Temperature (T) monitoring is essential across various domains, including biomedical applications, where respiratory rate (RR) can be estimated from the temperature difference between inhaled and exhaled air. Additive manufacturing, particularly fused deposition modeling (FDM), remains underexplored for this purpose despite the potential offered by conductive thermoplastic composites. Integrating conductive fillers into printable polymers provides an effective strategy to develop novel sensors. This study presents the fabrication and characterization of resistive sensors produced by FDM using thermoplastic polyurethane (TPU) filled with carbon black (CB). Three bio-inspired geometries—spider (SP), honeycomb (HC), and flap (FL)—are fabricated with three thicknesses (1, 2, and 3 layers). The electrical responses to temperature and relative humidity (RH) are analyzed to evaluate the influence of design on metrological performance. All sensors exhibit a synergistic behavior, with resistance increasing as T and RH rise. A parabolic response to T variations is observed, with resistance changes up to 125%. T sensitivity increases with thickness, with the HC geometry showing the highest values. Response times range from 15 to 23 s, with 8% hysteresis. In the 30%–70% RH range, sensors provide linear responses. Integration into a commercial face mask enables reliable RR detection during bradypnea, eupnea, and tachypnea.
| [1] |
P. R. N. Childs, J. R. Greenwood, and C. A. Long, “Review of Temperature Measurement,” Review of Scientific Instruments 71, no. 8 (2000): 2959–2978.
|
| [2] |
M. A. Cretikos, R. Bellomo, K. Hillman, J. Chen, S. Finfer, and A. Flabouris, “Respiratory Rate: The Neglected Vital Sign,” Medical Journal of Australia 188, no. 11 (2008): 657–659.
|
| [3] |
P. B. Lovett, J. M. Buchwald, K. Stürmann, and P. Bijur, “The Vexatious Vital: Neither Clinical Measurements by Nurses nor an Electronic Monitor Provides Accurate Measurements of Respiratory Rate in Triage,” Annals of Emergency Medicine 45, no. 1 (2005): 68–76.
|
| [4] |
A. Nicolò, M. Girardi, I. Bazzucchi, F. Felici, and M. Sacchetti, “Respiratory Frequency and Tidal Volume During Exercise: Differential Control and Unbalanced Interdependence,” Physiological Reports 6, no. 21 (2018): e13908.
|
| [5] |
M. J. Tipton, A. Harper, J. F. R. Paton, and J. T. Costello, “The Human Ventilatory Response to Stress: Rate or Depth?,” Journal of Physiology 595, no. 17 (2017): 5729–5752.
|
| [6] |
A. Nicolò, M. Girardi, and M. Sacchetti, “Control of the Depth and Rate of Breathing: Metabolic vs. Non-Metabolic Inputs,” Journal of Physiology 595, no. 19 (2017): 6363–6364.
|
| [7] |
A. Nicolò, C. Massaroni, E. Schena, and M. Sacchetti, “The Importance of Respiratory Rate Monitoring: From Healthcare to Sport and Exercise,” Sensors 20, no. 21 (2020): 6396.
|
| [8] |
S. Rolfe, “The Importance of Respiratory Rate Monitoring,” British Journal of Nursing 28, no. 8 (2019): 504–508.
|
| [9] |
J. Lindemann, R. Leiacker, G. Rettinger, and T. Keck, “Nasal Mucosal Temperature During Respiration,” Clinical Otolaryngology and Allied Sciences 27, no. 3 (2002): 135–139.
|
| [10] |
J. Fei and I. Pavlidis, “Thermistor at a Distance: Unobtrusive Measurement of Breathing,” IEEE Transactions on Bio-Medical Engineering 57, no. 4 (2010): 988–998.
|
| [11] |
C. Massaroni, A. Nicolò, D. Lo Presti, M. Sacchetti, S. Silvestri, and E. Schena, “Contact-Based Methods for Measuring Respiratory Rate,” Sensors 19, no. 4 (2019): 908.
|
| [12] |
C. Romano, D. Lo Presti, S. Silvestri, E. Schena, and C. Massaroni, “Flexible Textile Sensors-Based Smart T-Shirt for Respiratory Monitoring: Design, Development, and Preliminary Validation,” Sensors 24, no. 6 (2024): 2018.
