Development of a Pain Measurement Device Using 3D printing and electronic air pressure control

doi:10.21203/rs.3.rs-2908246/v1

This article describes the design of a system wireless pain monitor, also known as a pain meter, which will be used to diagnose people with fibromyalgia. As the test should be done simultaneously while a Magnetic Resonance Imaging (MRI) is being performed on the patient to observe the brain activity, the device can´t have metallic components. Solid modelling and additive manufacturing has been used for the manufacturing of the device and an electropneumatic control has been defined too, several prototypes have been manufactured and tested. The work focuses on the validation of the designed pain meter, built by Fused Deposition Modeling (FDM) technology in different materials and with different printers. The surface finishes and manufacturing tolerances of the critical parts were tested, and their suitability to the necessary function is verified.

A proper mechanical pain meter device has been designed, to be used in fibromyalgia diagnosis, compatible without steel components nor wires, so compatible with simultaneously MRI on the patient.

Rapid prototyping

FDM

functional prototypes

3D printing in medicine

experimental analysis

The difficult diagnosis of fibromyalgia requires techniques related to magnetic resonance imaging in which metallic elements cannot be used.
3D printing allows prototypes and final devices for medical use to be made without metallic elements.
All the phases of the design, control and validation of a pain meter for its manufacture by 3D printing and use in the diagnosis of fibromyalgia are presented.

Numerous references to 3D printing applications in medicine can be found in the literature [1–4]. In the field of medical devices and motivated by the interest of clinicians, new applications are designed for specific fields [5, 6]. The high competitiveness for small-scale production of complex shaped products and the ease of communication between engineers and clinicians with these prototypes makes 3D printing the most suitable technology [7]. One of the most commonly used rapid prototyping technologies is Fused Deposition Modeling (FDM). Advantages of this technology are: wide variety of materials available, quick material change, low maintenance costs, quick production of thin parts, a tolerance equal to + 0.1 mm overall, no need for supervision, no toxic materials, very compact size, low temperature operation [8]. Moreover, complex shapes can be achieved, with adequate strength for the application and without metal parts [9]. In this context, FDM has been selected for the pain meter prototypes manufacturing.

Fibromyalgia is a disease that causes musculoskeletal pain generalized and a painful sensation to the pressure in specific points, also known as sore spots. It brings associated pain in muscles and fibrous tissue [10]. Although it is a disease that is not talked about much, it is quite frequent and is suffered by between 4% and 6% of the world’s population, being more frequent in women. Currently, there are no specific laboratory or imaging tests for fibromyalgia [11].

In order to make a more specific diagnosis possible, clinical criteria of the American College of Rheumatology are followed. One of the recommended tests is known as the thumb test [12], based on the application of pressure and pinching on a thumb for a certain time. It is a simple and convenient way of both detecting the disease and measuring its degree (mild, moderate or high). This test subjectively measures the patient's sensation of pain. For a better diagnosis, it is proposed to relate it to the objective response observed in a simultaneous Magnetic Resonance Imaging (MRI) [13, 14].

A pain meter, so called dolorimeter [15], is an instrument used to measure pain threshold and pain tolerance. Dolorimetry has been defined as the measurement of pain sensitivity or pain intensity [16].

There are several types of pain meters on the market: the digital and analogical pain meters, the latter the most widely used today. They have metal components that do not allow use with an MRI machine.

The analogue pain meter (Fig. 1.a) is pushed over the skin, until the patient reaches their maximum pain tolerance. This type of device displays the pressure that has been applied on a dial although manual use makes it difficult to maintain the same pressure for a given time.

Digital pain meter (Fig. 1.b) is capable of collecting, storing and calculating statistics with the pressure applied. They are usually accompanied by computer software to collect the data. Other pain meter types have been proposed [17]. Nevertheless, no one of these devices are metal free to be introduced in a MRI machine.

The purpose of this work is to design a device with the ability to exert controlled pressure in a controlled time test on the thumb with no metallic parts, so that it can be used while a MRI is being performed to relate brain activity with the level of pain that the patient indicates to feel.

