Introduction

Currently, two dominant manufacturers of pressure measuring films can be found on the market: Sensor Product Inc. (product name: Surface Profiler Film®)1 and FujiFilm (product name: Prescale)2. The use of the Prescale film for measuring the pressure distribution is increasingly growing. According to the manufacturer (FujiFilm Company), to cover a wide pressure range (from 0.006 MPa to 300 MPa) they supply nine types of Prescale of two types: Two-sheet type and Mono-sheet type. To analyse the obtained results, they provide two products: Fuji Digital Analysis System for Prescale FPD-8010E and Pressure Image Analysis App FUJIFILM Prescale Mobile. Main applications are: checking the seal of cylinder head gaskets; assessment of rubber roll mounting quality; inspection for damage or alignment of rollers used for pressing electrode plates; checking roller pressure uniformity when attaching the cover glass to touch panels or LCDs and others2.

The Prescale films have been used in medical analysis for years3. Plantar pressure distribution is very important in various medical conditions (clubfoot, hallux valgus, trophic ulceration, rheumatoid arthritis, marfan syndrome, etc.)4. The most popular methods of plantar pressure measurement are: footprinting mat, foil pedobarography, optical pedobarography, photoelasticity, liquid crystals, capacitance mats, piezoelectric multielement plates, sensitive resistive mats, dye-based pressure sensitive films, and piezoelectric polyvinylidene fluoride foil5,6,7,8,9,10,11,12,13,14,15. Silvino et al.5 use the Harris and Beath footprinting mat for the diagnosis of pressure metatarsalgia and other disorders of the foot. The recording of peak pressures under the feet while standing or walking, by using foil pedobarography, was described in6. The mechanism in operation at an illuminated plastic to glass interface under load (optical pedobarography) was described in7. By using this technique the static and dynamic pressure distribution can be analysed. The quantitative data and new parameters representative for human posture (standing and walking) were proposed in8. The light reflected from a special sandwich plate produces an interference pattern which is a function of the contact pressure. The difference in the distribution of forces between walking and running was presented in9. The average duration of foot contact with the ground in running is decreased by 0.18 s in comparison to walking. Gerber10 proposes a system for measuring the dynamic pressure distribution under the foot. His improved system generates up to 160 pictures per second during the measurement of the pressure at a Nicol mat. Gross and Bunch11 use piezoelectric ceramic squares to measure vertical plantar stress during dynamic activities (in shoes). The value of discrete stress analysis in shoes can be used during their designing for sport activities. For normal walking Corrigan et al.12 determined the effect of the heel height on the foot loading. The height of the heel could contribute to overload of the distal forefoot, because the contact area between foot and ground decreases. Bennett and Duplock13 presented the use of the force plate device (for measuring discrete areas of pressure beneath the human foot). The results were in close agreement with other studies with comparable systems. The use of electronic transducers and the dye-containing microcapsules, helpful for people who have a qualitative or a quantitative change in sensation in hands, was presented in14. These two methods can help patientsto calibrate the remaining sensations. The use of piezoelectric polyvinylidene fluoride foil for foot pressure measurement outside the laboratory was described in15. The design of the whole system (the shoe insole, amplifier, and wireless system for measurement) was described along with an analysis of the results.

The Prescale and other films are often used to measure the bite force. It is a crucial parameter influencing the masticatory system. Maximum bite force can be obtained by using the pressure measurement films systems16,17,18,19. Imammura et al.16 use pressure-sensitive film (Occluzer) to examine masseter muscle activity of 10 people using the ANOVA and the Benferroni methods to analyse the results of the occlusal contact points at different values of voluntary contraction. Suzuki et al.17 determined an error discrimination rate of 16% between 50 wearers of non-adjusted complete dentures and 50 wearers of adjusted prostheses. For 262 participants Morita et al.18 analysed the masticatory performance for variables like physical status, oral conditions, and oral functions. All variables were analysed using univariate and multivariate analyses. Iwasaki et al.19 studied the association of the maximum bite force and an objective measure of oral function with morality in older adults. The maximum bite force was independently associated with all-cause mortality in older adults. The relationship between lip closing force, occlusal contact area, and occlusal force after orthognathic surgery in skeletal Class III patients was studied in20. Surgery can improve contact area, occlusal, and lip closing force. The comparison of three occlusion analysis methods was presented in21 (scanning of articulating paper marks, dental prescale occlusion analysis system, and a modified virtual occlusion construction method). Moroi et al.22 use the Dental Prescale to examine the influence of the magnitude of setback in sagittal split ramus osteotomy on the contact area and the bite force without relapse after surgery. Improvement in the biting force of patients after receiving implant-supported overdentures was presented in23. More uniform force transfer is obtained with the straight-geometric design of the trimming line24. The Prescale pressure measurement is widely used in the design of medical tools. Carassiti et al. used the pressure measurement film to determine the forces and pressure distribution applied by laryngoscope blades (GlideScope and Macintosh laryngoscopes) to the soft upper airway tissues25. The Prescale film increases the resolution of the pressure measurement (by using piezoelectric sensors on six points of the laryngoscope) done by26. Average pressure, force, and interface contact area were measured for new designed locking plates (two different plates) applied to fix fresh goat tibiae by Wei et al.27. By statistical analysis, they compare the parameters of these two plates geometry. The measurement of pressure and contact area in the joint can be done on dead patent or on laboratory stands. Prescale film inserted into the open radiocarpal joint of the forearm and hand (in Kienböck’s disease) was used to measure pressure in the joint. The films were analysed using a special optical densitometer to convert the colour density into a numerical value of the pressure28. The measurement of contact area and pressure in the tibiotalar joint after isolated medial malleolar fracture was presented in29. Research was carried out on ten cadavers. The differences were significant for the normal tibiotalar joint and with fractures of 2 mm displacement30. The actual mechanics of contact with an artificial tibiofemoral joint was studied by Liau et al.31. The FE model developed by the authors consisted of the experimental results (contact area and pressure measurements). Clark et al.32 determined the contact area, mean, and maximum pressures in the healthy feline patellofemoral joint. Short and long duration trials (2 s and 5 min) of loading force were measured. For long duration trials the contact area was up to 33% bigger in comparison to the short duration trials. Wilson et al.33 proposed using the Iscan system to measure forces and pressure in the natural patellofemoral joint instead of the Prescale film. To restore anterior inferior glenoid bone loss the autologous distal clavicle and autologous coracoid bone grafts were proposed34. Contact characteristics in fixation of anterior cruciate ligament hamstring grafts were presented in35. The pressure around the interference screw used in the experiment was measured by prescale film for direct fixation technique and bone wedge interposition with 8 and 9 mm interference screws. When comparing the Prescale film method to the real-time thin film pressure transducer, for measuring the tibiofemoral contact area in total knee arthroplasty, the result obtained by Prescale film was lower than for the K scan sensor36. The real time analysis by using sensors allows to measure and record contact areas and pressures under dynamic load, flexion, and torsion. Liau et al.37 proposed a two-dimensional finite element model of the Prescale film. Classical Hertzian theory was determined to validate the FE model. Pau et al.38 compare the contact area between the steel sphere and the cylindrical steel plane (sphere-plane contact). The two tested non-invasive methods (ultrasonic and Prescale film) had a good agreement in the size and shape of the nominal contact area. For dynamic contact and pressure higher than 1 GPa the ultrasonic method can be used. Zdero et al.39 compare the ultrasound measurement and Prescale film methods to determine the total contact area of the knee arthroplasty.

