The Foot 14 (2004) 192–197

New material for recording impression of foot forcustomized footwear applications

G. Saraswathy, Gautham Gopalakrishna, B.N. Das∗Shoe Design and Development Center, Central Leather Research Institute, Adyar, Chennai 600020, India

Received 26 August 2003; received in revised form 1 March 2004; accepted 16 March 2004

Abstract

Background: The orthopedic shoe is a therapeutic aid employed by the specialists for the patients with defective feet. It is custommolded for the individual foot using special techniques and design. Therefore, the role of mold making of the patient’s foot is a verycritical step for the successful outcome of the orthopedic/therapeutic shoe or for any type of custom orthoses.Objectives: To develop abiodegradable and cost-effective material for taking the impression of patient’s foot and standardizing the materials by physicochemical andexperimental methods.Methods: Starch-based dough was prepared using water as binder and other ingredients like plasticizer, preservativeand coloring agent. The developed foot impression material was characterized for physicochemical properties and used for taking impressionof patient’s foot and subsequent mold making.Results: The material binds with water and forms a paste like mass which can be shaped intoany form on the application of pressure and is stable until not disturbed.Conclusion: The developed material has proved to be suitable forrecording impression of foot for customized footwear applications and also found to biodegradable, repeatable over several impressionsand cost-effective as the material is developed from natural and locally available ingredients.© 2004 Elsevier Ltd. All rights reserved.

Keywords: Impression material; Custom molded footwear; Orthopedic shoe; Orthoses; Starch; Biodegradable; Recyclable

1. Introduction

Foot orthoses are currently used to redistribute pressuresat the shoe–foot interface[1,2]. Their use is particularlyprevalent in the treatment of foot disease associated withdiabetes and rheumatoid arthritis (RA) where mechanicalfactors play an important role in the development of neu-ropathic ulcers[3]. Abnormal loading patterns under thefeet and increased plantar pressures have been observed,for example, in patients with RA where there is little nervedysfunction[4] and in diabetic patients with few clinicalsigns of neuropathy[5]. Orthotic shoe insoles, which reduceelevated local pressures at the plantar foot–shoe interface,can be of substantial preventive and/or therapeutic value toa variety of patients[6–8].

Orthopedic custom made footwear is prescribed forthose who have abnormal foot because of any degenerativechanges of bone. Various researchers have studied the ef-fect of customized insoles on reduction of plantar pressuresin sites of wounds and previous neuropathic ulceration in

∗ Corresponding author. Tel.:+91-44-24420589 (O)/22354916 (R);fax: +91-44-24911589.

E-mail address: bndas@clrim.org (B.N. Das).

the diabetic foot[9]. The findings have showed significantreductions in peak vertical pressure and increased totalcontact area for the insole.

The orthopedic shoe is defined by the last (model ofpatient’s feet). The last, which determines the shape and cu-bical content of the shoe, is created by taking a negativemold of the foot[10–15]. Most of the methods of mold mak-ing have involved the use of plaster of Paris (POP) whichis still in use. The main drawback of using POP is that thepatient’s foot must remain in a fixed position within the POPfor approximately 30 min to allow the POP to set. The com-pressible foam impression systems reduce the time requiredto complete a foot impression, however, these systems aregenerally applied when a person is sitting and the foot issuspended. In all these methods, the resulting insoles tendto support the foot in a non-natural position, which is differ-ent from the foot position that results when weight is placedon the foot. They either do not produce a natural foot posi-tion or produce insufficient detail of the bottom of the foot.Accordingly, it is believed that such systems do not pro-vide all the advantages for which they have been promoted.The specific problems associated with the current methodsof taking foot impressions have led to the development ofthe new impression materials in our institute. In the present

0958-2592/$ – see front matter © 2004 Elsevier Ltd. All rights reserved.doi:10.1016/j.foot.2004.03.004

G. Saraswathy et al. / The Foot 14 (2004) 192–197 193

work, efforts have been made to develop a material whichwill give the impression of the foot in the weight bearingposition, produce sufficient detail of the bottom of the foot,allow for immediate casting of the mold, be easy to use,reusable and biodegradable.

2. Methods

2.1. Materials

Starch, sodium metabisulphate, red liquid dye, and water.

2.2. Preparation

About 1 l of water was treated with 45 g of sodiummetabisulphate (3% by weight of sodium metabisulphateon the weight of starch) and 40 ml of red liquid dye (4% byvolume of red liquid dye on the volume of water). The waterwas mixed well to form a clear solution. The prepared solu-tion was added slowly to about 1.5 kg of fine starch powder.The solution and powder were mixed uniformly and the pro-cess was continued till the dough (sample I) was obtained.The dough was made into different consistencies by varyingthe amount of solution used for making the dough. SamplesII and III were made with 1.25 and 0.75 l of the solution,respectively.

