A New Design Approach: Applying Optical Fiber Sensing to 3D-Printed Structures to Make Furniture Intelligent

: In the context of sustainability, the development of optical ﬁber sensing technology and 3D printing technology brings new sustainable manufacturing solutions for the furniture industry. Based on the current status and development situation of the application of optical ﬁber sensing technology and 3D-printed furniture, this paper proposes the concept of applying embedded optical ﬁber sensing technology to traditional furniture manufacturing as the intersection of traditional furniture design and 3D printing technology. This design method is applied in furniture design cases, the stability of the 3D-printed structure after the optical ﬁber is embedded in the structure is veriﬁed through experiments, and the integration of the embedded optical ﬁber light-inducing and monitoring technology is used to assess the intelligence of furniture. It is found that by applying optical ﬁber sensors to furniture intelligence, the real-time monitoring of the environment, temperature, humidity, and other parameters during the use of furniture can be achieved, thus improving the energy efﬁciency and comfort of furniture. This innovative design idea and method provides a new direction for the sustainable development of furniture products and encourages the furniture industry to move forward in a more environmentally friendly and intelligent direction.


Introduction
After nearly 40 years of development, the Chinese furniture industry has attracted much attention from the world furniture industry [1].In recent years, China's furniture product manufacturing technology has achieved rapid development and a swift progression of intelligent manufacturing driven by the level of science and technology, gradually changing from traditional manual manufacturing technology to information and digital manufacturing technology [2,3].China's intelligent furniture ushered in a new era of data-driven, artificial intelligence and other diverse developments.The emergence of each new technology will inevitably play a role in promoting the original production technology to a certain extent.As a member of the manufacturing industry, furniture products' design and production will also inevitably be affected by new technologies, materials and ideas [4].
People have a specific predilection for wooden materials because of the inherent natural qualities of wood, and as a result, wooden furniture has grown to be a significant and necessary component of the furniture industry.The production of traditional Chinese solid wood furniture is usually based on mortise and tenon construction, which involves processes such as cutting, processing, polishing, assembling, and painting; the process of making mortise and tenon construction is complex, and requires a high degree of precision [5].The traditional processing of Chinese furniture has not only limited the mass production of furniture, but there also has yet to be a breakthrough in terms of furniture modeling.With the development of 3D printing technology, more and more furniture designers are participating in this trend, and the application of 3D printing technology in the field of furniture is becoming more and more widespread.The effective combination of traditional furniture design and manufacturing methods with 3D printing technology allows the advantages of 3D printing technology, such as personalization, high precision, and rapid prototyping, to be incorporated into furniture design and manufacturing practices to meet user customization requirements better [6].
Intelligent furniture and 3D-printed furniture are well-developed in their respective fields, but how to make 3D-printed furniture intelligent is an unsolved problem.We propose the design concept of embedding optical fiber sensors into furniture through 3D printing, aiming to give traditional furniture a new intelligent function through the integration of technologies from different disciplines, using the advantages of 3D printing technology in complex forming and rapid forming, as well as personalization and high precision, and using embedded system technology and sensor technology to integrate 3D-printed parts with traditional furniture design and manufacturing in an organic way.It can improve the shape adaptability of innovative furniture products in terms of embedding hardware devices, realizing the integration and interaction of different intelligent functions, and designing innovative furniture that is in line with the development of the times, which not only facilitates production, but also facilitates the replacement of subsequent parts.The combination of new and traditional technologies also allows furniture to break away from traditional modeling theories and is of great relevance to the development of the furniture manufacturing industry.

