Manufacturing of insoles

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Method

Participant

To find a suitable candidate for the test the inclusion criteria was a person who could put load on the entire plantar surface of the foot. The exclusion criteria were no vascular diseases and no major structural foot deformities. This was decided through clinical experience by the acting orthotist at Skaraborgs Ortopedservice. They chose a pseudo-random candidate based on our criteria who participated in the study. A written and oral consent of participation was approved and signed by the participant before the study was initiated.

Production of insole samples

As described earlier there are three different manufacturing methods. The insoles for this study was produced from the same feet. The insoles manufactured using SM was obtained by Skaraborgs Ortopedservice from Cadmed Sweden. The VF insoles were also obtained from Skaraborgs Ortopedservice who used EVA form sheet from Mediroyal. Both insoles produced with VF and SM were made by the same orthotist at the mentioned P&O department.
The third manufacturing method is AM. To manufacture this type of insoles a structure sensor connected to an iPad was used. The milled insole received from the above company was scanned. Using this method, an almost identical insole was obtained and could be work within a CAD application such as Solidworks and Rhino. A mesh file is created when scanning an insole or a foot. A mesh file is a digital representation of a geometric model containing a collection of vertices, edges and faces that defines the shape. The faces usually consist of triangles because it simplifies the rendering. Opening a mesh file as a text document it includes the collection as mentioned above that  creates a grid visualized as a surface model. Rhino was used to edit the mesh file and refine the insole. In Rhino unnecessary polygons were deleted to get a smooth and clean surface. When this was done Solidworks was used to create a solid body from the surface and cavities in the heel could be applied. The imported surface was converted to a solid body. Trimlines were adjusted and the face up-side at the forefoot were flattened out. The plantar surface was also flattened out. To create the pattern for the cavities at the heel instructions described in the article (Davia-Aracil et al., 2018) was followed.
The chosen design for this study was a linear rectangular pattern distanced one mm from the top surface and two mm from the bottom surface. A two-dimensional sketch was done similarly to the described article and in relation to the features and settings of the 3D-printer at Jönköping School of Engineering in terms of geometry and size. The same tool to generate the cavities of the insole was not available, for this reason the cavities were made from the same principle. Through the sketch extrude cut was made with setting a distance from the plantar surface and a distance to the above surface. The model was cleaned up using chamfer to remove sharp edges. To 3D print the finished model an FDM 3D-printer at Jönköping School of Engineering was used. The name of the printer is Raise3D N2+ manufactured by the company Raise 3D in the USA, the serial number for the machine being used is 10111713009.
The selected material was TPE due to material alternatives that the school offered which worked with the 3D-printer. PRIMASELECT flex was used for the samples. PRIMASELECT flex is a thermoplastic copolyester (TPC) which is a group of TPE. The shore for the material is D 45 compared to the EVA sheet which has A Shore A scale is used for soft elastomers while shore D scale is used for thermoplastic.

Testing machine

To compare and evaluate the three different pieces of insoles an abrasion resistance machine constructed as a project according to the standard ASTM G133 at Jönköping School of Engineering was used. Together with this test, the ASTM G133 standard was used as a guideline. The test involves a pin that is rubbed back and forth in a straight line against the surface of the insole.

Outcome measures

To analyze the data from the test a scale, model Denver Instrument S- 114 with a capacity of measuring 110 g was used. The scale had readability and a standard deviation of 0.0001 g (Denver Instrument, n.d). With the help of the scale, it was possible to see how much material had been worn off for each session. The collected data was analyzed. These outcome measures were chosen as a compromise to the economic and timely limitations of the study. As the machines used were owned by the university no external cost was required for using them.

Test standard

The ASTM G133 standard together with pilot tests was used to determine the settings for the machine. This test is done by placing the test piece underneath the pin that will slide back and forth over the surface in a linear direction. The load is applied vertically downward through the attached pin against the horizontally mounted test piece of the insole. Friction occurs and abrades the material. The normal load, stroke length, frequency, duration and environment are selected before the process started. A cycle is defined as two strokes namely fourth and back (ASTM International, 2016).

Pilot tests

Pilot tests were executed in order to conclude whether the method of weighing the samples was a viable outcome measure, if this testing standard was applicable and if the settings were correctly made. Each sample was measured three times before and after the test, a mean value was then included. When the tests were done the samples were placed in a container and transported from the laboratory to the weighing room to reduce the risk of harming or contaminating the samples.