Much research on using Prescale film is connected with machine design. The authors40 used the mono-sheet type S of the Prescale pressure films to determine the contact zone between a railway vehicle and the rail. The use of the Prescale film to non-invasive experimental measure of the pressure distribution in the aerostatic thrust bearing was presented in41. The non-uniform pressure model to perform thermal analysis can lead to avoiding excessive wear of the dry clutches42. In developing a new clamping mechanism for a cylindrically structured polymer electrolyte membrane fuel cell, the Prescale film was used to measure average contact pressure and clamping pressure. The new clamping system had a higher contact pressure distribution (33%) and was less contact resistant (18%)43. The clamping pressure distribution is very important during the design of the PEM water electrolyser. Sealing materials (Teflon, Viton, ethylene propylene diene monomer rubber, and nitrile rubber) were studied in clamping tests with the use of Prescale film. The pressure area distribution depends on the thickness and stiffness of the gasket material44. The proton exchange membrane fuel cells were studied under different loading conditions by Jia et al.45. The interface and the pressure applied to the teat tissue by the teat cup liner during milking can be measured using a hollow artificial teat made of silicone with Prescale film46. The tooth instantaneous and total contact pattern, determined during the design of gears, are important parameters influencing the correctness of tooth meshing. Fudali et al.47 present an analysis of the tooth contact pattern and the contact line (along with a description of the method for determining these parameters based on film scans) for cylindrical gears with a sinusoidal profile. For double-enveloping worm gear the method and results of contact pattern were presented in48. These results were compared to CAD and FEM analysis, obtaining a significant convergence of the numerical and experimental methods, which confirmed the possibility of applying this method to the study of tooth contact pattern.

The production of piezoelectric sensors is limited by their complexity and cost. The flexible pressure sensors are widely used in robotics, smart devices, health, medicine, etc. By changing the concentration of each chemical element during the fabrication process the parameters of the sensors can be improved49. Chuai et al.50 designed, fabricated, and characterized chips which show a larger linear response range and a better linear output compared to touch mode capacitive pressure sensors.

One of the optical techniques in air is pressure-sensitive paint (PSP). This method uses the phenomenon of pressure with the partial pressure of oxygen51,52.

A more advanced method is the surface stress-sensitive film (S3F). It can simultaneously capture changes in surface pressure and shear force in the air and water53 The use of pressure sensitive paint measurement on the blade with actual cooling structure was presented in54. The characterisation of blade cooling failure was proposed by considering the span-wise distributions of film effectiveness and gas temperature.

Non-contact pressure film (optical pressure sensor) measures the pressure change in variation in fluorescent intensity of the microspheres in the film. The FE model of the film deformation under pressure was developed and calibrated by experimental method55.

To measure pressure in the gas foil journal bearings, polyvinylidene fluoride piezoelectric-film sensor technology can be used56. The gas film pressure distribution can be determined in the axial and circumferential directions by using the measured pressure data and firing algorithm.

Other research using Prescale film was done by Li et al.57. They conducted more than 240 experimental tests of the rock type (granite, limestone, shale, green sandstone, red sandstone) and fracture size in the real contact area by using pressure-sensitive films. The higher the normal stress, the more significant the lithological effect on the contact area was. In paper58 the real contact area in rock joints (sandstone joints) was presented. The normal stresses were set to 50 MPa. The real contact area increases hyperbolically with increasing normal stress in the joint. To study the behaviour of rock under static and dynamic load the split Hopkinson pressure bar can be used59,60.

The stresses transmitted by a compressed powder onto the base and side walls can be measured by a Prescale film or by the thin flexible capacitive sensor61.

In numerical studies many assumptions must be made: on the contribution of microcapsules to the microcapsules contribution on the sheet, on the property of base and the receiver, and on the material model (equivalent linear elastic model, linear Drucker-Prager material model)40.

The film manufacturer recommens that after a certaintime period - “as soon as possible”. The delay time can influence the measurement results. To ensure the appropriate measurement, it is important to perform the scan in time according to the manufacturer’s instruction and to report the scan delay62.

The traditional digital image correlation method is still widely used, but in recent years with machine learning63. Kalina et al.64 present a procedure in which calibration curves are determined by using an office scanner. This method can be used without using a specialized scanner and software, which are usually distributed by the pressure measurement film manufacturer.

In this publication the microstructural observation and measurements of the transfer sheet within Sensor Product Inc. SPF-D two-sheet pressure-measuring films were done to present changes in microstructure under applied force. Both qualitative and quantitative analyses were performed. A key finding was the observation of a consistent microcapsule rupture mechanism, regardless of the load force. The percentage of microcapsules initial damages was measured. A non-uniform distribution of microcapsules, which often formed “grape-shaped” clusters, was observed and measured. None of the previous research was focused on that issue. In addition, the research work included an analysis of the chemical composition of the active surfaces of both sheets.