Applications of 3D-Printed Furniture
3D printing technology, also known as additive manufacturing, is a technology that uses liquid or solid plastics, metals, and other highly viscous, malleable materials to create three-dimensional solid models that are consistent with virtual models by stacking and bonding them layer by layer [7].It is a mature, high-tech technology that incorporates a variety of cutting-edge technologies, such as 3D modeling, new material technologies, and automatic control technology [8].Since the emergence of 3D printing technology in the late 1970s, it has changed the way factories produce, and has brought about new changes in the furniture manufacturing process with its technological uniqueness and creativity [9].Kang [10] analyzed examples of 3D-printed furniture and lamps according to three categories, and proposed five points on the competitiveness of 3D printing technology in the furniture design industry.Feng and Wu [11] introduced the history of the development of 3D printing technology and the principles, advantages, and disadvantages of the five types of 3D printing technology, summarized its use in household products, and analyzed the development trend of 3D-printed household products.Svoboda et al. [12] presented the principles of 3D printing technology and showed several examples of furniture applications using additive manufacturing (3D printing technology-FDM method) in the university teaching process.
In addition to 3D-printed furniture, there is also a form of 3D-printed connectors, where panels or sticks are used as the raw material for furniture.3D-printed connectors are used to conveniently assemble and disassemble furniture for transport and end-of-cycle recycling, allowing users to assemble individual, multiform furniture shapes according to their needs.The Hungarian designer Olle Gellert has designed a 3D-printed connector for panel furniture called 'Print to Build', using polyethylene material.The emergence of the 'Print to Build' connector has changed how people perceive furniture connectors [13].They are no longer limited to the traditional hardware connectors and furniture production methods, but are a fundamental component in the design and assembly of the whole piece of furniture.Aiman et al. [14] used ABAQUS CAE software to analyze the structure of 3D-printed connectors for modular furniture.They concluded that furniture made from plywood assembled with connectors made by the FDM process is lightweight and has a high load-bearing capacity.Nicolau et al. [15] designed a 3D-printed connector in their paper and experimentally compared it with a mortise and tenon joint made of larch wood, proposing that the use of 3D-printed connectors instead of wooden joints in the assembly of furniture will become a design trend.The emergence of 3D-printed furniture connectors has simplified the process of processing wood for furniture and reduced manufacturing difficulties, shortening the processing cycle of wooden furniture products and reducing the cost of time and material waste.A new era in furniture manufacturing has been ushered in.

Intelligent Manufacturing of Furniture
With the development of Internet technology, the original lifestyle of consumers has been overturned, and the direction of furniture needs of young consumers has undergone essential changes, which has created more opportunities for cross-border integration between the furniture industry and the technology sector; 'intelligent furniture' is beginning to attract much attention.Intelligent furniture is based on modern furniture, effectively combining electronic intelligence, mechanical intelligence, IOT intelligence, and other intelligent components with furniture products, with self-sensing, intelligent, multifunctional, and other characteristics of a new type of furniture product [16].The implementation of its intelligent functions is mainly based on the embedding of hardware and the bearing of software [17].Mori et al. [18] propose an information support system based on a room-based behavioral measurement environment 'sensing room' and active image projectors, with sensors embedded in the floor and other furniture, and use these sensors to identify the state of the room's inhabitants.Haiyan et al. [19] introduced a method called functional module design, using the design of an intelligent wardrobe with dehumidification and anti-mold functions as an example.Rus et al. [20] explore using intelligent textiles in furniture with a sofa as an example, using capacitive sensors to recognize human posture.Gust et al. [21] present a draft concept of intelligent furniture that can determine and assess sitting posture in real-time, giving haptic feedback to the user through the deformation of the seat surface actuators.If the user sits in one position for a long time, the seat will cause the user to adjust the sitting posture through small movements to ensure muscle health.Xiong et al. [22] provide an in-depth analysis of the critical technologies applied in the development of intelligent furniture, such as embedded system technology for intelligent furniture items, sensor technology, and short-range wireless communication technology for smart home interconnection platforms, artificial intelligence, and intelligent interaction technology, pointing out the prospects for the development of intelligent furniture in China.Wu [23] designed a ZigBee system by combining wireless communication technology, Internet technology, and traditional Chinese furniture, and he verified the feasibility of the ZigBee system through experiments.He explored the possibilities of integrating everyday furniture and health monitoring at a technological level, with a view to protecting people's health to a specific extent, which at the same time has a catalytic effect on the development of the furniture industry.

