costs involves not only improving manufacturing processes but also increasing market demand and production volumes. This can too be challenging in the early stages of technology adoption [145,146].

2.5. Medical Standards and Regulatory Approval

Medical and/or biomedical FOSs must meet stringent regulatory standards set by organizations such as the FDA (Food and Drug Administration) and EMA (European Medicines Agency) [147,148,149,150,151,152]. This involves extensive testing to demonstrate safety, efficacy, and reliability.

Ensuring compliance with international standards for medical devices is critical. This includes adhering to ANSI (American National Standards Institute) /AAMI (Association for the Advancement of Medical Instrumentation) or ISO standards such as AAMI/ISO 10993, ISO 13485, AAMI TIR42: Technical Information Report (TIR), not a formal standard, but rather guidance for evaluating biocompatibility in alignment with ISO 10993. It helps manufacturers interpret biocompatibility requirements for regulatory compliance. [147,153,154,155].

Also, regulatory frameworks governing medical devices within the European Union include the EU Medical Devices Regulation (MDR) and the In Vitro Diagnostic Regulation (IVDR) [150,151,153]. The process of obtaining regulatory approval can be lengthy and complex, requiring a long lead time and resources. This can delay the introduction of new fibre optic sensor technologies to the market and their wider application. Table 4 outlines the regulatory standards governing the use of fibre optic sensors in medical applications.

3. Conclusions

FOSs have been transformative in healthcare, offering high accuracy and versatility in monitoring physiological and biochemical parameters including temperature, pressure, strain, and biochemical markers, enhancing diagnostics and patient outcomes. However, the widespread adoption of FOS technology in healthcare faces several critical challenges, including biocompatibility, miniaturization, durability, and robust signal processing. Addressing these challenges requires interdisciplinary collaboration across materials science, engineering, and medical practice to develop reliable, scalable, and clinically viable sensor systems.

Innovative advancements in biocompatible materials, fabrication techniques, and signal processing algorithms continue to push the boundaries of what FOSs can achieve in medicine. Standardization and regulatory approval remain key hurdles that must be overcome to facilitate their transition from research laboratories to commercial medical devices. Future research efforts should focus on enhancing sensor integration within existing medical systems, improving long-term reliability, and developing AI-driven analytical methods for accurate data interpretation.

By overcoming these challenges, fibre optic sensors have the potential to revolutionize biomedical sensing, paving the way for more precise diagnostics, personalized treatment plans, and improved patient outcomes. As the field advances, the synergy between optical sensor technology and emerging biomedical innovations will shape the future of healthcare, making real-time, minimally invasive, and monitoring an integral part of medical practice.