Materials and methods

Pressure measuring films

The pressure measuring films are an alternative to strain gauge maps. Their main advantages over strain gauge maps are:

  • smaller dimensions - especially the thickness of the film - so they allow the measurement of pressure distribution, in places inaccessible to electronic pressure maps,

  • the possibility of cutting to adjust the shape of the film - thus reducing the amount of film used,

  • for unit measurements, the cost of the film can be less than the cost of the strain gauge mat,

if the permissible loads are significantly exceeded during measurement and the sheet is destroyed - no measurement electronics are damaged that would require repair - and the value of the loss is equal to the price of the destroyed part of the sheet, However,, there also disadvantages of this solution, which include:

  • the films are single-use, which translates into additional costs, especially when it is necessary in case of the need to repeat the measurement,

  • in order to accurately analyse the films, it is necessary to have a suitable scanner that makes it possible to allows scanning the image of the film with sufficient accuracy. Also, it is necessary to have additional software that allows for the analysis of scanned images,

  • humidity and ambient temperature have a noticeable effect on the measurement results, so in the case of a series of measurements, it is necessary to carry them out under similar conditions,

  • additional requirements for the duration of the measurement (the measurement must be long enough for the correct colouring of the developer sheet),

  • expiration date - the sheets have a certain shelf life,

  • in the case of two-sheet films, there may be additional problems related to the need to embed the film so that the sheets do during the measurement they do not move one another each other on the measurement plane, during the measurement.

  • the mating surfaces of the parts should be properly prepared,

  • friction between the surfaces of the foil should be avoided,

  • when cutting the film some of the microcapsules rupture, so that the colouring substance of the film can get on the fingers of the person making the measurement. This can result in the film being stained when touched even before the actual measurement is performed.

Types of the pressure measuring films

Films can be classified according to two main criteria, i.e. the range of values of measured pressures and the number of sheets comprising one set. Thus, in the case of the latter criterion, we distinguish between films of the following types:

  • mono-sheet type - in this variant there is no separate transfer sheet,

  • two-sheet type, where one film acts as a transmitter (transfer sheet) and the other as a receiver (developer sheet). Only the receiver film is analysed.

Tables 1 and 2 show sequentially selected film parameters according to manufacturers: Sensor Products Inc., and FujiFilm.

Table 1 Selected parameters of pressure measurement films offered by sensor products Inc1.
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Table 2 Selected parameters of pressure measurement films offered by FujiFilm2.
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Fig. 1
figure 1

shows photographs of the two-sheet set, designated SPF-D.

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Figure 1 SPF-D film set where the left side shows the transmitter film roll and the right side shows the developer film roll.

The structure and principle of operation of two-sheet films

The two-sheet type film set consists of two types of sheets - i.e. a film that acts as a transmitter (Transfer Sheet), hereafter referred to as the transmitter, and a sheet that acts as a receiver (Developer Sheet), hereafter referred to as the receiver. Both sheets have a layer of polyester base with a thickness of about 4 mils (0.1016 mm).

The transfer sheet has a layer on which microcapsules are placed, which contain a substance that, when released, reacts with the colour developing layer. The colour fixing layer, in turn, is on the developer sheet. Figure 2 shows images of the microstructures of the two films before measurements were made.

Fig. 2
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Microstructure on the surface of the pressure measuring film: on the left—the transfer sheet and on the right—the developer sheet.

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Note that both films have two sides, i.e. glossy and matte. For two-sheet type films, place the sheets between the pressed surfaces, making sure that their matte sides are facing each other. As a result of pressing the matte surfaces of the parts against each other, rupture of the microcapsules in the contact zone occurs. Then, the reagent released from the microcapsules reacts with the developing layer of the developer sheet - as a result, the developer sheet is coloured. The principle of operation is illustrated schematically in Fig. 3.

Fig. 3
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Principle of operation of the two-sheet type films.

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Depending on the type of load (continuous or momentary), it is recommended that the load application time be a minimum of 5 s (for momentary load) and 2 min (for continuous load). Furthermore, it should be remembered that the films should be exposed to the environmental conditions (temperature and humidity) under which the measurement is made for a minimum of 30 min before the start of the measurement2. It is also worth noting that after the measurement is completed and the sheets are separated from each other, it is advisable to wait another 10–20 min before starting the analyses, as the dye substance can still react with the developing layer.

Fig. 4
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shows how to read the measured pressure based on the colour of the sample from Sensor Product Inc1.

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Figure 4 Relationships between sheet colour intensity and measured pressure for the Sensor Product Inc. films (The chart is for illustrative purposes only and helps to explain how to evaluate the pressure value. The actual curves can be found in the manufacturer’s brochure.)

It is worth noting here that the manufacturer FujiFilm, for a given type of film, provides families of characteristics for reading pressure values. It provides different curves for one type of sheet dividing them into graphs for continuous or instantaneous load measurements. In addition, for each group, it adds 4 curves depending on the prevailing conditions (temperature and humidity). An auxiliary chart is used to select the appropriate curve. Figure 5 shows a graph for instantaneous load measurements for FujiFilm sheets2.

Fig. 5
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Relationships between sheet colour intensity and measured pressure for the FujiFilm (The chart is for illustrative purposes only and helps to explain how to evaluate the pressure value. The actual curves can be found in the manufacturer’s brochure.)

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Experimental procedure

Within the framework of the conducted experimental research, two key stages can be distinguished. Stage 1 involved the preparation of specimens based on pressure film manufactured by Sensor Product Inc. (type SPF-D) and a test stand on which it was possible to load them with a preset load, the value of which was derived from the pressure film measurement range. Stage 2 of the research included the determination of selected parameters of the microstructure of the transfer sheet. As part of this stage, the microstructure of both the developer sheet and the transfer sheet was examined before loading. The results obtained served as reference samples to determine what changes in the microstructure of the transfer sheet occurred as a result of loading the samples in stage 1.