Once the FDM technique has been selected, the suitability of the material is checked and, in order to be able to generalise the use of the device, different types of printers are included in the study. The behaviour of different prototypes, based on pieces manufactured in four different 3D printers, with six different materials, Polylactic Acid (PLA), Acrylonitrile Butadiene Styrene (ABS), PLA Tough, ABS M30i, Polycarbonate (PC) and Nylon, with a wall thickness of 5 mm and high printing density, is tested. Hardness and surface roughness tests will be carried out and the pressure ratio that the device provide in use will be checked.

In order to carry out a specific evaluation of the patient's brain function, it is desired to design a device without metallic elements that can be inserted into an MRI machine. In this way, a real-time image of the fibromyalgia patient's brain activity while subjected to the painful stimulus can be obtained.

The specifications of the system were indicated by a neurologist and the following should be highlighted:

enter the MRI scanner.
possibility of selecting from a range of pressures manually.
select tests that have a predetermined pressure and a determined duration.
possibility of storing the results obtained with different patients during an evaluation.

Based on these specifications, several conceptual designs were proposed and finally, a prototype was designed that includes the following subassemblies:

Physical support that is to be held by the patient with one hand while in the MRI scanner. It must be light, comfortable to wear, and free of metal parts. Therefore, it was decided to design a device to be built by FDM. The device, with a pressure control system, exerts a controlled force on the patient's thumb over a given period of time. The cylinder rod for pressure on the thumb and the handle designed can be seen in Fig. 2.
4 mm pipe responsible for transporting the pressurised air from the main compressed air installation to the pain meter.
Pressure regulator. By means of a current control loop, it will be possible to transmit the pressure that is desired at the outlet to this regulator.
Microcontroller: used to generate a voltage to produce the current necessary to carry out the control of the pressure regulator. It also serves as a point of connection so that, from a smartphone, tablet or PC, the person in charge of carrying out the test can indicate what pressure is to be used.
Printed Circuit Board (PCB): Contains the signal conditioning circuitry needed to generate the current from the voltage obtained from the microcontroller.
Manual regulator: it is placed between the compressed air intake and the automatic regulator. Its purpose is to limit the air pressure that reaches the automatic regulator and thus avoid possible physical harm to patients, in the event of a failure in the rest of the system.

The operation of the pain meter is simple. It has to be held by the patient with one of their hands while keeping the thumb in the position shown in Fig. 3. Once in this position the patient is placed into the MRI scanner. The doctor decides how much pressure and how long the force should be exerted, with the patient indicating the level of pain on a scale from no pain to very severe pain.

3.1. Handheld device manufactured by FDM

As can be seen in Fig. 2, the handheld device has two FDM built main sections. On one side the thumb support area with the handle, and on the other hand the cylinder rod.

The thumb support area (Fig. 4c) is ergonomic. Its shape is not easily manufacturable by traditional methods, but FDM technology can be used to produce complex shapes. A rubber commercial handle on the base was added for more comfort, it is cheap and easy to acquire (Fig. 4d).

The other section designed, the pneumatic cylinder (Fig. 4a, piston; Fig. 4b, body), must work by providing precision force, under the air pressure selected. Even more, as it is the most critical part, it will be an independent part that will be easy to change if needed, since by use the force exerted can lead to a loss of precision.

The quality of the pieces built by FDM doesn’t have great accuracy nor good superficial quality. Therefore, the most critical part of this device built by FDM, is the precision of the piston.

Two O-ring (polymer NBR 70) were inserted in the two annular grooves on the outer diameter of the piston to guarantee the precision and avoid air loses.

3.2. Pneumatic Cylinder.

On the advice of neurology specialist, the force to be exerted on the thumbnail of the patient should be selectable to an upper limit of around 45 Newton (N). As normal value of air compressed lines available in hospitals is 6 bar, the internal diameter for the cylinder was stablish in 16 mm, so that varying the air pressure from 0 to 3 bar, the force acting on the patient’s thumbnail will vary from 0 to 50 N.

The surface finish and the hardness of the internal surface of the cylindrical hole are important properties that influence the proper functioning of the pneumatic cylinder, as well as the dimensions of the cylindrical hole and the piston and grooves for housing the O-rings.