Embedded Optical Fiber Sensing Based on 3D Printing Technology
Fiber Bragg Grating (FBG) sensing technology, a new type of sensing technology, has the advantages of being small in size, implantable, easy to integrate, and can conduct probe form sensing [24].It can be easily embedded into 3D-printed structures and achieve monitoring functions, and is widely used in environmental microclimate monitoring [24].When there is an excess of toxic and harmful gases or dust in the atmosphere, such as nitrogen oxides, carbon dioxide, sulfur dioxide, and cyanide, it can endanger people's health, as well as the survival of plants and animals.The purpose of environmental microclimate monitoring is to observe and analyze the source, distribution, quantity, and movement of harmful substances [25].At the same time, its low maintenance costs and good sensing performance make this technology important for improving the reliability and structural intelligence of 3D-printed structures.
In environmental monitoring, Zhou et al. [26] proposed a new optical fiber sensing system based on FBG sensors for monitoring methane concentration in the air.Yung-Li et al. [27] invented a high-frequency detection FBG sensing system based on an annular laser structure for long-range environmental monitoring.Yang [28] proposed a new method for dust concentration measurement based on FBG sensors, and built real-time monitoring software to achieve safe detection and early warning.Su [29] improved sensor resolution using an LPG-FBG cascade structure in gas-liquid environments.Zheng [30] designed an optical fiber grating demodulation system to detect the temperature in hazardous areas with an error of less than 1%.Lo Presti et al. [31] propose a plant wearable device based on FBG technology for monitoring plant growth and environmental parameters (e.g., temperature and humidity).
With the development of optical fiber sensing and 3D printing technology, it has become possible to directly embed optical fibers into materials based on 3D printing technology.As early as 2016, it was shown that embedded FBG sensors offer the possibility of real-time online monitoring of structural stresses, strains, and damage cracks during the manufacture or operation of 3D-printed smart structures [32].Leal-Junior et al. [33] measured three parameters, strain, temperature, and force, with only one FBG sensor by using the semi-embedding of FBG sensors in a 3D-printed structure.Fofei Pang's team at Shanghai University built an FPI-type optical fiber pressure sensor based on 3D printing technology [34].Zhao et al. [35] used 3D printing technology to embed FBG sensors inside and outside of PLA materials to experimentally investigate the effect of FBG sensors on the physical information transfer of human posture when placed at different locations in the material.
A survey of the literature reveals that optical fiber sensing has been relatively well developed for environmental monitoring, and there are many studies and experiments in the literature on embedded optical fiber structures.Previous experiments have verified the effectiveness and reliability of embedded FBG sensors for health monitoring in 3D-printed structures, which has a certain guiding significance for implementing and developing 3D-printed intelligent structures.Drawing on the application of optical fiber sensing technology in the field of environmental testing, combined with the exploration of 3D printing technology in the field of furniture, we have found through our research that although both intelligent furniture and 3D-printed furniture have been developed in their respective fields, the combined field of the two is still at a preliminary stage of exploration.In addition, because optical fiber sensing technology has generated such substantial achievements in the industry, we have attempted to incorporate it into the design sector.The use of 3D printing technology and embedded optical fiber sensing in wooden furniture components to achieve intelligent functionality is not only realistic and feasible, but the subsequent replacement of parts is also more convenient, breaking through the traditional modeling theory of furniture.

