Product characteristics versus customer perceptions on a health-related product

Abstract

Manufacturers are continuously designing health-related products with various styles and specific characteristics to challenge and outcompete each other. However, it is challenging to convince consumers that a new product’s attributes are better than similar existing products. The main challenge for manufacturers is how to create a product that improves on existing products. This research aims to analyze whether customers' purchasing decisions are aligned with the product’s applicable mechanical characteristics and to develop guidelines for supporting product design and development. The design team can use these guidelines to quickly and easily understand and support customer requirements. The study uses “famous types of heel inserts” as a case study, with quality function deployment, usefulness of product, factor analysis, ergonomics, reverse engineering, and finite element analysis as the key tools. The results can help users select the right health-related product and make small adjustments to their routines to improve their quality of life.

Keywords:

• Customer perception
• engineering design
• health-related product
• product design and development
• quality function deployment
• house of quality
• usefulness

1. Introduction

In the manufacturing world, precision and accuracy are commonly used to describe how well machines and humans work together with minimal errors. Precision refers to the act or instance of repetition, while accuracy is related to correctness. These two terms are crucial in product design and development to explain customer behavior, perceptions, and needs before starting the conceptual design platform in a systematic way. Precision or repetition is a strong foundation for purchasing decisions since frequently hearing, seeing, or reading information about a product can influence an individual to buy it without conscious thought. However, accuracy or correctness is also important to ensure that the consumer is satisfied with the product after using it for a while.

One of the main challenges in making purchasing decisions is misinterpreting factors such as how long it will take to achieve the desired results. Selective distortion of the intended message or initial attitude can occur during testing, leading to unexpected results. To address this issue, design engineers should consider questions such as how to assist customers in understanding what they really want, how to translate customer needs into engineering design, how to create better products and services, and which methods should be applied to reveal customers’ feelings for design.

Manufacturers must engage with customers to understand their needs and preferences by communicating clearly and effectively. Hidden issues or needs can be identified through interviews and conversations with target groups. These results can be used to analyze product specifications and concepts, and ultimately determine the development plan’s outline and contents to create the initial designs of the new prototype. Product design and development (PDD) can assist manufacturers in quickly achieving their targets (Akao, 2004; Rianmora & Phlernjai, 2020; Rianmora & Takaew, 2017; Rianmora, Infandra, et al., 2020; Rianmora, Mahitthiburin, et al., 2020; Ulrich & Eppinger, 2012).

The primary objective of this study is to assist the design team in developing heel inserts that effectively enhance comfort, support, stability, and practicality for users, while simultaneously minimizing the risk of foot pain and injury. Achieving this objective requires a comprehensive analysis of biomechanical factors and thorough testing and evaluation of the materials employed. Furthermore, it is crucial to integrate feedback from target users during the design process, encompassing considerations of size, shape, and materials. By prioritizing the development of efficient and user-centric heel inserts, this study can contribute to the advancement of foot pain and injury solutions. Additionally, the study emphasizes the significance of validating and verifying users’ perceptions concerning the pertinent mechanical and physical characteristics of the products, such as the materials and geometric shapes utilized. These factors are key determinants of product effectiveness in providing comfort, support, stability, and injury prevention. Consequently, it is vital to ensure that users’ perceptions correspond with actual product performance and characteristics, and to make any necessary adjustments or improvements based on feedback and evaluation.

2. Literature review

Product Design and Development (PDD) is a process that helps companies create new products through five main steps: concept development (CD), system level design, detail design, testing and refinement, and production ramp-up (Ulrich & Eppinger, 2012). During the “CD stage”, various activities are carried out to create, test, modify, and refine the physical characteristics of a new product to meet customer needs and keep up with market trends. Quality Function Deployment (QFD) is a planning tool that facilitates the entire product development stage by translating the Voice of the Customer (VOC) into engineering characteristics and appropriate testing methods (Akao, 2004). It enables manufacturers to focus on the characteristics of new or existing products or services from various perspectives, such as market segments, company needs, or technology-development requirements (Abdel-Basset et al., 2019; Carnevalli & Cauchick Miguel, 2008; Hauser & Clausing, 1988; Jaiswal, 2012). QFD produces charts and matrices, such as the House of Quality (HOQ), to aid in this process. By prioritizing each product or service characteristic and setting development targets, QFD assists in creating a product or service that meets customer requirements and satisfaction (Abdel-Basset et al., 2019; Chiadamrong & Tham, 2017; Hauser & Clausing, 1988; Zadry et al., 2015; Jaiswal, 2012; Kuo et al., 2014; Pinto & Paiva, 2010; Puglieri et al., 2020). Once the core product idea is obtained, the second stage of PDD, called “system level design”, is performed. Here, designers and engineers develop the product architecture in detail, and manufacturers determine which components to make or purchase and identify the necessary suppliers. The product architecture identifies the product in chunks or organizes the product features into groups of primary functional systems and subsystems.

