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Biodegradable force sensors are a specific type of technology designed to be environmentally friendly and capable of naturally degrading over time. These sensors are typically made from biocompatible and biodegradable materials, allowing them to break down and dissolve into nontoxic components when exposed to specific environmental conditions. Flexible biodegradable force sensors find applications in various fields where temporary force monitoring or sensing is needed, for example, in biomedicine, environmental monitoring, or agricultural applications. Like most flexible sensors, they can also be used in medical implants or wearable devices to monitor the forces applied during rehabilitation or assistive activities.
- Biodegradable force sensors
- sensors
- Carrageenan
1. Introduction
Flexible force or pressure sensors, known from 1900 [1], have currently become among the most demanded electronic components. The wide variety in their typical application fields set specific requirements for such sensors: they must be functional, inexpensive, meet environmental protection requirements, and be harmless to humans. Flexible force sensors are required to quantify the applied force or pressure, efficiently converting the mechanical deformation into an electrical signal [2]. The flexibility of the force sensor allows them to conform to irregular surfaces, allowing more accurate and precise force measurements [3]. Flexible force sensors typically consist of a flexible substrate material, such as polyimide, silicone, etc., that serves as the base for the sensing elements and conductive materials, such as carbon or silver particles [4]. The specific sensing mechanism may vary depending on the design, but common approaches include piezoresistive, capacitive, or optical sensing principles [5,6,7].
The operating range of force sensors generally depends on the properties of the material used and the mechanical design of the sensor [8]. A well-known example of such a sensor is a force-sensitive resistor (FSR) [9]. With the emergence of flexible force sensors, some of the FSR problems were solved, but new challenges were raised, and the best combination of substrate and conductive materials can still be improved. However, gel-based sensors typically suffer from significant hysteresis and sensor sensitivity variation with respect to time or changes in the environmental conditions. Therefore, the main challenge parameter for the piezoresistive flexible force sensors is repeatability and linearity, which could be ensured by improving the characteristics of the sensitive material or implementing smart signal acquisition [10] and processing techniques. Multi-criteria research on flexible force sensors leads to various unexpected solutions, such as the construction of flexible force sensors on a piece of paper [11].
2. Carrageenan in the Development of Biodegradable Force Sensors
2.1. Biodegradability of Biosensor Materials
It is worth noting that the research and development of biodegradable force sensors is ongoing, and advancements in materials science and sensor technology continue to improve their performance, sensitivity, and degradation properties. Reviewing the most exciting solutions for biodegradable polymer-based force sensors, the work of Yu et al. [17] is worth mentioning. Researchers have developed biodegradable force sensors using polymer-based materials, such as polylactic acid (PLA) or polyhydroxyalkanoates (PHA). These sensors can be integrated into medical implants or wearable devices to monitor the forces exerted during rehabilitation exercises or assistive activities. Ma et al. [18] presented another eco-friendly force sensor solution for temporary force monitoring based on the use of starch—a renewable and biodegradable material developed by incorporating conductive materials within a starch matrix.
The sustainable use of cellulose or cellulose-nanofiber-based materials for sensors is also an essential issue; for example, Su et al. [19] proposed that cellulose nanofibers derived from sustainable sources such as wood or plants could be used to produce biodegradable force sensors. These sensors exhibit good mechanical properties and can measure forces in various applications, including environmental monitoring or agricultural settings.
Silk-based force sensors are also considered a sustainable solution [20,21]. This natural protein fiber is suitable for the development of biodegradable force sensors, which can be implanted or integrated into medical devices to measure forces in biomedical applications. These sensors are biocompatible and gradually degrade with time.
Biodegradable gelatin, derived from collagen, has been investigated for creating a biodegradable force sensor by Wang et al. [22]. The authors defined that the incorporation of conductive materials or microstructures within gelatin matrices makes it possible to develop sensors to monitor forces in applications such as tissue engineering or drug delivery systems.
Generally, the use of biodegradable force sensors offers several advantages. First, they minimize environmental impact by reducing the accumulation of non-biodegradable waste. Second, they eliminate the need for sensor retrieval or removal after use, as they naturally degrade over time. This feature is particularly beneficial in applications where the sensors are implanted or deployed in hard-to-reach or sensitive environments. Finally, biodegradable sensors can provide real-time force data during their functional lifespan, enabling the monitoring and analysis of force-related parameters.
2.2. Application of Seaweed-Derived Carrageenan in the Development of Biodegradable Force Sensors and Combination with Metal Salts and Metal Oxides
Red algae are photosynthetic eukaryotes with an extracellular matrix consisting of a complex of supramolecular networks connecting cells. The structure of the extracellular complex depends on the algae species and the life-cycle stage. The main components of the extracellular complex are sulfated galactans such as agars, porphyrins, and carrageenans [23]. Thus, polysaccharides retain water and sometimes work as phycocolloids with gelling and viscosity properties depending on the biopolymer’s structural modifications. Gel formation and thermo-versability are the most critical features of carrageenan. Like all biopolymers, carrageenan undergoes aging and syneresis. Isopropyl alcohol or 2-propyl alcohol is used to precipitate carrageenan from the extraction liquid. Therefore, 2-propanol can be observed in the final samples produced, where this material acts as a surfactant [24].