Methods
We adopt a research method combining theoretical analysis and experimental research to design and apply the embedded optical fiber integrated 3D-printed structure and the health monitoring of the components after the embedded optical fiber.Firstly, in terms of sensing mechanism, we completed the design and optimization of the sensor structure and the establishment of the intrinsic theoretical model; secondly, in terms of model structure design, we completed the design of the 3D-printed structure with embedded optical fiber in wooden furniture for the previous sensor model function concept, and optimized the combination of parts to establish the complete model; thirdly, in terms of system construction, this was based on the 3D printing technology to bury the optical fiber into the furniture pre-defined structural locations, and the preparation process and material removal mechanism of the femtosecond laser-engraved optical fiber sensor were studied; finally, we constructed a distributed optical fiber sensing scheme and built an experimental platform to investigate the failure behavior and tolerance mechanism of the sensor in complex environments.The specific research framework is shown in Figure 1.

Technical Principles
The basic principle of optical fiber operation is based on the principle of the total reflection of light, and the basic structure of bare optical fiber is shown in Figure 2. The mathematical relationship between the center wavelength and the effective refractive index of Fiber Bragg Grating (FBG) is the basis for studying FBG sensing.From the coupled wave theory, when the phase-matching condition is satisfied, the Bragg wavelength of the grating is In the formula, n eff is the effective refractive index of the fiber core and Λ is the grating period.The reflected center wavelength signal is related to the grating period and the effective refractive index of the fiber core, so when the external measurement is caused by the Fiber Bragg Grating temperature, stress and magnetic field changes will lead to changes in the reflected center wavelength.That means that the change in the wavelength of the reflected light center of the Fiber Bragg Grating reflects the change in the external signal to be measured.Fiber Bragg Grating sensor structure schematic and spectral transmission characteristics of the characterization are shown in Figure 3. Based on the furniture structure, the embedded optical fiber sensor is developed based on 3D printing technology, and the optical fiber embedded fixed mounting structure is designed.At the same time, the preparation process and the removal mechanism of hard and brittle materials are used to solve the problem of high temperature and high-pressure tolerance of the integrated optical fiber probe by using femtosecond laser inscription of the optical fiber sensor to realize the preparation of the integrated optical fiber temperature and pressure integrated sensor.The magnitude of each corresponding physical quantity is detected according to the relationship of the parameters (e.g., light intensity, wavelength, amplitude, phase) with external factors.Finally, combined with the multi-source sensor structure, the modulated signal light is fed into the light detector and demodulated to obtain the measured parameters, and the magnitude of the physical quantities is displayed on an electronic screen [36].

Preparation and Embedding Process of Optical Fiber Sensors
To explore the technology of optical fiber embedding in plastic replacement parts of furniture structures based on 3D printing, firstly, we have to improve the melting and forming process and optimize the slicing scheme of the workpiece model.Secondly, we have to plan the filling trajectory for the filling position of the optical fiber sensor and complete the analysis of the mechanical characteristics of the optical fiber embedded sensor through the error analysis of the 3D-printed model and the structural fit of the wooden furniture.
Preliminary printing experiments on embedded optical fiber using plastic material doped with carbon fiber as the raw material are required in order to explore the tolerance mechanism of embedded optical fiber sensors in the complex environment of temperature, compressive stress, and other physical coactions of sensing and monitoring, studying the toughness and compressive resistance of the plastic replacement parts of the furniture structure, refining the processing, forming process for the performance test results, and finally completing the preparation of the embedded optical fiber sensor.

Temperature and Compressive Stress Sensing Testing of Optical Fiber Sensors
Two issues arise once optical fiber sensors are embedded in a 3D-printed structure made of doped carbon fiber: first, whether the material strength and structure change after the optical fiber sensor is buried, and second, whether the optical fiber sensor still has sensing capability.In order to solve the above problems, we need to experimentally reveal the sensing law of optical fiber sensors under the influence of numerous temperature and pressure stress parameters in multi-field coupled environments and further optimize the response sensitivity of the embedded optical fiber sensor.
In order to undertake dual parametric temperature and compressive stress sensing tests on replacement furniture constructions with integrated sensors, a high-temperature furnace and pressure pump were used to produce a temperature and compressive stress test environment.The replacement piece of embedded optical fiber is placed at one end of the pressure carrier tube and penetrates deep into the high-temperature field of the high-temperature furnace; the other end is connected to a pressure pump to provide pressure, and the optical fiber is exported to the photoelectric demodulation system through a sealing process to optimize the structural parameters by analyzing the statics characteristics of the sensor when temperature and pressure loads are applied under complex operating conditions.