The next stage in PDD is the “detailed design” stage, where the classified groups from the previous stage are used to generate a virtual model. To quickly create a CAD model from an existing object, reverse engineering (RE) techniques can be applied (Hsiao & Chuang, 2003; Sokovic & Kopac, 2006; Tamás et al., 1997). Rapid prototyping or manufacturing technologies can be used to create prototypes for functional testing or to check physical characteristics before moving on to the next stage, “testing and refinement”. Finite element analysis (FEA) is a key technique for predicting how a product will react to real-world conditions such as vibration, heat, fluid flow, and other physical effects (Cook, 1989). This analysis can be used to evaluate different designs and materials, optimize the designs, and reduce material usage. Ergonomics is also a critical consideration for product design, and simulation is often required to design products that are harmonious with ergonomic principles related to joints, muscles, and nerves (Zadry et al., 2015).

To avoid additional waste and investment in testing and designing on the real part, many prototypes have been generated and analyzed as models (Campbell et al., 1982). Simulation tests can help reduce costs for creating prototypes and test parts. In this research, the concepts of Product Design and Development (PDD) and Concept Development (CD) were studied and applied to determine the relationship between customer perception and engineering design, using various shapes and materials of heel inserts as an example (Cardoso et al., 2015). The expected results are presented as guidelines to help customers select the appropriate design and material of heel inserts for supporting their shoes (Chao et al., 2020). The engineer or designer is required to perform five main phases in sequence, with the requirements for achieving the Concept Development phase listed in Figure 1. The accuracy of the proposed design depends on the initial phase of PDD, Concept Development (CD), which is critical. If the fabricated prototype (obtained from testing and refinement) is not accepted by the customer, the manufacturers must analyze and return to the origin of the design process (Rianmora & Koomsap, 2011; Rianmora & Werawatganon, 2021).

Figure 1. Requirements for supporting Product Design and Development (PDD).

The first stage of PDD, which is the “concept design”, is a critical part of the production design development process. During this step, the target market requirements are identified, competitive products are reviewed, product specifications are defined, a product concept is selected, an economic analysis is conducted, and the development project is outlined (Guinta and Praizler, 1993). The key components of CD related to the proposed approach are VoCs and customer requirements. To develop the product concept in the CD stage, it is crucial to identify customer requirements through surveys and group discussions (Yick and Tse, 2013). By collecting data on customer experiences with similar products in the market, the product’s suitable characteristics and properties can be determined. These data will then be used to create a guideline platform for creating a prototype that meets customer requirements. Customer perceptions play a crucial role in their purchasing decisions, particularly for health-related products. They analyze and identify specific characteristics such as ingredients, advantages/disadvantages, or effects after using the product, as well as the manufacturing or expiration date before making a purchase. Some customers are influenced by social media advertisements, forum reviews, discussions with friends or family, and medical articles. When customers’ perceptions align with their actual experiences, their perception can transform into consciousness, positively impacting their satisfaction with the product.

3. Method of approach – case study: Heel inserts

This research focuses on heel inserts as a case study and aims to explore how well the launched product has achieved its goal and to recommend any design changes that would improve user perceptions. The study examines the link between customer perception and product characteristics by comparing practical purchasing decisions with mechanical simulations. The results will help determine whether customer purchasing decisions align with the applicable mechanical characteristics of the product. Seven main stages are required to achieve the research objective.