Stage 1: preparation and loading of samples at the test stand

The main purpose of the first stage of the research was to obtain a set of samples with a simple shape (flat rectangle) for further analysis of their microstructure properties, and the transfer sheets in particular. Type SPF-D pressure films (Table 1), manufactured by Sensor Product Inc., were used for the study. Specimens with a small load area were chosen due to the desire to minimise the impact of manufacturing errors of the components of the sample loading station on the measured pressure values. Each of the samples was loaded with a different force so that the measured pressure values (resulting from the set force and the area of contact of the surfaces pressed against each other) were within the range specified by the manufacturer. In addition, some samples were loaded so that the measured pressures were lower and higher than those included in the range. The schematic diagram of the specimen loading station is shown in Fig. 6.

Fig. 6
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Test stand schematic.

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As can be seen in Fig. 6, the pressure measuring films were placed between two gauge blocks, which were made according to EN ISO 3650 in accuracy class 1. The size of the smaller gauge block (30 mm x 9 mm) determined the area of the surface loaded with the given force. The pressure measurement films were trimmed to a size larger than the loaded area of the smaller gauge block. This procedure was aimed at, among other things:

  • eliminating errors resulting from trimming the pressure films to the size of the smaller gauge block,

  • avoiding damage or destruction of the microstructure of pressure films (especially transfer sheets) at the edges of the loaded area as a result of trimming the sheets with scissors or during sample handling (abrasion and compression of the edges with fingertips causes destruction of some of the microcapsules located near the edge of the sample),

  • improving contact conditions (edge phenomena) and reducing wear of gauge blocks,

  • making it possible to measure the microstructure at the edges of the contact area, so that imprinted and unimprinted areas can be compared directly on a single sample,

  • reducing sample preparation and setup time for the stand test.

To improve the pressure distribution and reduce wear, gauge blocks were placed between flat plates. This avoided direct contact of the gauge blocks with the head and table of the TOX® PRESSOTECHNIK used to apply the load.

Stage 2: examination of the microstructure of samples

Examination of the Microstructure of the Samples Microscopic observations of the film surface were made using a Hitachi S3400N scanning electron microscope mainly in backscattered electron mode (BSE-3D) as well as in secondary electron mode (SE), under high vacuum (< 1 Pa) and low vacuum (approximately 30–50 Pa), with an accelerating voltage of 15–20 kV. To enhance imaging, the surfaces of the examined sheets were coated with a thin layer of gold using a Cressington 108auto vacuum sputter coater. Observations and measurements were made to identify and characterize the morphology of components on the film surface, with magnifications ranging from 30x to 5000x. The chemical composition was also analysed in selected micro-areas on the surface of both types of films using the Thermo Scientific™ UltraDry EDS detector for chemical composition analysis by X-ray dispersive spectroscopy (EDS) and Thermo Scientific™ NORAN™ System 7 software. Three areas were considered in the observations and measurements:

  • Area 1 (undamaged state): Measurements of all spheroidal particles were conducted in at least six different areas to calculate their average values as a reference for subsequent measurements.

  • Area 2 (transition zone): The transition between undamaged and damaged areas was observed to illustrate the differences resulting from the applied force that causes contact between the sheets and the rupture of some microcapsules.

  • Area 3 (contact zone): Measurements of both intact and ruptured particles in contact with the developer sheet surface were performed.

Quantitative evaluation was conducted to determine the proportion of damaged particles relative to all identified particles in the measured area and relative to the calculated average of all identified particles. The proportion of undamaged particles was calculated similarly. On the basis of the measurements, a chart was compiled showing the dependence of the proportion of damaged particles, categorized by size classes, relative to all identified particles. Observations and quantitative measurements were made at a magnification of 200x, with each analysed area having defined dimensions. Digital processing of microphotographs taken with a Hitachi S3400N scanning electron microscope (SEM) was carried out using Leica Application Suite V4.13.0 software. Figure 7 shows photographs of the test stand used to examine the microstructure of pressure measuring films.

Fig. 7
figure 7

Examination stand photos: (a) Hitachi S3400N scanning electron microscope, (b) Cressington 108auto sputtering machine.

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Results and discussion

Developer sheets scans obtained in the first stage of the experiment

This part of the article shows the developer sheets scans obtained in the first stage of the experiment. The load application time was 120 s. The experiment was carried out under ambient conditions: temperature 23 °C humidity 53% RH. Additionally, after the experiment, the films were left to expose the laboratory ambient conditions for a period of 30 min.

Table 3 shows the results of the samples that were selected for the second stage of the experiment. In addition, the table shows the results of analysing the scans of these sheets using a developed programme based on image processing.

Table 3 Developer sheets scans obtained during the first stage of the experiment.
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Microstructure of the transfer sheet: qualitative analysis

Microstructure of the Transfer Sheet – Qualitative Analysis: Based on the observations of scanning electron microscopy (SEM), the presence of particles with spherical morphology was confirmed to be the dominant phase on the surface of the transfer sheet (Figs. 8, 9, 10, 11, 12 and 13).

The surface exhibited microcapsules of varying sizes (diameters), distributed chaotically with a clear tendency to form local aggregates with a grape-shaped morphology (Figs. 9, 10, 11, 12 and 13). This clustered form indicates non-uniform distribution and the presence of areas with increased microcapsule density locally within micro-areas.

Fig. 8
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Surface of the transfer sheet, showing the distribution and morphology of two distinct phases: spheroidal microcapsules (gray areas) and smaller, irregularly shaped microcrystals (bright areas).

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Fig. 9
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Micro-area fragment on the transfer sheet surface, showing the spherical morphology of microcapsules, with varying sizes and random distribution as aggregates or individual particles: (a) magnification 300x at 50 Pa; (b) magnification 400x, < 1 Pa; BSE-SEM mode.

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Fig. 10
figure 10

Micro-area fragment on the transfer sheet surface, showing the spherical morphology of microcapsules, with varying sizes and distribution, as well as very fine particles of the second phase (bright areas): (a) magnification 500x at 50 Pa; (b) magnification 500x, < 1 Pa; BSE-SEM mode.