The Design of 3D-Printed Furniture with Embedded Light-Guiding and Monitoring Integrated Fibers
Based on the above analysis and understanding of optical fiber sensors and 3D printing technology, we take the innovative design practice of seating furniture as an example to explore the application of 3D-printed structures based on embedded optical fiber sensing and traditional wood crafts in innovative furniture design, realizing the intersection of old and new processes.

Extraction of Design Elements
The full moon surrounded by dark clouds is the design's inspiration for the stylistic elements.By drawing inspiration from the stability of the full moon and the haziness of the dark clouds, the design gradually develops into a dignified and atmospheric circle and flowing lines, which are then used to create the motifs of the 'Bright Moon' and the 'Diffuse Breeze', which are used on the seat back.The backrest's shape is echoed and complemented by the other joints of the seat by the use of a significant amount of linear wood and curved shapes, and the overall design is smooth and rounded, expressing a sense of serenity and relaxation (Figure 4).

Programme Presentation and Design Concept
The scheme (Figure 5) uses 'Bright Moon' and 'Diffuse Breeze' as the main design elements, combines traditional Chinese patterns and furniture shapes, and uses traditional Chinese mortise and tenon construction with 3D printing technology to complete the production of complex backrest shapes.The new Chinese seating furniture is designed to be durable, beautiful and practical.
The backrest adopts a curved shape of flowing clouds, bringing a sense of staggering height and depth, and because of its complex and varied shape, the structure of the backrest is made using 3D printing.The structure is embedded with integrated optical fibers for light attraction and monitoring, and the light attraction can realize the design theme of 'Bright moon'.The furniture is aesthetically pleasing in its light and shadow lines, while at the same time enabling microclimate monitoring of the environment, completing the functional fusion of modern technology and classical craftsmanship.The main body of the furniture is in the original wood color of light white oak, with beige upholstered cushions, creating a place of lightness and tranquility in the noisy modern living environment.

Material Selection: White Oak and PLA
Taking into account the overall effect and structural properties and following the special properties of the wood, such as anisotropy, we finally chose white oak for the main furniture.The white oak has an elegant overall color, with a clear and distinctive mountain-shaped grain, and is finished with a clear water lacquer to reflect the texture of the wood itself.Oak is dense, hard, and durable, with a high mechanical strength that is not easily worn and is clean and delicate to the touch.
The circular structure of the backrest is made of Polylactic Acid (PLA) and is printed using Fused Deposition Modeling (FDM) technology.PLA is a biodegradable thermoplastic derived from renewable resources, and is more environmentally friendly than other plastic materials.Moreover, mixing a quantitative amount of wood fibers in PLA, such as bamboo, birch, cork, and pine, allows for a woody-feeling material to be created, and the printed models feel similar to wood to the touch.Meanwhile, adding carbon fibers to traditional 3D printing plastics (e.g., nylon, ABS, or PLA) can increase the toughness of 3D printing materials [37][38][39][40], and the fiber-reinforced composites doped with carbon fibers are more similar to the thermal expansion coefficients and mechanical toughness of wood, which is more suitable for the partial replacement of the wooden structure.The combination of wood and plastic is not only a collision of materials to complete the visual experience, but also an exploration of new materials on a technical level.