3.1. Stage 1: Select the product of interest

To illustrate the proposed concept, a health-related product, namely heel inserts, is chosen as the case study in this research. The selection of this product type is based on several reasons. Firstly, health-related products are considered basic or physiological needs, according to Maslow’s Hierarchy of Needs Pyramid (Figure 2) (McLeod, 2017), which is about maintaining homeostasis. Secondly, people spend a considerable amount of time researching the advantages and disadvantages of such products. Lastly, the selling price of such products can be easily influenced by seasonal events or demand fluctuations throughout the year. Maslow’s Hierarchy of Needs is often depicted as a hierarchical pyramid with five levels, with the four lower levels representing physiological needs and the top-level representing growth needs. It is widely accepted that lower-level needs must be met before higher-order needs can impact behavior.

Figure 2. Maslow’s Hierarchy of need by Saul McLeod, 2017 (McLeod, 2017).

Furthermore, in the current market, both online and physical, women’s shoes come in various styles and patterns. Therefore, it is essential to investigate how to make the right decision when selecting a heel insert that fits perfectly with these shoes and identify any underlying issues.

3.2. Stage 2: Understand the concept of customer purchasing decision

The experiences a customer has with a product have a significant impact on their future purchasing decisions. Reliable information about a product that is shared with the customer influences their perception of the product. This perception is based on the purchasing and usage experiences of the customer. A product that meets customer requirements and expectations has a higher chance of surviving in a competitive market. Quality products tend to sell themselves without the need for advertisements. However, when it comes to health-related products, customers tend to be more cautious and want to analyze the product’s characteristics and properties before making a purchase. Issues with a product can be major or minor, depending on the type of product. A major fault occurs when a product does not work as intended. A minor fault is when the product’s appearance or finish is affected. Customers may feel uncomfortable purchasing health-related products without understanding the product’s advantages, disadvantages, and potential effects on their bodies. Providing guidelines for selecting a product can help customers understand the functions and details of the materials used in the product. In the case of a health-related product such as heel inserts, it is essential to understand how to select the appropriate design and material to provide the required support for a particular type of shoe.

3.3. Stage 3: Define the scope of the decision

During the initial stages of product design and development, the research team developed a set of questionnaires to assess the target customers’ perceptions of the heel insert product. A variety of factors contribute to the overall experience with a heel insert product, as illustrated in Figure 3. The population under study consists of working-age women between the ages of 25 and 45 who reside in Bangkok Metropolitan, Thailand. This is an infinite population because the exact number of women within this age range in Bangkok Metropolitan cannot be determined. The target users in this group prefer to spend at least two days a week shopping for fashionable accessories, shoes, decorative items, and cosmetics.

Figure 3. Customer perceptions and aspects contribution to the whole experience with a heel inserts’ product.

In order to make the research to be more robust, the appropriate number of respondents (i.e., the people who have filled in the survey or it represents only part of the group of people or target group whose opinion or behavior we are concerned about.) is determined by assigning a margin of error of 5% and a confidence level of 95%, the population size (N) was 300. Therefore, the sample size (n) should be 169 (Glen, 2022; Sangren, 1999).

(1) $\mathit{Samplesize},\mathit{n}=\mathit{N}\times \left(\frac{\frac{{\left(\mathit{z}\right)}^2\times \mathit{p}\times \mathit{q}}{{\left(\mathit{e}\right)}^2}}{\left(\mathit{N}-1\right)+\frac{{\left(\mathit{z}\right)}^2\times \mathit{p}\times \mathit{q}}{{\left(\mathit{e}\right)}^2}}\right)$(1)

$\mathit{n}=300\times \left(\frac{\frac{{\left(1.96\right)}^2\times 0.5\times \left(1-0.5\right)}{{\left(0.5\right)}^2}}{\left(300-1\right)+\frac{{\left(1.96\right)}^2\times 0.5\times \left(1-0.5\right)}{{\left(0.5\right)}^2}}\right)$

$\mathit{n}=\mathit{\hspace{0.17em}}169$

where,

n is the minimum size that is needed to estimate the true population proportion with the required margin of error and confidence level.

N is population.

Z is critical value of the normal distribution at the required confidence level.

e is the desired level of precision

(the margin of error, or confidence interval).

p is the estimated proportion of the population which has the attribute to the question (i.e., sample proportion).

q is 1 – p.