|
| [13] |
K. Storck, M. Karlsson, P. Ask, and D. Loyd, “Heat Transfer Evaluation of the Nasal Thermistor Technique,” IEEE Transactions on Biomedical Engineering 43, no. 12 (1996): 1187–1191.
|
| [14] |
H. Akre and O. Skatvedt, “Advantages of Measuring Air Flow in the Pharynx With Internal Thermistors,” European Archives of Oto-Rhino-Laryngology 257, no. 5 (2000): 251–255.
|
| [15] |
E. Jovanov, D. Raskovic, and R. Hormigo, “Thermistor-Based Breathing Sensor for Circadian Rhythm Evaluation,” Biomedical Sciences Instrumentation 37 (2001): 493–497.
|
| [16] |
M. K. Marks, M. South, and B. G. Carter, “Measurement of Respiratory Rate and Timing Using a Nasal Thermocouple,” Journal of Clinical Monitoring 11, no. 3 (1995): 159–164.
|
| [17] |
H. Xue, Q. Yang, D. Wang, et al., “A Wearable Pyroelectric Nanogenerator and Self-Powered Breathing Sensor,” Nano Energy 38 (2017): 147–154.
|
| [18] |
S. A. Pullano, A. S. Fiorillo, I. Mahbub, S. K. Islam, M. S. Gaylord, and V. Lorch, “Non-Invasive Integrated Wireless Breathing Monitoring System Based on a Pyroelectric Transducer.” 2016 IEEE Sensors (IEEE, 2016), 1–3.
|
| [19] |
Y. P. Huang, M. S. Young, and C. C. Tai, “Noninvasive Respiratory Monitoring System Based on the Piezoceramic Transducer's Pyroelectric Effect,” Review of Scientific Instruments 79, no. 3 (2008): 035103.
|
| [20] |
A. Gautam, K. Kinjalk, A. Kumar, and V. Priye, “FBG-Based Respiration Rate Sensing With Arduino Interface,” IEEE Sensors Journal 20, no. 16 (2020): 9209–9217.
|
| [21] |
A. Manujło and T. Osuch, “Temperature Fiber Bragg Grating Based Sensor for Respiration Monitoring.” Photonics Applications in Astronomy, Communications, Industry, and High Energy Physics Experiments 2017 (SPIE, 2017). 10445, 362–367.
|
| [22] |
F. Taffoni, D. Formica, P. Saccomandi, G. Pino, and E. Schena, “Optical Fiber-Based MR-Compatible Sensors for Medical Applications: An Overview,” Sensors 13, no. 10 (2013): 14105–14120.
|
| [23] |
V. Mishra, N. Singh, U. Tiwari, and P. Kapur, “Fiber Grating Sensors in Medicine: Current and Emerging Applications,” Sensors and Actuators, A: Physical 167, no. 2 (2011): 279–290.
|
| [24] |
C. Massaroni, A. Nicolo, M. Sacchetti, and E. Schena, “Contactless Methods for Measuring Respiratory Rate: A Review,” IEEE Sensors Journal 21, no. 11 (2021): 12821–12839.
|
| [25] |
Z. Zhu, J. Fei, and I. Pavlidis, “Tracking Human Breath in Infrared Imaging.” Fifth IEEE Symposium on Bioinformatics and Bioengineering (BIBE’05 (IEEE, 2005), 227–231.
|
| [26] |
S. Y. Chekmenev, H. Rara, and A. A. Farag, “Non-Contact, Wavelet-Based Measurement of Vital Signs Using Thermal Imaging.” The First International Conference on Graphics, Vision, and Image Processing (GVIP) (Citeseer, 2005), 107–112.
|
| [27] |
F. Q. Al-Khalidi, R. Saatchi, D. Burke, and H. Elphick, “Facial Tracking Method for Noncontact Respiration Rate Monitoring.” 2010 7th International Symposium on Communication Systems, Networks & Digital Signal Processing (CSNDSP 2010) (IEEE, 2010), 751–754.
|
| [28] |
I. Gibson, D. Rosen, B. Stucker, et al., Additive Manufacturing Technologies (Springer, 2021). 17.
|
| [29] |
O. Abdulhameed, A. Al-Ahmari, W. Ameen, and S. H. Mian, “Additive Manufacturing: Challenges, Trends, and Applications,” Advances in Mechanical Engineering 11, no. 2 (2019): 1687814018822880.