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Fig. 11
figure 11

Micro-area fragment on the transfer sheet surface with visible clusters of microcapsules of varying sizes, forming grape-like morphology aggregates, and very fine, irregularly shaped particles (bright areas): (a) magnification 500x, SE-SEM mode, < 1 Pa; (b) magnification 500x at 50 Pa, BSE-SEM mode.

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Fig. 12
figure 12

Micro-area fragment on the transfer sheet surface, showing microcapsule aggregates: (a) magnification 1000x; (b) visible partial damage to a microcapsule in the form of a microcrack, magnification 1500x, BSE-SEM mode at 50 Pa.

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Fig. 13
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Micro-area on the transfer sheet surface, showing a cluster of aggregated spherical microcapsules of varying sizes: (a) magnification 2000x, SE-SEM mode; (b) visible irregular microcrystals of the second phase (bright areas), magnification 2000x, BSE-SEM mode.

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On the basis of microscopic observations of the transfer sheet surface in macro- and micro-areas, it was determined that the functional layer deposited on a transparent PET (polyester) substrate exhibits distinctly heterogeneous morphology due to the presence of several types of dispersed particles. The active structure consists of spherical microcapsules with the diameters ranging from approximately about 1.0 to 40.0 μm, characterized by a smooth surface and good separation from the matrix, as well as inorganic microcrystals with irregular shapes and sizes ranging from approximately 0.5 to 5 μm, distributed irregularly throughout the layer volume.

To identify the chemical composition and characterize the phase components of the active layer of the Fuji Prescale system transmitting film, a micro-area analysis was performed using the scanning electron microscopy (SEM) method with X-ray detection (EDS). The measurements were made on a general scale, in the area of approximately 600 × 500 μm at 200× magnification (Fig. 14; Table 4), and point-wise, in selected microstructures at 1000× magnification – including individual microcapsules and bright microcrystals (Figs. 15 and 16; Tables 5 and 6).

In the general analysis, including a mixture of morphological elements, the dominant share of carbon (41.5%) and oxygen (43.5%) was identified, indicating the presence of a polymeric organic matrix and a microcapsule core liquid. Among the inorganic elements, a significant share of calcium (9.4%), sodium (3.8%), and sulphur (1.6%) was observed, suggesting the presence of mineral functional additives – probably calcium salts and sodium sulphur compounds. This composition represents the chemical average of the entire functional layer, containing both capsules and inorganic dispersed particles.

In the point analysis performed on the surface of a single microcapsule (over 1000×), a very high share of carbon (79.9%) and oxygen (9.3%) was observed, which clearly confirms that the capsule probably consists of organic material – both in the shell and in the liquid core. The significant content of sodium (7.7%) and sulphur (2.3%) may indicate the presence of organic compounds containing sulphur and sodium atoms, used as components of the dye or auxiliary agents (e.g., emulsifiers, solvents). After calculating the composition without carbon and oxygen, these elements become dominant components, indicating a significant share of inorganic additives in the organic environment of the capsule.

In turn, microanalysis of bright microcrystals, visible as highly contrasting objects in the SEM-BSE image, showed a very high share of calcium (97.8% after excluding C and O) and the presence of oxygen and carbon in the full composition, with a complete lack of sulphur. On this basis, it was assumed that these are particles probably consisting of calcium carbonate (CaCO₃), occurring in the form of sharp-edged microphases. Their functions may be: rheological stabilisation of the active layer, structural filling, or control of microcapsule dispersion. The presence of CaCO₃ was also confirmed in the general analysis, suggesting its wide distribution in the layer.

The combined qualitative and semi-quantitative analysis indicates that the Fuji Prescale system transmitting film consists of a heterogeneous mixture of: (1) organic microcapsules, probably containing colouring compounds and sodium-sulphur additives, (2) a polymer matrix binding the active layer elements, and (3) dispersed inorganic particles, mainly calcium carbonate, performing mechanical and stabilising functions. The composite nature of the layer and the high content of organic and mineral components indicate the functional integration of different material phases oriented at ensuring the film’s reactivity under pressure and maintaining structural coherence.

Fig. 14
figure 14

Micro-area on the surface of the transfer film, where the EDS-SEM chemical composition analysis was performed and the EDS spectrogram from this area, BSE-SEM mode at 30 Pa, magnification 200x.

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Fig. 15
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Micro-area on the surface of the transfer film, where the EDS-SEM chemical composition analysis was performed, and EDS spectrograms from the analysis in this area, BSE-SEM mode, magnification 1000x.

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Fig. 16
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Micro-area on the surface of the transfer film, where the EDS-SEM chemical composition analysis was performed, and EDS spectrograms from the analysis in this area, BSE-SEM mode at 30 Pa, magnification 3000x.

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Observations performed by scanning electron microscopy (SEM) in the backscattered electron detection mode (BSE) also showed that a very large number of small, strongly contrasting microcrystals are present in the structure on the surface of the developer sheet. These crystals appear as bright, irregular inclusions embedded in a resin matrix and are morphologically similar to those previously observed in the transmitting film layer. On the basis of their morphological characteristics and particular chemical composition (high calcium content), it can be assumed that these are the same mineral particles, probably calcium carbonate (CaCO₃). However, in the receiving film they are much more numerous and seem to be more strongly integrated with the polymer matrix, which may result from differences in the technology for producing the carrier layer and its mechanical and visual functions. The even distribution of these microphases and their embedding in the resin matrix indicate their durable and structural nature in the composition of the developer film (Figs. 17 and 18).