Based on the calculation, a sample size of around 169 would be sufficient to achieve the desired confidence level. However, in practice, a sample of 172 respondents was obtained from the target population (300 people) of women’s fashion floor customers in malls and department stores in Bangkok. These respondents were asked about their attitudes towards purchasing heel inserts using both digital and paper-based surveys.

The proposed approach was demonstrated using heel inserts as an example product. The key decisions in product development are the shape and material of the heel inserts, particularly for supporting everyday shoes or shoes that are worn for long periods. To define the scope of the decision-making process and select the appropriate insert design, two main issues were researched.

3.3.1. DEFINE issue I: The type of everyday shoes worn for working (or 8-hour period)

Shoes often feature leather or polymer rubber outsoles that can absorb the force applied during walking or standing. However, during long periods of wear, continuous force distribution occurs around the heel areas, leading to fatigue and pain, which can affect working performance. To create the conceptual design of heel inserts, it was important to first analyze the types of shoes and popular styles worn by the target users, taking into consideration cultural influences. Questionnaires were used to gather this information. Before administering the questionnaires, the definitions of shoe types were taken into consideration (MasterClass, 2021). After obtaining the results from the target users, it was found that 87% of them consulted reviews and friends before making purchasing decisions. These findings are summarized in Table 1. According to the questionnaire results, the most popular shoe style among the target users was flat shoes for women, followed by court shoes with a heel height of less than 3.5 cm. The reasons for selecting these types of shoes were related to the negative effects of wearing elevated heel shoes with a height greater than 3.5 cm. These shoes can cause toe problems, as the weight and shock resulting from walking activity are transmitted directly to the bony structure of the body and foot. While women feel more comfortable wearing flat shoes, heel pain can become a serious problem after wearing them for a long day. As a result, many women have attempted to apply heel inserts to alleviate these pains. However, finding the right heel inserts can be quite challenging.

Table 1. The type of everyday shoes worn for working

Shoe Types		No. interviewees
		(Total = 172)
Flat shoes (Ballet Flats)		48
Flat shoes (Man style)		10
Court shoes with less than 3.5-cm-heel height (Mid heel)		34
Stiletto shoes with over 3.5-cm-heel height (High heel)		22
Sandals with less than 3.5-cm-heel height		25
Slingback with over 3.5-cm-heel height		20
Sneakers		13

3.3.2. DEFINE issue II: The way to relieve fatigue and stress on the feet after wearing flat shoes for a long day

After identifying and exploring the issues and problems of interest related to shoes, the researchers found that flat shoes with a ballet-flats style were an interesting style to be considered, as they were selected by a higher percentage of participants compared to other shoe styles. In order to design and develop a new pattern of heel inserts that would be suitable for this shoe style, the researchers conducted a new set of questionnaires targeted towards women who wore flat shoes. The questions focused on how to relieve fatigue and stress on the feet for a long day. From the responses of the 50 participants (Table 2), it was found that a majority of women, 58%, decided to change their posture or bodily movement to relieve fatigue and stress on the feet. However, these were personally experienced methods based on their individual situations and levels of pain. Factors such as the type of activity during the day, posture while working, number of hours spent standing or walking, age, or body mass index could affect the level of fatigue experienced. The results show that using heel insoles for relieving fatigue and stress is not a popular option for the target group. This is because they believe that wearing shoes with proper heel support can improve overall foot health and adding heel inserts may cause discomfort or awkward sliding of the inserts inside the shoes.

Table 2. The way to relieve fatigue and stress on the feet after wearing flat shoes for a long day

The ways to relieve fatigue and stress on the feet for a long day	No. interviewees (Total = 50)
Change posture/bodily movement	29
Stop working for a while	16
Apply the medicine/See the doctor	3
Change the shoes	2

3.4 Stage 4: Analyze the characteristics of the heel inserts for the flat shoes

After defining the target user group and scope of the decision in the previous stage, the proposed research focused on studying a flat-shoes type, since 58% of the target customers reported changing posture or bodily movement to relieve fatigue and stress on the feet for a long day. To analyze activity and obtain results for the self-administered questionnaires, three steps were taken: consideration of drafted ideas (Figure 4), collection of detailed data (Figure 5), and specification of requirements (Figure 6 and 7). Once the basic requirements and characteristics of the health-related product were obtained from the confidential self-administered answers, another set of questionnaires was established to gather opinions, perceptions, satisfactions, and expectations of customers on existing competitive products and the newly designed product. To analyze the characteristics of heel inserts for flat shoes, three main issues were researched and raised.