Implementation of Intelligent Functions
Our design method is to make furniture with an intelligent function through the embedded optical fiber sensing technology, and the design scheme can realize the function of low brightness light guide, microclimate monitoring, and sedentary time detection.
The light-guiding function is realized through optical fibers.The optical fiber light guide can adjust the softness of the light intensity; the light source beam will be sent to the predetermined place with the path after entering the optical fiber, and multiple strands of optical fiber converge to form a flexible light band.One strand of the optical fiber is picked out and fused to the fiber with the FBG sensor to form the integrated optical fiber for light guiding and monitoring.Through 3D printing technology, the whole strand of optical fiber is embedded into the 3D printing structure of the seat backrest with the help of the embedded structure in Figure 6c to complete the fabrication of the furniture backrest with sensing and monitoring functions, build a complete sensing system, and embed the LED display on the left panel of the three panels of the seat surface as the display module of the sensing system.The temperature-sensing function of the FBG sensor is used to monitor the indoor environment temperature and microclimate data such as temperature, humidity, and carbon dioxide content are displayed on the terminal LED display, and when the indoor air content of hazardous gases exceeds the specified value, the terminal will give a reminder of the danger.
When the sensor is subjected to external stress, its wavelength will change accordingly, so we use the stress-sensing function of the FBG sensor to identify whether the person is sitting on the furniture to record the time that the person sits on the seat and give the user a reminder when the time exceeds the limit value, to minimize the harm of being sedentary to human health.

Drawing and Printing 3D Models
First, we carried out several structural designs for the mortise and tenon structure of the 3D-printed part in combination with the wooden structure, selected the optimal combination solution, and determined the mortise and tenon structure of the 3D-printed structure in combination with the backrest and seat surface.Then, we used 3ds MAX software (version 2016) to create a 3D model of the 3D-printed structure according to the dimensions (Figure 6a) and planned the location of the optical fiber sensor filling.We imported the model file of the workpiece to be prepared into the computer, used Cura software (version 15.02.1) to complete the slicing (Figure 6b), optimize the slicing scheme in terms of contact angle and support surface design, set the printing parameters according to the usage requirements of the structure, and planed the optimal filling path according to the pre-embedded position of the optical fiber sensor.
We prepared a multifunctional integrated optical fiber sensor by fusing a conduction fiber to the fiber with the FBG sensor in advance.Then, we imported the printing program into the 3D-printed device (Figure 7), adjusted the printer parameters, and selected a material filling density of 60% for printing.
Considering the small size of the 3D-printed device in the laboratory, we selected a section of the structure to complete the experimental verification and completed the process of embedding the optical fiber sensor during the printing process, precisely controlling the pre-embedded position of the multifunctional optical fiber sensor; the optical fiber was fixed in the 3D-printed sample through the structure shown in Figure 6c.The optical fiber-embedded fixed mounting structure had circular holes of 0.6 mm radius at both ends and rectangular holes of 0.4 mm × 0.6 mm in the middle (Figure 6c), the optical fiber was tensioned and fixed in position during printing until it was completely covered by the printing material, and the sensor was always kept in a pre-stressed state during embedding and curing to ensure its sensing performance and accuracy.Finally, we eliminated the redundant support structure after printing to complete the production of 3D-printed parts with light-directing and monitoring functions (Figure 6d).

Validation Experiments for Temperature
The equipment used in the experimental process is a Fusion Machine (Figure 8a), a constant temperature drying furnace (Figure 8b), and a spectrometer (Figure 8c).Firstly, we fused the embedded optical fibers to the optical fiber patch cables to facilitate the subsequent connection to the spectrometer (Figure 9a).Then, we placed the print in a thermostatic drying chamber with the print suspended in the air and the thermometer located above the device (Figure 9b) to be able to ensure that the workpiece was heated evenly, and led the optical fiber from the side of the chamber to connect to the spectrometer (Figure 9c).Due to the influence of ambient temperature, the lowest temperature is 20 degrees Celsius at room temperature, so we set the temperature test range to 20 degrees Celsius to 60 degrees Celsius.After setting the temperature control program, we began to monitor the value of the spectrometer.
The presence of a spectrum is proof of the successful embedding of the fiber (Figure 9d), followed by a performance test to record wavelength changes at different temperatures, and we obtained the spectrograms at different temperatures (Figure 10a).
Next, we collected data and used Origin software (version 2022) for data processing.We conducted two experiments and compared and analyzed the results separately.As seen in Figure 10, the errors of the two experiments are very small and almost negligible, which proves that the optical fiber sensors are stable after being embedded in the 3Dprinted structure.After that, we analyzed the temperature and wavelength data of the two measurements with linear fitting, and the correlation coefficients were 98.9% and 99.1% (Figure 10b), which proved that the wavelength and the temperature had a good linear relationship, which indicated that the sensor performance was stable and could be used for detecting the temperature of the structural components.