Six different materials and four different printers have been used in this study. Different qualities of printers have been used to validate the functionality of the device, referencing both the material used and the printing machine. A number in parentheses has been assigned to each printer for reference in this article. Prusa i3, referenced as (1) in Table 1, with open heating bed, low cost printer, working with PLA. Two medium-high range printers, close heating bed, with temperature control, HP Designjet 3D (2) working with ABS and Ultimaker (3) working with PLA Tough. Finally, 3D Fortus 450 m Stratasys high range printer (4), working with PC, ABSM30i and Nylon.

3.3. Measurement of surface hardness of printed parts

Some articles provide data on the hardness of the materials used in this study [18, 19]. However, given the variability depending on the FDM parameters, in this study Shore D durometer has been used to measure the hardness in all the prototypes.

Table 1 shows the hardness data obtained from the literature and the data resulting from the measurements carried out. Ten Shore D hardness measurements have been carried out and the mean value, deviation and variance are shown. All the materials tested had similar Shore D hardness values, between 75 and 80.

Table 1

Shore D hardness measured on the different prototypes. The material, the printer used and the hardness proposed by the literature are referenced. The number in brackets accompanying the material indicates the printer used in each case.
Material	3D printer	Shore D Literature data	Shore D Average	Deviation σ	Variance σ²
PLA (1)	Prusa i3	83	73.50	6.70	44.94
ABS (2)	HP Designjet 3D	100	78.00	1.89	3.56
PLA Tough (3)	Ultimaker	79	79.00	1.05	1.11
ABS M30i (4)	3D Fortus 450 m Stratasys	80	78.70	2.83	8.01
PC (4)		80	79.90	2.73	7.43
Nylon (4)		80	74.30	1.70	2.90

ABS (Acrylonitrile Butadiene Styrene); PLA (Polylactic Acid). PC (Polycarbonate).

3.4. Cylinder bore surface roughness.

The roughness has been measured on the inner face of the cylindrical hole, in four positions separated by 90º, and in the longitudinal direction, coinciding with the displacement of the piston. A MarSurf M300 roughness meter has been used (Fig. 5).

There are clearly appreciable differences in the roughness of the surfaces, and the best results are obtained for those built in PLA Tought (3) and ABS M30i (4).

3.5. Proposed geometry for cylinder, and O-rings

The chosen geometry for the pneumatic cylinder are showed in Fig. 6. The dimension of the hole diameter d = 16 mm, and the O-rings used are the most critical as their contact concerns the functionality of the device. Two types of O-ring were used: O-ring type A) in Fig. 6 was mounted on groove diameter of 11.0 mm, and O-ring type B) was mounted on groove diameter of 11.3 mm. These combinations were used in order to reduce friction and pressure losses. As in other engineering problems, numerous combinations could have been used.

In order to verify the adequacy of the dimensions proposed for the manufacture of the pneumatic cylinder built by FDM without any subsequent mechanisation, all the prototypes were built from the same computer-aided design files (*.stl), with the geometry described in Fig. 6.

After printing the first prototypes and trying to assemble them, it could be seen a great variation on the printers tolerances. Measurements verification of the diameter of the hole in different positions is showed in Fig. 7. The variability of these dimensions can be observed in Fig. 8.a) respect to the .stl file and in Fig. 8.b) can be appreciate the relationship between them.

Although the variability of the diameters exists, these measurements vary longitudinally for the proposed dimensions. Pistons fit without interference in the cylinder holes, except in some prototypes built in PLA (1). Some cases of interference appear which have required the outer diameter of the piston to be sanded.

Materials and printers with less dimensional variability coincide with those with the best surface finish. In addition, best results are obtained for those built in PLA Tought (3) and ABS M30i (4).

3.6. Measurement of the force exerted by the pneumatic cylinder as a function of the pressure exerted for different geometries and materials

The force exerted on the thumb by the pain meter has been measured on a MTS test machine with a load cell capable to measure up to 100 N (Fig. 9).

Figure 10 for the O-ring with configuration A and Fig. 11 for the O-ring with configuration B, show the forces obtained for pressures ranging from 0.5 bar to 3 bar, so that force exerted varies from zero to a maximum around 50 N. To arrive at these designs, it is necessary to take the variability in the dimensions of the cylinder hole (see Fig. 8) into account as well as the surface finish of the surface of said hole (roughness, Fig. 5), the dimensions of the elastic ring used and the diameter of the annular groove in the piston.