Figure 4. Step 1 – Drafted idea consideration.

Figure 5. Step 2 – Detailed data.

Figure 6. Step 3 – Matrix question.

Figure 7. Step 3 – Contingency question.

To analyze the characteristics of heel inserts for flat shoes, three main issues were researched and raised.

3.4.1 ANALYZE issue I: The characteristics of the heel inserts that the users would like to purchase

To analyze the characteristics of heel inserts for flat shoes, a set of questions regarding the mechanical and physical characteristics of the inserts were launched to 300 people. However, only 168 target users responded (as shown in Table 3). The key considerations focused on their perceptions and experiences. They preferred heel inserts that were odorless and lightweight, which could reduce pain immediately after wearing. The material of the inserts was important as it should keep the shoes fresh, hygienic, and comfortable even during a long day.

Table 3. The required characteristic of heel insoles

The required characteristic of heel insoles	No. respondents (Total = 168)
Reducing the pain immediately after wearing shoes with inserts	40
No odor	36
Light weight	35
Easy for cleaning	26
Durable	24
Use various types of material	7

i. Result for ANALYZE issue I

The survey results showed that the most important factor for target users when selecting heel inserts was their ability to “reduce pain immediately after wearing shoes with inserts”. This was followed by the requirement for the inserts to be “odorless” and “lightweight”. However, it is important to conduct further research and testing to determine the feasibility and effectiveness of incorporating these factors into the design of the heel inserts. It is crucial to investigate whether the factors or considerations mentioned by the target users are related or necessary for the heel inserts. This is because these factors could have an impact on the overall design and functionality of the heel inserts. Therefore, it is vital to ensure that the product meets the needs and expectations of the target users.

3.4.2. ANALYZE issue II: The most interesting material to support heel during a long day (perception)

This section discusses the appropriate materials used for heel inserts, as well as the statistical method used to measure the difference between observed and expected frequencies of a set of events or variables. Figure 8, shows the various heel inserts currently available in the market.

Figure 8. Sample shapes of heel inserts which were varied in material properties.

To minimize bias towards specific brands, the researchers converted the images of different brands into grayscale when presenting them on the questionnaires. The decisions of the target customers regarding which heel insert style to purchase are presented in Table 3 and 4. At this stage, the key designs and specific conditions of each heel insert style are explained and analyzed. A total of 168 interviewees completed the survey questions, and they were specifically looking for heel inserts that could help alleviate stress during long periods of standing or walking (as listed in Table 5). It was observed that the “Gel” material is currently trending in heel inserts.

Table 4. The most interesting shape to support heel during a long day

Type	Shapes	No. of respondents (Total = 40)
1		0
2		4
3		35
4		1

Table 5. The most interesting material to support heel during a long day

Types of material	No. respondents (Total = 168)
Gel	40
Gel-Silicone	33
Foam	33
Silicone	7
Rubber	5
Have no idea	50

Additionally, Figure 9–12. present the summarized results of the survey conducted among the target customers regarding the four main designs of heel inserts. The results indicate that approximately 87.5% of the target users (around 35 respondents) preferred model No.3 as the desired shape of the heel inserts that they would like to purchase the most. Based on the questionnaire results, three models (No. 2, No. 3, and No. 4) were selected for further data analysis, where the shape and material of the heel inserts were studied in detail. To establish guidelines for selecting the appropriate heel inserts, the design team analyzed the physical shapes and real colors, as well as the materials used in the production of the product.

Figure 9. Model No.1 – Tapering down shape and multi-curved structure with 2 layers of gel and soft fabric on top surface.

Figure 10. Model No.2 – Tapering down shape and firm structure with foam layer and soft fabric on top surface.

Figure 11. Model No.3 – Tapering down shape with U-shape heel cup style and silicone material.

Figure 12. Model No.4 – Even thickness, small-circles texture on top and oval shape with gel material.