First test

To decide what load needed to be applied to the test piece, a calculation of plantar pressure distribution was made. Knowing the pressure level beneath the forefoot during the gait and the diameter of the pin attached to the test machine, the machine load could be calculated. The diameter of the pin was set to 8.5 mm. The area of a half sphere is:
A = 4πr² / 2
A = 4 × π × 4.25² / 2 = 1.135 cm².
The forefoot was divided into five different regions: the medial/MTP1, the central/MTP2, the lateral MTPs, the hallux and the lesser toes. According to the article the forefoot pressure underneath these areas was estimated to be the following values in percentage of total weight (TW) per cm2 (Castro et al., 2012):
Lateral MTPs = 5 % TW/cm².
Central/MTP2 = 5.1 % TW/cm².
Medial/MTP1 = 4.2 % TW/cm².
Lesser toes = 2.4 % TW/cm².
Hallux = 4.6 % TW/cm².
Following formula was used to calculate the forefoot mean pressure which resulted in 4.26 % TW/cm2.
X̅= (X1 + … + Xn) / n
According to a report from Statistiska Centrabyrån, the mean weight of a Swedish 35-44 year old person was 77.2 kilogram (Statistiska centralbyrån, 2018).
m = 77.2 × 9.81 = 757.332 N
To calculate the machine load needed:
F = P × A = (0.0426 × 757.332) × 1.135 = 36.617759532 N
Six samples (approximately 20×50×10 mm) were cut out from a slab of unheated flat EVA. The same abrasion testing machine as described below was used. The samples that were tested were weighed before and after the test using a scale. After the test, the samples were blown dust free using compressed air before weighing. As the outcome measure had been confirmed and deformation of the sample had been identified in previous tests, the effect of test time on polymer behavior was of interest. Therefore, the third sample was not weighted before testing due to a difference in weight was not sought after. Samples were also photo documented (see appendix 10.3).

First sample

The first sample was tested at 2 Hz, with a load of 3.4 kg and a stroke length of 25 mm at 60 min.

Second sample

The second sample was tested at 2 Hz, with a load of 1.7 kg and a stroke length of 25 mm at 90 min.

Third sample

The third sample was tested at 2 Hz, with a load of 3.4 kg and a stroke length of 25 mm at 90 min.

Second test

Due to the limited setting of the machine the ideal pin could not be used (see section 6.6). A compromise to this was to use a pin of the same dimension as before (see section 4.6.1) but changing the interfacing area from hemispherical to flat circular with rounded edges. The minimum load that could be applied for the machine was measured by a load cell to 2.9 N. Since the load was lower than before (see section 4.6.1) the material would not deform as much. For this reason, a flat circular surface would result in a bigger interfacing area compared to a hemispherical pin head.
The roughness of the pin was approximately 23.67 µm. As this was right between the two grit sizes a grit size of 240 was used to roughen the pin in a linear fashion to assimilate the plain weave type of a sock.
Before each test was performed, the pin was roughened with the sandpaper mentioned before (see section 2.9.2). Samples were also photo documented (see appendix 10.4).

First sample

For the first sample of the second test a load of 2.9 N, a testing time of 1h, a stroke length of 35 mm and a frequency of 2 Hz was set.

Second sample

For the second sample of the second test a load of 2.9 N, a testing time of 2h, a stroke length of 35 mm and a frequency of 2 Hz was set.

Main test

After the pilot tests were completed the settings were established. Samples were produced with the dimensions (62.4 × 47.8 × 10 mm). Two test samples were constructed of each manufacturing type (VF, SM and AM). To get the right thickness of the SM samples, polypropylene (PP) was used as a base plate. All the samples were blown dust free and weighted before and after the test. The samples were weighted three times and their mean was included. The difference in weight before and after the test and also the proportion were calculated with the following formulas:
weight = X̅weight-after – X̅weight-before
proportion = (        weight / X̅weight-before ) × 100
For the machine settings, a load of 2.9 N was applied, a stroke length of 35 mm, a frequency of 2 Hz and with an attached pin of 8.5 mm in diameter with a roughness of approximately 23.67 µm (roughened manually with sandpaper with a grit size of 240). Time was set to first one hour and then two hours for respective sample in specific manufacturing group. Samples were also photo documented (see appendix 10.5 – 10.7).