After calculating the elemental composition of the surface of the developer sheet, excluding carbon and oxygen, the dominant component was calcium (64.1% by mass), which indicates the presence of an expanded mineral phase, probably in the form of calcium carbonate (CaCO₃). Significant shares of titanium (5.8%) and zinc (5.2%) suggest the presence of titanium dioxide (TiO₂) and zinc compounds, which may perform pigmentary, optical, or auxiliary functions in the colour development mechanism. Aluminium (5.4%) and silicon (5.6%) occur in comparable amounts, which may indicate the presence of aluminosilicates or mineral admixtures that modify the properties of the carrier layer. The presence of sodium (8.9%) and sulphur (2.3%) should be interpreted as the effect of material transfer from the transfer film, because the tested sample came from the use of the system after the trial test (Table 7). This means that these elements may represent components of the dye or chemical carrier mechanically transferred to the surface of the developer film as a result of contact under pressure. The surface structure formed in this way indicates the complex nature of the active layer, which integrates mineral components with deposits of reactive residues from the transfer film.

Fig. 17
figure 17

Micro-area on the surface of the developer sheet, where the EDS-SEM chemical composition analysis was performed, and EDS spectrogram from the analysis in this area, BSE-SEM mode at 30 Pa, magnification 3000x.

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Fig. 18
figure 18

Developer sheet surface in an undamaged state, not subjected to applied forces: (a) magnification 200x; (b) magnification 1000x; (c) magnification 2000x; (d) magnification 5000x, BSE-SEM mode.

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Table 4 Content (% by mass) of elements identified during EDS analysis in the micro-area shown in Fig. 14.
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Table 5 Content (% by mass) of elements identified during EDS analysis in the micro-areas shown in Fig. 15.
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Table 6 Content (% by mass) of elements identified during EDS analysis in the micro-areas shown in Fig. 16.
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Table 7 Content (% by mass) of elements identified during EDS analysis in the micro-area shown in Fig. 17.
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The entire structure is embedded in a thin layer of polymeric binder, likely acrylic or polyvinyl in nature, which serves as a binding matrix. This layer exhibits slight surface micro-roughness and non-uniform thickness, particularly near clusters of larger particles. The spatial arrangement of microcapsules and their distribution suggest a controlled application technique (e.g., roll coating or screen printing), with local areas of increased density and regions with reduced fill levels. The microstructural characteristics indicate a complex multiphase composite system optimized for the mechanical release of dye.

Microcapsules on the transfer sheet surface, under the influence of the applied force in contact areas with the developer sheet, undergo mechanical damage and rupture with varying intensity depending on the magnitude of the force (Figs. 19, 20, 21, 22, 23, 24, 25, 26 and 27) and with different percentage contributions based on their size. Furthermore, it was noted that in non-loaded areas, damaged particles are present sporadically and randomly Figs. 12(b), 13 and 25. Quantitative evaluation included undamaged areas to validate and determine the average proportion in observed measurement areas, accounting for a reference average of initially damaged microcapsules. The micro-areas of contact on the surface of the transfer were evaluated, by measuring the diameter and number of visible intact and damaged particles (Figs. 25, 26 and 27). Measurements were performed on SEM micrographs recorded at 200x magnification in BSE mode, considering a minimum of six analysed micro-areas.

Fig. 19
figure 19

Transfer sheet surface in the transition zone between the undamaged (non-contact) and damaged areas due to tool interaction and contact with the developer sheet: (a) applied force 2.75 kN, magnification 30x; (b) applied force 3.08 kN, magnification 50x, BSE-SEM mode at 50 Pa.

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Fig. 20
figure 20

Transfer sheet surface in the transition zone between the undamaged (non-contact) and damaged areas due to tool interaction and contact with the developer sheet: (a,b) applied force 2.61 kN, magnification 100x, (b) higher contrast, BSE-SEM mode at 50 Pa.

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Fig. 21
figure 21

Transfer sheet surface in the transition zone between the undamaged (non-contact) and damaged areas due to tool interaction and contact with the developer sheet at an applied force of 3.08 kN: (a) magnification 200x, BSE-SEM mode, < 1 Pa; (b) higher contrast, BSE-SEM mode at 50 Pa.

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Fig. 22
figure 22

Varying degrees of microcapsule damage of different sizes in randomly selected micro-areas on the transfer sheet surface under the influence of two different applied forces: (a,b) 0.48 kN; (c,d) 3.08 kN, magnification 1000x, BSE-SEM mode at 50 Pa.

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Fig. 23
figure 23

Varying degrees of microcapsule damage of different sizes in randomly selected micro-areas on the transfer sheet surface under the influence of different applied forces: (a) 0.48 kN; (b) 0.92 kN; (c) 1.30 kN; (d) 1.56 kN; (e) 1.86 kN; (f) 2.20 kN, magnification 500x, BSE-SEM mode.

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Fig. 24
figure 24

Varying degrees of microcapsule damage of different sizes and distribution in randomly selected micro-areas on the transfer sheet surface under the influence of different applied forces: (a) 2.24 kN; (b) 2.61 kN; (c) 2.75 kN; (d) 3.08 kN, magnification 500x, BSE-SEM mode.

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Fig. 25
figure 25

Micro-area on the transfer sheet surface with an example of the identification and marking of microcapsules, categorized as intact (red) and damaged (blue) during the diameter measurement process: (a,b) magnification 200x, BSE-SEM mode at 50 Pa.

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Fig. 26
figure 26

Examples of different micro-areas on the surface of the transfer sheet selected for measuring the degree of microcapsule damage, classified into two types: (1) intact and (2) damaged, under the influence of various applied forces: (a) 0.48 kN; (b) 0.92 kN; (c) 1.30 kN; (d) 1.56 kN; (e) 1.86 kN; (f) 2.20 kN; BSE-SEM mode, magnification 200x.

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Fig. 27
figure 27

Examples of different micro-areas on the transfer sheet surface selected for measuring the degree of microcapsule damage, categorized as intact and damaged, under the influence of different applied forces: (a) 0.48 kN; (b) 0.92 kN; (c) 1.30 kN; (d) 1.56 kN; (e) 1.86 kN; (f) 2.20 kN, BSE-SEM mode, magnification 200x.

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On the basis of macroscopic and microscopic observations of the developer sheet surface, it was determined that the active layer responsible for the colour reaction with the dye released from the transfer sheet consists of a thin functional coating deposited on a transparent polyester substrate. This layer exhibits a heterogeneous surface morphology due to the presence of numerous fine particles ranging from approximately 0.2 to 5 μm in size, embedded in a thin polymeric matrix (Fig. 18).