The key benefits of using “Silicone” and “Gel” materials over “Foam” for heel inserts stem from the fact that they are not water-absorbent, and users can clean them with liquid detergents and dry them in the shade, away from direct sunlight. This makes them highly resistant to moisture and creates an environment that is unfriendly to fungi, mold, and other microorganisms that can negatively impact the quality of the heel inserts, as well as the user’s health and comfort. Based on the perceptions and experiences of customers, the design team analyzed and identified the direction for a new product by extracting and revealing the key benefits from existing models one by one. One possible option is to use a soft fabric material on the top surface of the gel heel inserts with a tapering down shape, as illustrated in Figure 13.

Figure 13. The first concept discussion – Tapering down shape with gel layer and soft fabric or leather on top surface.

Furthermore, a leather top surface could also be considered as an alternative to the soft fabric one, as it is more desirable than stretch fabric. Leather is stiff enough to reduce the cushioning effect of the underlying gel, which can make the heel inserts too soft and not firm enough during walking or standing. Additionally, the rougher surface of the leather may help to reduce the added vertical heel motion in the shoe, as it provides more friction. This topping style is very interesting as it can help users to stand or walk more firmly once inserted inside flat shoes. However, with a flat-shaped surface on top, it can be challenging to maintain step balance and foot strength compared to the “U-shape heel cup style”, and a soft fabric or leather top surface may generate odors, fungi or mold after use. They are also prone to tearing apart from the main platform easily, unlike the “Soft fabric on top surface of foam body.” The first concept of heel inserts with a soft fabric or leather top surface discussed in the early stages of conceptual design and development was not suitable for supporting customers’ requirements while walking or standing with flat shoes for a long time, as it is quite challenging to stay on the platform and fix the positions of the feet to match the center of the inserts.

ii. Result for ANALYZE issue II

Based on Table 5, “Gel material” was the most preferred material for making heel inserts among the 168 interviewees, as it was selected by 40 women. On the other hand, rubber material was not well-liked due to its strong smell. Surprisingly, “no idea” was the most common answer (50%) among the respondents, indicating that they did not know exactly what they were selecting and just followed the suggestions of salespersons or influencers. Thus, it is essential to introduce proper guidelines for purchasing heel inserts, particularly for supporting a flat-shoes style during a long day. To determine the preferred shape of heel inserts, a new set of questionnaires was sent to 50 experienced people, and 40 answers were received. The results revealed that Type No.3 was the most preferred shape among the respondents, with 35% of the votes. Those who chose this type stated that its “U-shape heel cup style” comfortably supported their heels and was softer and more flexible than the other types. Overall, the study highlights the importance of proper guidelines for purchasing heel inserts, particularly for those who are not familiar with the different types of materials and shapes available in the market. The study also indicates that “Gel material” and Type no.3 shape are the most preferred options among the respondents.

3.5. Stage 5: The chi-square goodness of fit test

The chi-square goodness of fit test (Miguel & Oscar, 2014) was used to determine if each requirement for the characteristics of the heel insole is equal, with all variables considered as main factors. The results obtained from the Minitab software are shown in Figure 14.

Figure 14. The results of Chi-square Goodness-of-fit test form Minitab.

After conducting the chi-square goodness of fit test using Minitab, the null hypothesis (H0) was rejected with a P-value of 0.00, indicating that the distribution of each requirement of the characteristic of heel insole is not the same and certain variables should be considered. Based on the observed values, the characteristics selected as guidelines for creating the heel inserts are “reduce pain”, “light weight”, and “no odor”, as they had higher observed values compared to expected values.

H0.

The distribution of each requirement of the characteristic of heel insole is same. All of variables are considered as the main factors (O_i = E_i).

H1.

The distribution of each requirement of the characteristic of heel insole is not the same. Some of variables are considered as the main factors (O_i$\ne$E_i).