|
| [30] |
J. Gardan, “Additive Manufacturing Technologies: State of the Art and Trends,” International Journal of Production Research 54, no. 10 (2016): 3118–3132.
|
| [31] |
U. M. Dilberoglu, B. Gharehpapagh, U. Yaman, and M. Dolen, “The Role of Additive Manufacturing in the Era of Industry 4.0,” Procedia Manufacturing 11 (2017): 545–554.
|
| [32] |
T. Q. Trung and N. E. Lee, “Flexible and Stretchable Physical Sensor Integrated Platforms for Wearable Human-Activity Monitoringand Personal Healthcare,” Advanced Materials 28, no. 22 (2016): 4338–4372.
|
| [33] |
Y. Khan, A. Thielens, S. Muin, J. Ting, C. Baumbauer, and A. C. Arias, “A New Frontier of Printed Electronics: Flexible Hybrid Electronics,” Advanced Materials 32, no. 15 (2020): 1905279.
|
| [34] |
P. F. Flowers, C. Reyes, S. Ye, M. J. Kim, and B. J. Wiley, “3D Printing Electronic Components and Circuits With Conductive Thermoplastic Filament,” Additive Manufacturing 18 (2017): 156–163.
|
| [35] |
V. S. Turkani, D. Maddipatla, B. B. Narakathu, B. J. Bazuin, and M. Z. Atashbar, “A Carbon Nanotube Based NTC Thermistor Using Additive Print Manufacturing Processes,” Sensors and Actuators, A: Physical 279 (2018): 1–9.
|
| [36] |
M. Soni, M. Bhattacharjee, M. Ntagios, and R. Dahiya, “Printed Temperature Sensor Based on PEDOT: PSS-Graphene Oxide Composite,” IEEE Sensors Journal 20, no. 14 (2020): 7525–7531.
|
| [37] |
S. W. Kwok, K. H. H. Goh, Z. D. Tan, et al., “Electrically Conductive Filament for 3D-Printed Circuits and Sensors,” Applied Materials Today 9 (2017): 167–175.
|
| [38] |
P. Lakkala, S. R. Munnangi, S. Bandari, and M. Repka, “Additive Manufacturing Technologies With Emphasis on Stereolithography 3D Printing in Pharmaceutical and Medical Applications: A Review,” International Journal of Pharmaceutics: X 5 (2023): 100159.
|
| [39] |
M. B. Kumar, P. Sathiya, and M. Varatharajulu, “Selective Laser Sintering.” Advances in Additive Manufacturing Processes (Bentham Science Publishers, 2021), 28–47.
|
| [40] |
P. Calvert, “Inkjet Printing for Materials and Devices,” Chemistry of Materials 13, no. 10 (2001): 3299–3305.
|
| [41] |
V. Correia, C. Caparros, C. Casellas, L. Francesch, J. G. Rocha, and S. Lanceros-Mendez, “Development of Inkjet Printed Strain Sensors,” Smart Materials and Structures 22, no. 10 (2013): 105028.
|
| [42] |
T. N. A. T. Rahim, A. M. Abdullah, and H. Md Akil, “Recent Developments in Fused Deposition Modeling-Based 3D Printing of Polymers and Their Composites,” Polymer Reviews 59, no. 4 (2019): 589–624.
|
| [43] |
V. Saroli, M. Pinnelli, C. Romano, S. Silvestri, E. Schena, and C. Massaroni, “Design, Development and Characterization of a Single-Layer 3D-Printed Strain CB-TPU Sensor.” 2025 IEEE International Workshop on Metrology for Industry 4.0 & IoT (MetroInd4. 0 & IoT) (IEEE, 2025), 265–270.
|
| [44] |
U. I. Cicek, D. J. Southee, and A. A. Johnson, “The Development and Characterisation of 3D-Printed Multi-Material Thermistor,” Additive Manufacturing 94 (2024): 104510.
|
| [45] |
Z. Wang, W. Gao, Q. Zhang, et al., “3D-Printed Graphene/Polydimethylsiloxane Composites for Stretchable and Strain-Insensitive Temperature Sensors,” ACS Applied Materials & Interfaces 11, no. 1 (2018): 1344–1352.