As PLA Tough and ABS M30i showed the best surface results, a vegetable grease without metallic components that can be used inside the MRI has been tested in order to verify if it improves its performance (see Fig. 11, PLA Tough Oiled and ABS M30i oiled). It is observed a softer response in PLA Tough, but no significant difference in ABS M30i. The device can be simplified by its use in non-lubricated conditions.

Variance and covariance of forces obtained for the different devices are shown in Fig. 12. PLA Tough seems to be the best option, providing the most stable results.

3.7. Device control elements.

Once the device has been designed, it is necessary to establish both electronic control of the device and a control app to facilitate its use.

The industry offers multiple possibilities to make pressure regulations. The SMC model ITV3030 has been selected. This model has a control pressure ranging from 0.05 bar to 5 bar with a DC control signal type of 4 mA to 20 mA (https://www.smc.eu/es-es).

This application requires a microcontroller that have the ability to generate PWM waves, Wi-Fi and Bluetooth connectivity and small size. One of the main characteristics that differentiates some microcontrollers from others is its connectivity, and not many microcontrollers have it. Some of the devices having this property are the Raspberry Pi, Arduino MKR1000, and LoLin NodeMCU v3 ESP8266; this one was chosen. It has very high capacities since it integrates WiFi module for communication, ADC module for reading analogue signals and generating PWM waves, 14 configurable pins as input/output, 4 Mbyte memory, supports external power supply voltages ranging from 4.5V to 9V and USB powered. In addition, it has a compact module with reduced dimensions of 30 x 57mm, which make it easy to mount almost anywhere near the regulator pressure to be controlled.

The system developed will provide two possibilities for the communication between the smartphone or tablet and the electronic control of the device, Wi-Fi and Bluetooth (Fig. 13). Figure 14 shows the interface of the developed application, where the doctor can evaluate the pain test and the patient’s pain appreciation.

An application has been developed for the processing and storage of patient and pain sensation data. The doctor discusses the scale with the patient at the start of the test and can relate the pressure exerted, the patient's response and relate both to the simultaneous response seen on the MRI. It is necessary to stablish the pain scale in order to study Fibromyalgia [20, 21]. Figure 14 shows the interface of the developed application for this purpose.

The same printed device has been studied on three printers: one low-end, two mid-range and one high-end. Better quality printers also have a better surface finish (lower Ra), and less dimensional variation. However, one of the mid-range printers (see Table 2), shown to be fully valid for the manufacture of this device in any hospital, it not being necessary to switch to a high-performance printer.

Its dimensional variations require combinations of O-ring groove diameter and the size of the O-ring itself, in order to achieve a compromise between friction and air leakage that allows repeatability in force tests and the application of the desired force.

The type of stimulus to be used, intensity and time of the pressure exerted on the patient's thumbnail, requires different forms of study. Some researchers prefer randomised time and intensity series. In other cases, an increase in intensity accompanied by shorter application times is established. Table 2 shows 6 pre-designed tests relating pressures exerted and application times for a device built by FDM in PLA Tough material, working in oiled or dry conditions.

Table 2

Working pressure and expected force acting on thumbnail
Test number	Air Pressure [bar]	Maximum time [s]	Force on thumbnail [N]
Test number	Air Pressure [bar]	Maximum time [s]	Prototype: PLA Tough (Oiled)	Prototype PLA Tough (Dry)
1	0.5	25	9.45	9.05
2	1	20	13.05	12.17
3	1.5	15	22.45	19.13
4	2	10	30.06	27.07
5	2.5	5	39.14	35.58
6	3	5	50.15	44.10
			O-ring B_Dg = 11.30

The roughness of the printed surfaces and the dimensional tolerances of the pneumatic piston, responsible for exerting the force on the thumb, affect the value of the effort and the necessary repeatability, varying from one printer to another and even between different materials in the same printer (Fig. 5). Nevertheless, it is possible to find suitable designs, combining the printed parts with adequate dimensions of the piston and the O-rings used, which allow ensuring the reliability of the measures for their use in a pain meter (Table 2).