And the test statistic is

(2) $\mathit{\chi }2=\sum _i^k\frac{{\left(\mathit{Oi}-\mathit{Ei}\right)}^2}{\mathit{Ei}}\mathit{df}=\mathit{k}-1$(2)

where,

O_i = the observed number of cases in category i

E_i = the expected number of cases in each category, for each of the k categories,

i = 1,2, 3, … ., k, into which the data has been grouped

Use the 0.05 level of significant

3.5.1. Stage 6: Translating voice of the customer (VoC) to engineering design

In this study, the Quality Function Deployment (QFD) tool was utilized to assist researchers in identifying important engineering factors or attributes for an alternative design of heel inserts. While researchers may create and classify engineering specifications for a new design based on their viewpoints, some aspects of product characteristics may be subjectively designed (Ilham et al., 2021; Lai et al., 2008; Srizongkhram et al., 2021; Wiegers et al., 2011; Cardoso et al., 2015; Chao et al., 2020; Guinta et al., 1993, Zadry et al., 2015). The translation of demands into a physical form for a new design can be challenging, and QFD can help alleviate issues with “loss in translation”. By utilizing QFD, important engineering factors of foot inserts for lady shoes were identified in this research.

i. Quality function deployment (QFD)

The Quality Function Deployment (QFD) tool is a valuable asset for design teams, enabling them to incorporate quality considerations early in the design process and throughout the system’s entire life cycle. QFD is a customer-driven planning process used in new product design and development to define customer requirements (CRs) and translate them into engineering characteristics (ECs) to maximize customer satisfaction within budget constraints. The voice of the customer (VoC) is critical in QFD, as it directly influences subsequent procedures and resource deployment.

For this study, the researchers have created and classified engineering specifications of foot inserts for lady shoes. The top five specifications considered and selected for this study are weight, dimension, material used, mean lifetime, and shape. These engineering specifications represent the functionality and manufacturing point of view and are directly related to product design and characteristics. The House of Quality (HoQ) method is used as the framework to define the relationship between customer requirements and engineering requirements (i.e., product specifications). The initial stage of the HoQ framework involves asking the question, “What do customers want from the product of interest?” This question leads to the extraction of specific characteristics of a new design, such as the materials used and mechanical properties required to release stresses during all-day wear. This systematic approach ensures that the customer’s needs and preferences are incorporated into the product design, resulting in a product that is both functional and appealing to the target market.

ii. Usefulness of products

The “Usefulness of Products” approach can be applied in this research to provide an alternative perspective and guideline on which components of the existing product or design should be selected as references for modification in support of new design and development (Sarkar & Chakrabarti, 2011). The key component of this approach is the “Usefulness (U) formula”, which requires specific parameters, as explained in Equation 3(3) $\mathit{Usefulness}\mathit{\hspace{0.17em}}\left(\mathit{U}\right)=\mathit{Level}\mathit{of}\mathit{importance}\mathit{\hspace{0.17em}}\left(\mathit{L}\right)\mathit{x}\mathit{Rate}\mathit{of}\mathit{popularity}\mathit{of}\mathit{use}\mathit{\hspace{0.17em}}\left(\mathit{R}\right)\mathit{\hspace{0.17em}}\mathit{x}\mathit{Rate}\mathit{of}\mathit{use}$(3) .

(3) $\mathit{Usefulness}\mathit{\hspace{0.17em}}\left(\mathit{U}\right)=\mathit{Level}\mathit{of}\mathit{importance}\mathit{\hspace{0.17em}}\left(\mathit{L}\right)\mathit{x}\mathit{Rate}\mathit{of}\mathit{popularity}\mathit{of}\mathit{use}\mathit{\hspace{0.17em}}\left(\mathit{R}\right)\mathit{\hspace{0.17em}}\mathit{x}\mathit{Rate}\mathit{of}\mathit{use}$(3)

(4) $\mathit{Rate}\mathit{of}\mathit{use}=\mathit{Level}\mathit{of}\mathit{importance}\mathit{\hspace{0.17em}}\left(\mathit{L}\right)\mathit{x}\mathit{Rate}\mathit{of}\mathit{popularity}\mathit{of}\mathit{use}\mathit{\hspace{0.17em}}\left(\mathit{R}\right)$(4)

It is important to note that the perceived usefulness of a product can only be validated through its actual use. Therefore, the proposed method for measuring usefulness takes into account the product’s usage in terms of popularity and duration, as well as its level of importance to users. By considering these factors, the method aims to provide a more comprehensive and accurate assessment of a product’s usefulness.

Popularity of use is one of the factors that affect the usefulness of a product. A product that is used by many people within a certain time period is considered more useful than those that are used less frequently. This can be measured by the rate of popularity.

Usage duration refers to the length of time a product is used. A product that has been used for a longer period is considered more useful than one that has been used for a shorter time. The usage duration is usually measured in hours per day.