Verification Experiments of Structural Strength
The experiments were initially expected to perform dual parametric temperature and compressive stress sensing tests on the embedded optical fiber structural part under complex working conditions.However, due to the small size of the 3D-printed structural part and the insufficient weight of the weights, the sensor's perception of the stress on the structural part was too weak.
The wavelength could only fluctuate in the range of 1529.1058-1529.399nm, so we decided to change the experimental approach and use the INSTRON-5969 Universal Testing Machine (Figure 11) to carry out three-point bending experiments on the material after embedding the optical fiber.The test environment temperature was 20 • C, the size of the test specimen was 250 mm × 15 mm × 30 mm, the span between the two supports was 210 mm, and the loading speed was calculated by the formula: In the formula: R-Loading speed (mm/min); l-Test span (mm); h-Thickness of sample (mm).
According to Equation (2), the loading speed of the machine is 5.439 mm/min.At this loading rate, the material was loaded at a uniform speed.We recorded the degree of bending of the material for every 10 mm change in displacement during the experiment (Figure 12) and simultaneously recorded the load, the change in displacement at the center of the specimen span, and the maximum load-displacement curve at the time of damage.We analyzed and processed the experimental data, and Figure 13 gives a comparison of the three-point bending experimental curves of the normal printed sample and the printed sample with the embedded optical fiber sensor.According to Figure 13, it can be seen that the difference in the maximum bending load that the two samples of the same size can withstand is not large; P1 is 1654.53235N and P2 is 1690.70105N, and their gap mainly originates from the problem of the precision of the 3D printer: the printing of the material via the cold contraction produced a small edgewarping phenomenon, while the two samples had small errors in the printing thickness of 14.9 mm and 15.2 mm, respectively.This lack of precision caused experimental error, but the error was within the controllable range and did not affect the analysis of the strength of the material.Figure 13 shows that the force-displacement curve of the sample with the embedded optical fiber sensor starts to decline with unstable fluctuations in stage II, which is different from the descending curve of the ordinary printed sample.At the end of the test, it was found that this was due to the error caused by the offset of the force in one of the supports that support the sample (as shown in the red circle in Figure 12).However, in this paper, no specific analysis of the stresses and strains was involved, so this error does not affect the following data calculations.
The bending load of the material is one of the important parameters to measure the strength of the sample, and the formula calculates the bending load: In the formula: According to Equation (3), the bending strengths of the normal printed sample and the printed sample with an embedded optical fiber sensor are 77.21151MPa and 78.89938 MPa, respectively, and the two figures are very close to each other, which proves that the embedded optical fiber sensor in the 3D-printed sample does not affect the change in the sample's structural strength.