In addition, the final cost of the prototype represents a much lower total cost than a commercial pain meter that cannot be used simultaneously with an MRI because of its metallic elements. This prototype cost includes the physical support of the device obtained by additive manufacturing in ABS, the PCB control printed circuit, two pressure regulators (one manual and one automatic), a variable power supply and the Bluetooth module.

The main features of this pain meter system should be highlighted:

It has the ability to exert variable force on the finger, defined by a neurologist.
Ergonomic design, which allows it to be used in both hands (left or right).
It is a reliable piece of equipment, capable of providing a smooth and continuous movement.
Repeatability of thumb force at the same air pressure.
Control system allows the test to be preselected in time and pressure.
The time to be applied for each pressure can be selected by control system.

This study shows the design of a pain meter, without metallic components, which allows the device to be used while the patient is simultaneously subjected to an MRI scan.
The designed device is useful in the diagnosis of Fibromyalgia.
It has been shown that a pain meter based on a piston actuated with pressurised air, manufactured directly by FDM technology, allows the guarantee of known forces with proven repeatability.
The pain meter designed, has been tested on different materials and on various FDM printers.
While the design is feasible, experience indicates that for other printers or other materials, prior calibration of the forces exerted on the thumb would be required.

Funding

The authors wish to acknowledge to the University Institute of Industrial Technology of Asturias (IUTA), Spain, for the financial support of this investigation.

Competing Interests

The authors have no relevant financial or non-financial interests to disclose.

Author Contributions

All authors contributed to the study conception and design. 3D Modelling of the pain meter was performed by Jose M. Sierra Velasco and Jose L. Cortizo Rodríguez, control system was developed by Juan Díaz González, data collection was performed by Jose M. Sierra Velasco , Mª del Rocío Fernandez Rodríguez and analysis was realized by all the authors. The first draft of the manuscript was written by Mª del Rocío Fernandez Rodríguez and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.

Data Availability

The authors confirm that the data supporting the findings of this study are available within the article. However, the authors undertake to expand the information upon reasonable request.