2.3. Experiment

The foot impression developed was packed tightly in acontainer of dimension 30 cm× 15 cm× 6 cm. The impres-sion of the patient’s foot was taken in a standing positionby pressing with gentle force (Fig. 1). After the impres-sion of foot (sole and side region) had transferred onto thematerial the foot was removed without disturbing the ma-terial (Fig. 2) and the impression was immediately castedwith POP. The POP model of the foot was removed fromthe material after it set and air dried at room temperature.The impression material was reused for the same purpose.After POP had set, it was removed from the material.The impression was disturbed and the surface was madeflat.

2.4. Characterization

2.4.1. Moisture contentInitial weight of the prepared material was taken (Wo). The

accurately weighed sample was dried in an hot air oven untilthe weight remained constant (at 60◦C for 3 h). The weightof the dried sample was found (W1). Percentage moisturecontent of the material was calculated from the formula,

W1 − Wo

Wo× 100

Fig. 1. Method of taking the impression of patient’s foot using thedeveloped material.

2.4.2. Set timeImpression of subject’s feet was made using the samples

and kept in the open at room temperature without castingwith POP and observed for 24 h. The set time was takenwhen the materials resisted to deform on application of gen-tle force but still had moisture. He moisture content of thesamples was measured as above.

2.4.3. Drying timeThe material, which were kept to observe set time, were

continually observed for a further 24 h to find drying time,that is until it loses all its moisture.

2.4.4. Shape retention and dimensional stabilityThe shape and dimension of the impression was taken

immediately after it was made and after it set. Any changein the shape of the impression made over the material wasmeasured with scale (Fig. 3). For determination of dimen-sional stability, the measurements of the POP model of thefoot were compared with that of the foot which created theimpression.

2.4.5. Volume shrinkageThe change in volume of the impression made was

measured from the amount of water it required to fill the

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Fig. 2. The impression of foot including sole and side region that hadtransferred onto the material.

Fig. 3. Dimensions used for the measurement of impression, POP modeland the foot, A: length, B: width of ball, C: width of instep, D: width ofheel.

depression at the time immediately after taking the impres-sion and after it had set.

2.4.6. Shelf-lifeVisual investigation of the samples was carried out every

24 h from the day of preparation, for any bacterial/fungalgrowth on the surface of the material. The samples werestored at 14◦C in a refrigerator.

2.4.7. Temperature stabilityThe samples were stored at different temperatures, such

as room temperature 20 and 14◦C and observed for anychange. Sample I was also tested by thermo-gravimetricanalysis (TGA). TGA was performed with a thermal analy-sis (TA) TGA 2900 thermo-gravimetric analyzer at a heat-ing rate of 10◦C/min under nitrogen atmosphere from roomtemperature to 500◦C. About 7 mg of sample was taken formeasurement.

2.4.8. Mechanical behaviorBehavior of sample I under stress was studied using In-

stron model 4501. The material was subjected to varioustypes of stress such as compression and needle penetration.The material was packed inside the glass tube of inner di-ameter 1 cm, height 2 cm and compressed with glass rod of1 cm diameter with cross head speed 5 mm/min. In case ofneedle penetration test, a needle of 4.5 mm in diameter wasused. The difference between the above two experiments is,in the first case the material was closed, but in the secondcase it is open, so the material was allowed to escape out.In both tests, the behavior of the material, on application offorce, was studied.

3. Results

3.1. Physico-chemical properties

3.1.1. Optimum moisture contentThe moisture content of the material prepared with 1 l

of solution (sample I) was found to be 71%, whereas thoseprepared with 1.25 l (sample II) and 0.75 l (sample III), werefound to have 86 and 66% of moisture content, respectively.

3.1.2. Set timeAt room temperature, in open air, all the samples were

found to set within 24 h (Table 1).

Table 1Percent moisture content of the samples prepared

Sample I (%) Sample II (%) Sample III (%)

Immediatelyafter made

71 86 66

After 24 h 37 40 30After 48 h 0 0 0

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Table 2Dimensions of the foot impressions made using the developed foot im-pression material

Sample Length(mm)

Projected width (mm)

Ball Instep Heel

Immediatelyafter made

I 250 95 85 62

II 252 96 86.5 62III 249 96 86 63

After set I 251 94 85 61II 245 88 77 54III 249 95 85 62

Table 3Comparison of dimensions of POP model with foot

Dimensions Length(mm)

Projected width (mm)

Ball Instep Heel

POP model createdusing sample 1

250 85 68 58

Foot which createdthe impression

248 82 66 56

3.1.3. Drying timeUnder the same conditions as above, all the samples were

found to dry within 48 h (Table 1).

3.1.4. Shape retention and dimensional stabilityThe shape and dimensions of the foot impression, mea-

sured immediately after made and set time, were found to bethe same in samples I and III. Sample II was found to loseits shape with time and the dimensions measured after settime were found to be less than that measured immediatelyafter the impression made (Tables 2 and 3).

3.1.5. Volume shrinkageThe amount of water required to fill the impression, im-

mediately after made and set time was found to be the samein samples I and III. In sample II, the amount of water re-quired to fill the impression after set time was found tobe less than that required immediately after the impressionmade (Table 4).

Table 4Determination of volume shrinkage

Impression Sample Quantity of waterrequired to fill theimpression (ml)

Immediately after made I 480II 485III 490

After set I 485II 430III 495

Fig. 4. Thermogram of starch-based foot impression material.