BSE-SEM images reveal the presence of moderately contrasting particles exhibiting varied morphologies—irregular granular and spheroidal. These particles are relatively uniformly distributed, although local microaggregates are observed. EDS analysis indicated a predominance of calcium (bright particles) within a matrix rich in carbon and oxygen (Table 7).

The binding matrix, in which the active particles are suspended, is likely composed of a thin-layer polymer (e.g., acrylate or epoxy resin) with a thickness of a few micrometres. This layer exhibits slight micro-roughness and a characteristic microporous surface structure, facilitating dye absorption (Figs. 17 and 18). In areas of the developer sheet subjected to localized pressure, distinct surface cracks were observed in the active layer, resembling a mud-crack pattern. These cracks are superficial, irregular, and branched, forming a network of separated segments. Their presence indicates the brittle nature of the active layer and its limited ability to undergo plastic deformation under localised pressure. This phenomenon may be due to localized mechanical overloading, low elasticity of the polymeric binder, and residual internal stresses in the thin film coating (Figs. 28 and 29).

Fig. 28
figure 28

Developer sheet surface in a damaged state, showing changes due to applied force and contact with the transfer sheet surface: (a) magnification 100x; (b) magnification 200x; (c) magnification 500x; (d) magnification 1000x, BSE-SEM mode at 30 Pa.

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Fig. 29
figure 29

Comparison of the developer sheet surface: (a) in the damaged area, in the contact zone with the transfer sheet under applied force; (b) in the transition zone between the damaged and undamaged areas, without contact with the transfer sheet, magnification 50x, BSE-SEM mode; (c) magnification 500x; (d) magnification 1000x, BSE-SEM mode at 30 Pa.

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The observed cracks are typical of brittle, thin layers containing a crystalline granular phase (e.g., calcium, silicon, aluminum compounds and their mixtures), particularly when the coating exhibits low internal cohesion. They may result from the following factors:

  • rapid expansion under pressure,

  • weakening of the binder due to contact with the dye or moisture,

  • residual stresses from the curing or drying process.

Microstructure of the transfer sheet: quantitative analysis

Microstructure of the Transfer Sheet – Quantitative Analysis: Based on the microscopic images, a statistical analysis was conducted to determine, among other things, the influence of loading on the number of ruptured microcapsules. Figure 30 presents the percentage distribution of intact microcapsules measured in randomly selected micro-areas from 6 different samples of load force (in the non-loaded part, with dimensions of 640 × 480 μm). Figure 31 shows the minimum, maximum, and calculated average values with standard deviation for intact microcapsules based on Fig. 30. The coefficient of variation for the calibration samples was 1.4%.

Fig. 30
figure 30

Summary of measurement results for the number of all microcapsules in undamaged areas, expressed as a percentage (proportion) of intact microcapsules relative to their total number.

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Fig. 31
figure 31

Summary of the average number of intact microcapsules in undamaged areas relative to the average number of all measured microcapsules, including minimum and maximum values.

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The average number of intact microcapsules was approximately 900 in an area of 640 × 480 μm, accounting for about 98% of all identified microcapsules. Consequently, approximately 2% of the microcapsules were initially damaged, and this value was subtracted from subsequent measurements.

Figure 32 presents the measurement results of the average number of damaged microcapsules relative to the calculated average number of all identified microcapsules within the examined micro-area on an SEM micrograph with a surface area of 640 × 480 μm. The obtained average values of damaged microcapsules were simultaneously reduced by 2%, which corresponds with the statistically determined proportion of initially damaged microcapsules. This correction was based on SEM microscopic observations combined with measurements performed using quantitative metallography techniques.

Fig. 32
figure 32

Relationship between the applied force and the proportion of measured ruptured microcapsules relative to the average number of all identified and measured microcapsules (approximately 900).

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Figure 33 schematically presents the statistical distribution of the proportion of intact microcapsules for a given force, classified by their diameter (Table 8).

Table 8 Classification of microcapsules based on their size.
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Figure 33a–f shows the statistical distribution of intact microcapsule classes according to different applied force values. Each graph shows the results only for one random sample (from six) for different load forces.

Fig. 33
figure 33figure 33

Statistical distribution of intact microcapsules relative to the average number of all microcapsules (888), categorized by size classes.

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The analysis revealed that regardless of the applied force, a similar percentage distribution pattern of intact microcapsule classes was observed. It was determined that the largest proportion, for all forces tested, consisted of microcapsules in classes II–V (3–15 μm). Microcapsules in classes I and VI–X accounted for less than approximately 13% of all intact microcapsules identified and measured. Figure 33g summarizes the average percentage proportion of individual size classes of intact microcapsules during tests for a force of 2.75 kN, along with the standard deviation. The distribution of individual measurements is nearly identical to the average distribution for all measurements at a given force (coefficient of variation below 5%).

In comparison, Fig. 34 presents the same relationship as Fig. 33, but for ruptured microcapsules (for selected applied force values).

Fig. 34
figure 34

Statistical distribution of ruptured microcapsules relative to the average number of all microcapsules (900), categorized by applied force and size classes.

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For ruptured microcapsules there are no significant differences when their numbers are related to the average number of microcapsules from all samples Fig. 35(a), or related to the average number of microcapsules for each sample Fig. 35(b).

Fig. 35
figure 35

The percentage of ruptured microcapsules in relation to the: (a) average number of microcapsules, (b) average number of microcapsules for each sample.

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High degree of correlation between the microstructure of the transfer sheet and the appearance of high-resolution macroscopic images of the colored developer sheet (Fig. 36) is an important observation made as a result of the research.

Fig. 36
figure 36

Illustrative comparison of microcapsule distribution on transfer sheet film and the character of developer sheet film coloring.