Culmone, C.; Smit, G.; Breedveld, P.: Additive manufacturing of medical instruments: A state-of-the-art review. Additive Manufacturing, Volume 27, 2019, Pages 461-473, ISSN 2214-8604, https://doi.org/10.1016/j.addma.2019.03.015.
D Printing in medicine, Ed. Kalaskar, D.M. Elsevier. 2017. ISBN: 978-0-08-100717-4.
Aimar A, Palermo A, Innocenti B. The Role of 3D Printing in Medical Applications: A State of the Art. J Healthc Eng. 2019 Mar 21; 2019:5340616. doi: 10.1155/2019/5340616. PMID: 31019667; PMCID: PMC6451800.
Erfan Rezvani Ghomi, Fatemeh Khosravi, Rasoul Esmaeely Neisiany, Sunpreet Singh, Seeram Ramakrishna, Future of additive manufacturing in healthcare, Current Opinion in Biomedical Engineering, Volume 17, 2021, 100255, ISSN 2468-4511, https://doi.org/10.1016/j.cobme.2020.100255.
Sierra, J.M., Rodríguez, J.I., Villazon, M.M., Cortizo, J.L. and Fernandez, M. R. (2020).: Evaluation of a rapid prototyping application for stomas. Rapid Prototyping Journal, Vol. 26 No. 9, pp. 1525-1533. https://doi.org/10.1108/RPJ-07-2019-0181
Rodriguez-García et al (2021).: Transanal endoscopic surgery with a 3D printed device. Techniques in Coloproctology (2021) 25:965–969. https://doi.org/10.1007/s10151-021-02456-1.
Schubert C, van Langeveld MC, Donoso L.A. Innovations in 3D printing: a 3D overview from optics to organs. Br J Ophthalmol 2014; 98(2):159–61.
Gajdoš, Ivan, et al.: Surface Finish Techniques for FDM Parts. Materials Science Forum, vol. 818, Trans Tech Publications, Ltd., May 2015, pp. 45–48. doi:10.4028/www.scientific.net/msf.818.45.
Mertz L.: Dream it, design it, print it in 3-D: what can 3-D printing do for you? IEEE Pulse. 2013 Nov-Dec; 4(6):15-21. doi: 10.1109/MPUL.2013.2279616. PMID: 24233186.
Qureshi A G, Jha S K, Iskander J, et al. (October 11, 2021) Diagnostic Challenges and Management of Fibromyalgia. Cureus 13(10): e18692. doi:10.7759/cureus.18692
NIH, National Institute of Arthritis and Musculoskeletal and Skin Diseases, https://www.niams.nih.gov/health-topics/fibromyalgia/diagnosis-treatment-and-steps-to-take. U.S. Department of Health and Human Services. Accessed February 2023.
Wolfe, F.; Smythe, H.A. et al.: Criteria for the classification of Fibromyalgia. The American College of Rheumatology. Reprinted from Arthritis and Rheumatism. Vol 33. Nº2. Feb. 1990.
Gracely RH, Petzke F, Wolf JM, Clauw DJ.: Functional magnetic resonance imaging evidence of augmented pain processing in fibromyalgia. Arthritis Rheum. 2002 May;46(5):1333-43. doi: 10.1002/art.10225. PMID: 12115241.
Wagemakers S.H.; van der Velden, J.M.; Gerlich, A.S.; Hindriks-Keegstra, A.W.; van Dijk, J.F.M.; Verhoeff, J.J.C.: A Systematic Review of Devices and Techniques that Objectively Measure Patients' Pain. Pain Physician. 2019 Jan;22(1):1-13. PMID: 30700064.
Tousignant, N.: The Rise and Fall of the Dolorimeter: Pain, Analgesics, and the Management of Subjectivity in Mid-twentieth-Century United States, Journal of the History of Medicine and Allied Sciences, Volume 66, Issue 2, April 2011, Pages 145–179, https://doi.org/10.1093/jhmas/jrq024.
Seung Ho Lee,1 Ok-Kyun Kim,1 Hyung-Hwan Baik,1 and Ji-Hye Kim; “Development of a Pain Measurement Device Using Electrical Stimulation and Pressure: A Pilot study”, Journal of sensor; Volume 2018 |Article ID 6205896. https://doi.org/10.1155/2018/6205896.
Hedayatyanfard K, Sadeghi SM, Habibi I.: Introducing a Device for Measuring Pain Intensity; a Letter to Editor. Emerg (Tehran). 2018; 6(1): e32. Epub 2018 May 17. PMID: 30009234; PMCID: PMC6036536.
Vian, Wei Dai and Denton, Nancy L.: Hardness Comparison of Polymer Specimens Produced with Different Processes (2018). ASEE IL-IN Section Conference. 3. https://docs.lib.purdue.edu/aseeil-insectionconference/2018/tech/3 .
Zhiani Hervan S, Altınkaynak A, Parlar Z.: Hardness, friction and wear characteristics of 3D-printed PLA polymer. Proceedings of the Institution of Mechanical Engineers, Part J: Journal of Engineering Tribology. 2021;235(8):1590-1598. doi:10.1177/1350650120966407.
Ahlers SJ, van Gulik L, van der Veen AM, van Dongen HP, Bruins P, Belitser SV, de Boer A, Tibboel D, Knibbe CA.: Comparison of different pain scoring systems in critically ill patients in a general ICU. Crit Care. 2008;12(1):R15. doi: 10.1186/cc6789. Epub 2008 Feb 16. PMID: 18279522; PMCID: PMC2374638.
Bhardwaj, P.; Yadav, R.K.: Measuring pain in clinical trials: Pain scales, endpoints, and challenges. 2015 International Journal of Clinical and Experimental Physiology DOI: 10.4103/2348-8093.169965 g

No competing interests reported.

Development of a Pain Measurement Device Using 3D printing and electronic air pressure control

Status:

Version 1

Abstract

Figures

Article highlights

1. Introduction

2. Pain Measurement Device Design Specifications

3. Pain Measurement device designed.

3.1. Handheld device manufactured by FDM

3.2. Pneumatic Cylinder.

3.3. Measurement of surface hardness of printed parts

3.4. Cylinder bore surface roughness.

3.5. Proposed geometry for cylinder, and O-rings

3.7. Device control elements.

4. Results and discussion

5. Conclusions

Declarations

References

Additional Declarations

Status:

Version 1