Carrageenan [25] is a unit of naturally derived polysaccharides extracted from algae. Using established procedures for the acquisition of the carrageenan biopolymer, after algae extraction, polysaccharides are precipitated using ethanol or isopropanol [26,27]. In this way, the seaweed Furcellaria lumbricalis can be successfully used as a source for the development of a biodegradable force sensor matrix, in which electrically conductive elements (particles) are encapsulated at evenly distributed distances and approach each other corresponding to the applied force, thus, changing the electrical resistance [28,29].
Therefore, carrageenan, which has a history of use for human purposes, was rediscovered in sensing technologies when the application of rubber-like materials and gels in sensor production became available. In general, conductive hydrogels, as a separate class that changes their electrical properties in response to the applied force, are attractive due to their self-healing, good conductivity, and flexibility [30].
The chemical structure of carrageenan located in the outer cell wall and the intracellular matrix of algae tissue [31] depends on the species of red algae and extraction methods. Carrageenans are linear anionic sulfated polysaccharides or sulfate galactose biopolymers composed of galactose and anhydrous-galactose and are divided into types according to repeating disaccharide units [32]. The major subtypes of carrageenan are kappa, iota, and lambda. Kappa or K-carrageenan forms strong gels, iota or i-carrageenan forms soft gels, and lambda carrageenan does not make a gel. Carrageenan subtypes differ in the number of sulfate groups in the hexose scaffold skeleton of plant molecules and contain some negatively charged sulfate ester groups per disaccharide unit [33].
In some cases, the extraction result may be kappa–iota hybrids of carrageenan. As a hydrophilic colloid, K-carrageenan is popular as a gelling agent in many manufacturing fields and has thermal response behavior as a sol-gel transition and gelation ability [34].
As a biopolymer, carrageenan carries out such properties as an antioxidant, antibacterial agent, anti-coagulant, and immune modulator [33]. The gelling properties of carrageenans depend on the structure of its molecule. One way to gelatinize is to apply polyvinyl alcohol (PVA) to provide the elasticity of the carrageenan biopolymer [35]. As an anionic polysaccharide, carrageenan can absorb the surface lipid droplets at pH3 to pH6 better than other marine polysaccharides. Emulsions of carrageenan with NaCl are stable, have high viscosity, and are applicable for different purposes [36].
The structural properties of carrageenan gels can be tuned by changing the salt concentration by adding KCl and CaCl2 to kappa and iota carrageenans, and their mixtures [37]. Different concentrations of salts are necessary for the gelation of ι-carrageenan, κ-carrageenan, and Ca2+ ions, causing the stiffer k-carrageenan network induced by K+ ion [37]. Morris E.R. [38] investigated the influence on carrageenan and reported that the presence of K+, Rg+, Cs+, and high concentrations of Na+ influences gel formation. Smidsrod also investigated the gelling mechanism [39]. The elastic modulus of κ-carrageenan gels with Ca2+ was investigated by P. MacArtain [40].
As is known, all polysaccharides as anionic biopolymers interact with water via hydrogen bonding. This means that in a biopolymer, the solvent works only on a hydrogen-bonded network, and this network retains unpolymerized molecules in its structure. The Flory–Huggins (FH) [41] theory describes the behavior of polysaccharides, two-phase systems, solvent separation, etc. Many models in the literature are dedicated to defining the elastic pressure of biopolymers, which, together with osmotic pressure and swelling ratios [42] defining the strength and stability, are named as the main characteristics of such materials. The uniaxial compression can be described by the theory of non-linear elasticity [43,44].
Due to many variations in the structure of carrageenan and its combination with other materials, many reports have been dedicated to carrageenan application in sensors. Tao [45] reported on the tactile biopolymer-based sensor made of carrageenan cross-linked with polyacrylic acid (PAM) as a pyramidal biopolymer with good mechanical properties. Adding metals to biopolymers containing PAM chains enhances the mechanical properties of the biopolymer. Qiang Zheng [46,47] reported about a κ-carrageenan/polyacrylamide biopolymer with remarkable mechanical performance as a strain sensor and good stability by using Zr4+ ions, cross-linked with PAM. The k-carrageenan with the nano-TiO2–anthocyanin layer is an indicator. On the contrary, the TiO2–agar layer works due to the strong adhesion between these two hydrophilic colloid layers [48].
Zhifeng Pan proposed stretchable sensors based on carrageenan for self-healing carrageenan-based sensors [49]. C.M. Costa [50] announced a 3D-printed carrageenan-based sensing device with multi-wall carbon nanotubes. M.M. Khodaei [51] presented a compound based on carrageenan–metformin nanoparticles produced using Fe3O4 magnetic particles. They mentioned carrageenan as an excellent material for sensors. Youshiyuki Nishio investigated the nano incorporation of iron oxides as magnetic compounds [52]. Chang [53] presented the successful application of carrageenan as a resistive switching layer for resistive memory devices. It is necessary to note that the composition of carrageenan with Fe2O3 nanoparticles was only mentioned as a gas sensor for medical purposes and investigated by analyzing carrageenan as a biological study object [54].
Marine polysaccharides have wide applications in industry, and there are reports of various development routes for this biopolymer. The transformation of the enchanted copolymerization of the K-carrageenan structure by microwave irradiation in aqueous medium results in polymers with enchanted characteristics [38,55]. Carrageenan is commonly applied as an emulsifier, stabilizer, thickener, and gelation agent in cosmetics and foods around the world. However, in the European Union (EU), its use in the food industry is restricted by legal regulations. However, carrageenan was treated as an antiviral material that could be used for medical purposes [33]. The antivirus feature of carrageenans is useful in designing products for natural environments.
This entry is adapted from the peer-reviewed paper 10.3390/s23239423
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