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As a result of the analysis in Fig. 36, it can be seen that the shape and position of the micro-areas (microcapsule agglomerates), marked in purple in the figure on the left, are very similar to the spotted discoloration preserved on the developer sheet (figure on the right). It is also worth noting that the spots visible in the figure on the right are characterized by varying color intensity, which is directly related to the amount of active substance released as a result of the microcapsules rupture. Furthermore, this coloration, when considered locally (in relation to a single spot), is usually more intense than the overall, averaged color of the entire sample. This phenomenon may make it difficult to read the pressure, because even in the case of a sample subjected to relatively low pressure, there may be local micro-areas where significantly higher pressure values are read. This issue is partially discussed in64. In the cited work, it can be clearly seen that the scans of the samples show areas with a much more intense color than the calculated average color of the sample It is therefore worth noting that the results of the research described in this article make it possible to explain this phenomenon by accurately characterizing the microstructure of the transfer sheet.

In addition to the qualitative characterization presented above, the microscopic investigations carried out in this study required extensive SEM observations and direct manual counting of intact and ruptured microcapsules across multiple micro-areas. For each individual analyzed micro-area, more than 900 separate counts were performed, including the identification, classification and measurement of every microcapsule, and six such micro-areas were examined for each applied load level. The collected dataset was subsequently subjected to statistical processing and correlated with the applied force as well as with the colour-component analysis obtained from the scanner images of the developer sheet.

The SEM observations revealed that microcapsules rupture in a characteristic manner, forming circular or crater-like structures with a clearly defined edge. Importantly, the mechanism of rupture was found to be identical for all applied forces; only the intensity of the process, expressed as the proportion of ruptured microcapsules, increased with load, as confirmed by the statistical results presented in the quantitative analysis. A particularly noteworthy observation is the initial stage of damage captured in Fig. 12b, showing that the rupture begins with a linear cut along the capsule diameter before full opening occurs. To the best of our knowledge, this early failure phase has not been previously documented.

It was also demonstrated that the local distribution of microcapsules affects the microscopic statistics within individual micro-areas; however, this heterogeneity does not influence the macroscopic imprint, where the distribution becomes statistically averaged. Instead, the uniformity of the pressure imprint and the intensity of colour on the developer sheet depend primarily on experimental conditions, such as surface parallelism, uniformity of load application, and the condition of the contacting surfaces. Despite these practical factors, the results confirm that the applied pressure-sensitive films exhibit high sensitivity and accurate reproduction of the contact interface, both in terms of load intensity and spatial distribution.

A topic that certainly requires further research and discussion is the determination of the microstructure properties of mono-sheet films. Another important issue is to determine the influence of the microstructure properties of the mating surfaces between which pressure measuring films are placed. Understanding the influence of the surface roughness parameter will certainly enrich our knowledge of this pressure measurement method.

Conclusions

This study provides the first comprehensive microstructural characterization of the transfer sheet used in two-sheet pressure measuring films (Sensor Products Inc., SPF-D), combining qualitative observations with quantitative measurements of microcapsule damage as a function of applied load. While previous research on pressure-sensitive films has focused almost exclusively on macroscopic aspects—such as pressure distribution, contact mechanics, and calibration procedures—this work demonstrates that the fundamental mechanisms governing color development originate from microstructural changes inside the functional layer. The results obtained in this study fill an important knowledge gap by explaining how local material behavior influences measurement repeatability, accuracy, and the interpretation of imprints.

Quantitative analysis showed that the transfer sheet contained on average around 900 microcapsules in a single measured area of approximately 640 × 480 μm, with ~ 2% of them already damaged in the as-received state, regardless of sample location. This baseline value is essential for correctly interpreting the proportion of ruptured microcapsules after loading. For all applied forces, the rupture process followed a consistent and repeatable mechanism, indicating that microcapsule failure is primarily controlled by their diameter and local packing density rather than by variations in applied force alone.

Across all loading scenarios, microcapsules in size classes II–V (3–15 μm) represented the dominant fraction of both intact and ruptured particles, confirming that this population is structurally the most relevant for film response. The proportion of damaged microcapsules increased systematically with force (after correction for the 2% initial defect level), providing the first experimentally validated microstructural correlation between applied load and the extent of capsule rupture. These findings support the assumption—commonly used in numerical simulations—that microcapsule damage can be treated statistically rather than deterministically, and that the activation of the colour-forming reaction derives mainly from the behaviour of mid-sized capsules.

Qualitative SEM analysis revealed a chaotic and non-uniform distribution of microcapsules, including the presence of characteristic grape-shaped clusters. Although such heterogeneity does not significantly affect the global rupture mechanism, it may influence the localization and sequence of microcapsule failure at the microscale. This structural irregularity should therefore be considered when interpreting slight variations in macroscopic imprint uniformity or when constructing micro-mechanical numerical models intended to simulate film behavior.

The study also identified clear morphological differences between the transfer and developer sheets. The transfer sheet is a multiphase composite comprised of organic microcapsules embedded within a polymer matrix and supplemented by mineral particles—mainly CaCO₃—serving structural and rheological functions. In contrast, the developer sheet features a thin brittle functional coating containing mineral pigments and exhibiting mud-crack-like damage under load. These observations help to explain several practical limitations of pressure films reported in the literature, such as sensitivity to humidity, long-term stability, and the manufacturer-specified measurement accuracy of approximately ± 15%. The presence of an initially damaged microcapsule fraction and the non-uniform capsule distribution additionally justify this accuracy level from a microstructural perspective.

From a practical standpoint, the results provide valuable implications for users of two-sheet pressure measurement systems. Understanding the microstructural mechanism of microcapsule rupture explains why consistent contact conditions, strictly controlled load application time, and surface preparation are critical for obtaining repeatable results. Moreover, the statistically determined rupture characteristics may support the development of improved calibration procedures and enhance the accuracy of numerical modelling approaches used in contact mechanics research.

In summary, this work demonstrates the novelty and importance of microstructural investigations for interpreting the behavior of pressure-sensitive films. By combining detailed material characterisation with quantitative rupture statistics, the study clarifies the underlying mechanisms responsible for color formation, identifies the factors limiting measurement accuracy, and provides new knowledge that has not been addressed in previous publications. This microstructural insight strengthens the physical basis for both experimental and numerical applications of pressure measuring films and contributes to the development of more reliable and better-informed pressure measurement methodologies.