|
| [46] |
C. Massaroni, V. Saroli, Z. Aloqalaa, D. L. Presti, and E. Schena, “Extrusion-Based Fused Deposition Modeling for Printing Sensors and Electrodes: Materials, Process Parameters, and Applications,” SmartMat 6, no. 4 (2025): e70027.
|
| [47] |
K. Rajan, M. Samykano, K. Kadirgama, W. S. W. Harun, and M. M. Rahman, “Fused Deposition Modeling: Process, Materials, Parameters, Properties, and Applications,” The International Journal of Advanced Manufacturing Technology 120, no. 3 (2022): 1531–1570.
|
| [48] |
S. C. Daminabo, S. Goel, S. A. Grammatikos, H. Y. Nezhad, and V. K. Thakur, “Fused Deposition Modeling-Based Additive Manufacturing (3D Printing): Techniques for Polymer Material Systems,” Materials Today Chemistry 16 (2020): 100248.
|
| [49] |
V. Saroli, C. Massaroni, S. Silvestri, and E. Schena, “Advancing Flow Rate Measurement: Innovations and Challenges in Spirometry and Mechanical Ventilation,” IEEE Sensors Reviews 2 (2025): 76–85.
|
| [50] |
T. Han, S. Kundu, A. Nag, and Y. Xu, “3D Printed Sensors for Biomedical Applications: A Review,” Sensors 19, no. 7 (2019): 1706.
|
| [51] |
A. Bandyopadhyay, S. Bose, and S. Das, “3D Printing of Biomaterials,” MRS Bulletin 40, no. 2 (2015): 108–115.
|
| [52] |
V. Shanmugam, M. V. Pavan, K. Babu, and B. Karnan, “Fused Deposition Modeling Based Polymeric Materials and Their Performance: A Review,” Polymer Composites 42, no. 11 (2021): 5656–5677.
|
| [53] |
P. K. Penumakala, J. Santo, and A. Thomas, “A Critical Review on the Fused Deposition Modeling of Thermoplastic Polymer Composites,” Composites, Part B: Engineering 201 (2020): 108336.
|
| [54] |
A. Plymill, R. Minneci, D. A. Greeley, and J. Gritton, “Graphene and Carbon Nanotube PLA Composite Feedstock Development for Fused Deposition Modeling.” Published online 2016.
|
| [55] |
Q. Wei, R. Yang, X. Zhao, J. Zhou, Y. An, and S. Yang, “Fused Deposition Modeling of Carbon-Reinforced Polymer Matrix Composites: A Comprehensive Review,” Polymer Composites 44, no. 9 (2023): 5313–5345.
|
| [56] |
E. M. Palmero, D. Casaleiz, J. de Vicente, et al., “Composites Based on Metallic Particles and Tuned Filling Factor for 3D-Printing by Fused Deposition Modeling,” Composites, Part A: Applied Science and Manufacturing 124 (2019): 105497.
|
| [57] |
M. Sajid, J. Z. Gul, S. W. Kim, H. B. Kim, K. H. Na, and K. H. Choi, “Development of 3D-Printed Embedded Temperature Sensor for Both Terrestrial and Aquatic Environmental Monitoring Robots,” 3D Printing and Additive Manufacturing 5, no. 2 (2018): 160–169.
|
| [58] |
J. G. Jeon, G. W. Hong, H. G. Park, S. K. Lee, J. H. Kim, and T. J. Kang, “Resistance Temperature Detectors Fabricated via Dual Fused Deposition Modeling of Polylactic Acid and Polylactic Acid/Carbon Black Composites,” Sensors 21, no. 5 (2021): 1560.
|
| [59] |
S. A. S. A. Saufi, M. Y. M. Zuhri, M. L. Dezaki, et al., “Compression Behaviour of Bio-Inspired Honeycomb Reinforced Starfish Shape Structures Using 3D Printing Technology,” Polymers 13, no. 24 (2021): 4388.
|
| [60] |
X. Xu, L. Heng, X. Zhao, J. Ma, L. Lin, and L. Jiang, “Multiscale Bio-Inspired Honeycomb Structure Material With High Mechanical Strength and Low Density,” Journal of Materials Chemistry 22, no. 21 (2012): 10883–10888.