Ethical considerations

Ethical issues regarding the study may be that the participant feeling discomfort during casting whether plaster or a foam box is used but the procedure will not expose the participant to any physically or mentally harmful risks. The patient shall not feel obliged to take part in the study. Regardless if the patient will take part of the study or not, their healthcare should not be affected. From a social and global perspective, there is a chance that AM could streamline the production of insoles within the O&P business. (see Appendix 10.2).

Discussion

Main result

The population inclusion criteria for this study is described in section 4.1 and appendix 10.2. The reasoning behind this inclusion was in order to be able to generalize the results to a larger population. Because vascular diseases and other illnesses can affect the human body in a variety of different ways it could affect the results to include this group of candidates.
For the main findings, a possible explanation for the weight increase could be derived from metal particles generated when abrading the pin contaminating the samples and thereby invalidating the results. This was regarded by cleaning the pin with a wet piece of cotton before applying it to the machine. For further studies, the machine should be cleaned with acetone or other cleaning agents and a pin that cannot contaminate the sample should be used to gain more relevant results.
It is reasonable to assume that material would be worn off when abrading the surface of the samples. This contradicts the results of the main test. As some of the samples gained weight between the weightings it is difficult to conclude which of the materials from the different manufacturing methods had the best abrasion resistance.
Another common explanation for differing results is measuring errors due to human or machine. In the case of the present study, this would not be the explanation as described in section 4.4 due to the standard deviation of the scale not exceeding the difference identified between the weightings.
It was noticed that the mean weight of the AM sample was a lot higher than those of the VF samples (SM is not applicable for comparison as they were modified with a PP base). When calculating the mean density of the AM sample (see appendix 10.11) it was also noticed that AM had a higher mean density than the VF samples as follows:
ρAM = 1.1305 g/cm³
ρVF = 0.31 g/cm³ (appendix 10.1)
If an insole was created according to the principles described herein the weight of it might be a problem for users with muscle imbalances. Thereby further studies about other materials or internal, weight saving structures should be conducted.

tructural optimization

In the O&P business two of the three categories mentioned in section 2.7 about structural optimization are frequently worked with. Size and shape of orthotics are fitted to match the goals of the orthotist and to the patient. The third category, topology optimization is not currently worked with due to nowadays manufacturing methods. This could be solved by AM and a well-developed Application Programming Interface (API) for the O&P business.
For EVA, internal structures are not necessarily due to the low weight of the material and the high shock absorbing properties. AM can produce insoles with similar properties but at a very high cost of the machine and material. Using this type of machine with these types of materials would result in very expensive insoles, something that is not viable for the O&P business or the healthcare system. SM and VF are unable to adjust internal structure, a property AM possesses. The optimal settings of the internal structure can be decided using topology optimization if the correct parameters are included and determined. Such parameters could for example be the patterns mentioned by Davia-Aracil et al. (2018) and others. The linear rectangular pattern was chosen out of a few different ones. Which one of these gives the best shock absorbing property is unknown and there was no motivation behind the choice. As it is unknown, this area still needs further studies for how different patterns relate to properties such as shock absorption, mass and fatigue resistance to constant load pounding.
Materials for AM is a large market where multiple manufacturers produce a lot of different filaments in various properties. This results in a lot of options for potential insole materials which all would need to be tested for quality in respect to for example abrasion and fatigue resistance in further studies. Another topic not tested for in this study is shore. The shore is related to the internal structure as was mentioned in section 2.6. However, it is unknown exactly how the internal structure affects the shore. This could be tested using a shore durometer or using simulation based on topology optimization as mentioned before.

Table of contents
Abstract 
1. Introduction
2. Background 
2.1 History of insoles
2.2 Application and function of insoles
2.3 Manufacturing of insoles
2.4 Cost differences
2.5 Additive manufacturing
2.6 Internal structure
2.7 Structural optimization 5
2.8 Materials
2.9 Mechanics
3. Aim 
4. Method
4.1 Participant
4.2 Production of insole samples
4.3 Testing machine
4.4 Outcome measures
4.5 Test standard
4.6 Pilot tests
4.7 Main test
4.8 Ethical considerations
5. Results 
6. Discussion 
6.1 Main result
6.2 Structural optimization
6.3 Pilot test
6.4 Samples
6.5 Machine
6.6 The pin
6.7 Method
6.8 Outcome measurement
6.9 Manufacturing of insoles
6.10 Ethical consideration
6.11 Friction
6.12 Further research
7. Conclusion 
8. Acknowledgements 
9. References
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A material study of insoles Manufactured using different methods