Experimental Results
We verified the feasibility of our design method through embedded printing experiments, temperature sensing experiments, and structural strength experiments.First, we initially completed the embedding process of the optical fiber sensor on the sample of PLA material through the embedded printing experiment.Second, the temperature-sensing experiment proves that the optical fiber sensor still has good sensing performance after being embedded in the 3D-printed structure, and also confirms from the side that the embedded process does not affect the sensing and monitoring of temperature by the optical fiber sensor, and, similarly, we can launch the scheme that can be used for environmental microclimate monitoring.Finally, in the strength experiment, the maximum bending loads of the ordinary sample and the sample with an embedded optical fiber sensor were measured to be 1654.53235N and 1690.70105N, respectively, which are greater than the force exerted by a person on the backrest of the furniture, and thus the material structure can satisfy the normal function of the furniture.In addition, in the strength experiment, the bending strength of the test sample is about 78 MPa, while the bending strength of wood is generally in the range of 50-110 MPa, such as the bending strength of Poplar, which is about 73 MPa; taking into account that the density of the print material of this sample is 60%, there is still room for improvement.Therefore, it is reasonable to estimate that after increasing the print density, the bending strength of the printed sample can continue to increase, gradually approaching the maximum bending strength of wood, so that the printed material can be used as a substitute for wood structure to withstand the gravity pressure well.In summary, these experimental results prove the feasibility of our proposed design method of applying embedded optical fiber sensors to 3D-printed components of furniture.

Conclusions
In this paper, we propose the concept of an integrated embedded optical fiber light guide and monitoring and its application to furniture design.The multifunctional integrated optical fiber sensor is embedded into the furniture parts through 3D printing technology, giving the wooden furniture an intelligent function in a new form.The advantages of 3D printing technology in the design and intelligent manufacturing process have brought multiple possibilities and opportunities for design and manufacturing.Its organic combination with traditional wood craftsmanship is becoming a breakthrough in furniture design innovation, and the superimposed application of optical fiber sensing technology is an innovative expression of the way furniture is made.
Replacing furniture components with 3D-printed structures embedded with optical fiber sensors reduces the difficulty of replacing wooden furniture structures on the one hand.On the other hand, the composite sensors guide light while also providing real-time monitoring of the environmental microclimate, enabling an intelligent leap forward in the basic functionality of wooden furniture.
We have confirmed through temperature experiments that the embedded optical fiber sensor still has sensing properties, and through strength tests, we have determined the destructive force of the material and confirmed that the strength of the material does not have a significant change when optical fibers are embedded within, providing data to support the theoretical concept and fully demonstrating the technical feasibility.In our experiments, we found that the measurement sensitivity of the optical fiber sensor after embedding is different from that of the sensor before embedding, which is affected by various aspects such as the printing material, the filler geometry, and the filler density of the material, and this will be the direction of our subsequent continued research.
Through the combination of embedded optical fiber sensing technology and 3Dprinted structures, furniture parts can be easily replaced and repaired, which not only improves the structural stability of the furniture and extends its service life, but also helps to reduce the number of discarded items of furniture, improve resource utilization, and reduce the waste of materials and environmental pollution.We hope that this technology and design concept will provide new ideas for other areas of design, encourage research on multidisciplinary design, and promote practical applications.In the context of the digital era, only by actively exploring the application space of new technologies in the field of furniture can we complete the creation of designs that highlight the characteristics of the times.The integration of digital technology and traditional manufacturing will provide a broader space and unlimited possibilities for creative thinking in the field of furniture.

Figure 2 .
Figure 2. Schematic diagram of the basic structure of bare optical fiber.

Figure 3 .
Figure 3. Schematic diagram of fiber grating structure and transmission spectral characteristics.

Figure 4 .
Figure 4.The evolution of design motifs.

Figure 5 .
Figure 5. Details of the design scheme.

Figure 6 .
Figure 6.(a) Mortise and tenon construction; (b) model slicing solutions of 3D-printed structures; (c) the optical fiber-embedded fixed mounting structure; (d) embedded optical fiber sensors during the printing process.

Figure 12 .
Figure 12.(a) Variation in bending degree of ordinary samples; (b) Variation in bending degree of embedded optical fiber samples.
σ-Bending strength of the sample (MPa); F-Bending damage load of the sample (N); l-Test span (mm); b-Width of sample (mm); h-Thickness of sample (mm).