|
| [61] |
F. K. Ko and J. Jovicic, “Modeling of Mechanical Properties and Structural Design of Spider Web,” Biomacromolecules 5, no. 3 (2004): 780–785.
|
| [62] |
A. P. Yoganathan, Z. He, and S. Casey Jones, “Fluid Mechanics of Heart Valves,” Annual Review of Biomedical Engineering 6, no. 1 (2004): 331–362.
|
| [63] |
M. L. Studebaker, “The Chemistry of Carbon Black and Reinforcement,” Rubber Chemistry and Technology 30, no. 5 (1957): 1400–1483.
|
| [64] |
D. J. Martin, G. F. Meijs, P. A. Gunatillake, S. P. Yozghatlian, and G. M. Renwick, “The Influence of Composition Ratio on the Morphology of Biomedical Polyurethanes,” Journal of Applied Polymer Science 71, no. 6 (1999): 937–952.
|
| [65] |
E. Segal, R. Tchoudakov, M. Narkis, and A. Siegmann, “Thermoplastic Polyurethane–Carbon Black Compounds: Structure, Electrical Conductivity and Sensing of Liquids,” Polymer Engineering & Science 42, no. 12 (2002): 2430–2439.
|
| [66] |
J. Cao and X. Zhang, “Modulating the Percolation Network of Polymer Nanocomposites for Flexible Sensors,” Journal of Applied Physics 128, no. 22 (2020): 220901.
|
| [67] |
V. K. S. Shante and S. Kirkpatrick, “An Introduction to Percolation Theory,” Advances in Physics 20, no. 85 (1971): 325–357.
|
| [68] |
S. Kirkpatrick, “Percolation and Conduction,” Reviews of Modern Physics 45, no. 4 (1973): 574–588.
|
| [69] |
N. Hu, Y. Karube, C. Yan, Z. Masuda, and H. Fukunaga, “Tunneling Effect in a Polymer/Carbon Nanotube Nanocomposite Strain Sensor,” Acta Materialia 56, no. 13 (2008): 2929–2936.
|
| [70] |
P. Sheng, “Fluctuation-Induced Tunneling Conduction in Disordered Materials,” Physical Review B 21, no. 6 (1980): 2180–2195.
|
| [71] |
Y. Liu, E. Asare, H. Porwal, et al., “The Effect of Conductive Network on Positive Temperature Coefficient Behaviour in Conductive Polymer Composites,” Composites, Part A: Applied Science and Manufacturing 139 (2020): 106074.
|
| [72] |
X. Zhang, J. Li, J. Lin, et al., “Highly Stretchable Electronic-Skin Sensors With Porous Microstructure for Efficient Multimodal Sensing With Wearable Comfort,” Advanced Materials Interfaces 10, no. 8 (2023): 2201958.
|
| [73] |
V. Saroli, E. Schena, and C. Massaroni, “Fully Printed Variable Orifice Flowmeter With Built-in Temperature Compensation for Application in Mechanical Ventilation,” Advanced Sensor Research 4, no. 6 (2025): 2400196.
|
| [74] |
D. Xiang, X. Zhang, Y. Li, et al., “Enhanced Performance of 3D Printed Highly Elastic Strain Sensors of Carbon Nanotube/Thermoplastic Polyurethane Nanocomposites via Non-Covalent Interactions,” Composites, Part B: Engineering 176 (2019): 107250.
|
| [75] |
T. Xu, W. Shen, X. Lin, and Y. M. Xie, “Mechanical Properties of Additively Manufactured Thermoplastic Polyurethane (TPU) Material Affected by Various Processing Parameters,” Polymers 12, no. 12 (2020): 3010.
|
| [76] |
M. A. Ragolia, A. Di Nisio, A. M. Lanzolla, G. Percoco, M. Scarpetta, and G. Stano, “Thermal Characterization of Electrical Resistance of 3D Printed Sensors.” 2021 IEEE International Instrumentation and Measurement Technology Conference (I2MTC (IEEE, 2021), 1–6.
|
| [77] |
F. Ursi and G. De Pasquale, “Comparative Characterization of FDM Structures With Electrically-Conductive Sensing Elements Under Static, Dynamic and Thermal Loads,” Scientific Reports 15, no. 1 (2025): 26877.
|
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