Table 5Parameters calculated from compression test

Load atmaximum(kN)

Load atmaximum(N)

Displacementat maximum(mm)

Stress atmaximum(MPa)

Strain atmaximum

Mean 0.813 813 2.425 12.78 0.857

3.1.6. Shelf-lifeThe samples stored at 14◦C in refrigerator were found to

show no undesirable odors or microbial growth during theobservation period of 2 months.

3.1.7. Temperature stabilityThe samples were found to become hard, 24 h after prepa-

ration at room temperature and at 20◦C if kept open. Theywere found to be stable with the retention of moisture ifstored in a closed container at 14◦C. The nature of TGAcurve is a clear indicator of the number of stages of thethermal degradation. In the thermogram (Fig. 4), a one stepweight loss was observed between 297 and 327◦C with themaximum weight loss at 314◦C. The total weight loss ob-served was 68%.

3.1.8. Mechanical behaviorThe parameters calculated from compression test and

needle penetration test are tabulated inTables 5 and 6,respectively. The load required to make maximum displace-ment of the material was defined as load at maximum. Inthe compression test, 813 N of load was required to makea maximum displacement of 2.425 mm. As the material

Table 6Parameters calculated from needle penetration test

Load atmaximum(kg)

Load atmaximum(N)

Maximumdisplacement(mm)

Energy tobreakpoint (J)

Mean 7.81 76.610 20.5 0.061

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had no place to escape out, the glass tube cracked andbroke in the walls. The initial volume of the material was1570 mm3. The volume after compression was 1380 mm3.Therefore, the reduction of volume was 190 mm3. In theneedle penetration test, the load required to make maxi-mum displacement was 76.61 N. During the experiment,the material escaped out of the glass tube.

4. Discussion

The fine powder of starch that was used in the experi-ment bound well with water and formed a dough-like mass(semi-solid) with a specific amount of water that gave a de-sirable consistency, so that the material was shaped into anyform, which was stable until not disturbed. As the shapeand dimensional stability of the impression was dependenton the moisture content of the material, optimization ofmoisture content for better performance of the material wasdone by changing the amount of water used for preparationand it was optimized to 71% (sample I). If the moisturecontent is more, there is a chance of change in dimensionsof the impression whilst removing the foot from the ma-terial. Therefore, maintaining optimum moisture contentis important. Sample III, containing 66% of moisture, re-quired a heavier load to be given by the subject to get thefoot impression. The sample, with an upper limit of 86% ofmoisture content, had given a good impression with the nor-mal load on foot, on standing. Though the impression madehad lost the shape with time, it would not be a problem if itwas immediately cast, which is normal practice in a clinicalsetting.

The material became dry if kept in the open air, and re-tained moisture if packed in a closed container and in a re-frigerator. With time, the material was spoiled by microbesif not stored properly. The impression created formed a per-manent set which is stable if not disturbed externally. Themeasurement of dimensions of impressions, immediately af-ter made and set time, had showed that there was only aslight change which is acceptable. Sample I could be castimmediately after it set too. The measured dimensions ofthe POP models, and the foot, had shown that the mate-rial had produced the accurate negative impression of thefoot.

In the thermo-gravimetric analysis, the weight loss wasdue to the loss of bound water and the starch itself, whichhappened around 300◦C. It was confirmed that the majorsolid constituent of the material was starch. From the loadand displacement curve, the material can be described asductile—soft (Fig. 5). There was only negligible change involume of material during compression. Cracking of theglass tube at the end of the test had showed that the materialhad not compressed but tried to escape out. This means thatwhile making an impression of the foot, the material will notchange in volume, but only try to escape out. Therefore, thecontainer of the material should have enough space to oc-

Fig. 5. The load vs. displacement curve of compression test of thestarch-based foot impression material.

cupy the volume of material that gets displaced on pressingthe foot whilst taking the impression. From the needle pen-etration test, it was confirmed that there is no compressionof the material on application of force, only displacement ofthe material, according to the volume of the object pressedinto it. Further, a less amount of the load was required forpenetration of the needle into the material. It showed thatthe force to be applied by the subject, to take a foot im-pression, will be less. The person does not need to strainmuch and the impression will be the same as the normalfoot.

The material was proved to be cost-effective by its re-cyclable use. The same material was used again and againuntil it retained a good consistency for making a foot im-pression on the same day, or even later, if stored properly.During the experimental period, the foot impression mate-rial was stored in a refrigerator and used not more than fivetimes within 2 months.

5. Conclusion

It has been concluded that the new impression material hasthe desired characteristics over the existing materials, suchas POP plaster and phenolic resin foams. The impressionwas taken in the weight-bearing position and the mold wascast immediately after being made. Disposal of the materialwas found to be easy as the basic material is starch whichis highly biodegradable. Therefore, the new impressionmaterial can be used successfully for taking the impressionof a patient’s foot, for making an orthopaedic/therapeuticshoe or for any type of custom orthosis, as the materi-als and methods are recyclable, cost-effective and easyto use.

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