Level of importance is also a crucial factor in measuring the usefulness of a product. The impact of a product on the lives of its users determines its level of importance. Some products are essential, while others are not, and products that are more important to society have a higher value for usefulness. Table 6 shows the five levels of usefulness of a product.

Table 6. Level of importance of products

Code	Points in a scale of 5	Level of importance	Type of importance	Product examples
A	5 (>4.0–5.0)	Extreme	Life saving drugs, life support systems Patient life support system Medical equipment, medicine	Mechanical ventilator, defibrillator, heart/lung bypass machine (Oxygen cylinder, pace makers)
B	4 (>3.0–4.0)	Very high	Essential for daily activities Compulsory daily activities	Water, Taking food, Using restroom
C	3 (>2.0–3.0)	High	Accommodation Social communication Banking transactions	House, clothes, internet, computer, smartphone, EDC machine, Pen, Belt, Spectacles, Shoes
D	2 (>1.0–2.0)	Medium	Household appliances Machines for daily needs	Air conditioning system, refrigerator, washing machine
E	1 (0.0–1.0)	Low	Recreation activities Entertainment systems Recreation systems	Television, Comics, Books, Computer games, Bowling, Go-carting

3.5.2. Stage 7: Applying reverse engineering and finite element analysis to physically check the relationship between shapes and materials

To investigate the relationship between the material used and the shape of the heel inserts, the researchers utilized finite element analysis (FEA) (Jia et al., 2008). FEA was used to analyze the internal stress that occurs as the load on the heel inserts is increased. In order to provide guidelines for users to select the appropriate heel inserts for flat shoes, 3D CAD models of four reference heel inserts were generated using a 3D laser scanner (HandyScan 3D: Model REVscan). These models were referred to as “specimens” in this study and are shown in Figure 8. To test the functional performance of the specimens, five different types of materials (latex foam, silicone gel, EVA foam, original gel, and others) were assigned to each specimen for force-distribution simulations. All specimens were tested for characteristics such as deformation after applying a load and direction of force distribution using finite element analysis (FEA). To input the specimen into the FEA, the researchers utilized the reverse engineering (RE) technique, which involves three main steps: data acquisition, surface reconstruction, and surface fitting (Besl & Jain, 1985; Eilam, 2005; Vezzetti, 2007). The 3D laser scanner (HandyScan 3D: Model REVscan) was used to capture surface details of the four samples, and the resulting 3D CAD models were used in the FEA application in the “Solidworks” platform. The FEA was applied to investigate the internal stress that occurred after applying a load.

4. Results

Accurate translation of communication between customers and manufacturers is crucial for meeting the customer’s requirements while also ensuring that the engineering specifications are met and the manufacturing costs are minimized. This proposed research can be divided into four main sections: quality function deployment and house of quality, usefulness of products, simulation of the characteristics of various shapes and materials of heel inserts, and analysis of material selection and cost.

4.1. Quality function deployment and house of quality

The questionnaire results indicated that the target customers desired a product that was lightweight, easy to clean, durable, had no odor, and could provide immediate pain relief using various materials. Three references were used to support the design stage, as shown in Figure 15. The HoQ was used to list all expectations and requirements in the left column, with engineering viewpoints located beneath the roof of the HoQ, as illustrated in Figure 16. The relationships between customer requirements (What) and engineering specifications (How) were represented by numbers 9, 3, 1, and none, with higher values indicating strong, moderate, weak, or no relationships, respectively. Relevant engineering characteristics of foot inserts were used to select engineering specifications. The researchers assigned interrelationships between these factors (i.e., functional requirements), which were classified into three levels using symbols: positive correlation (+), negative correlation (-), and no correlation (O) between factors for the HoQ.

Figure 15. Three references used for supporting the design stage.

Figure 16. House of quality with relationship matrix between customer requirement and functional requirement.

To clarify, in the example provided in this study, the researchers analyzed the correlation between pairs of factors listed in the HoQ. For the first pair, “Weight” and “Dimension” of the heel inserts, a positive correlation was identified since a lighter weight generally corresponds to smaller dimensions. The plus symbol (+) was placed in the correlating matrix to indicate this relationship. However, for the second pair, “Dimension” and “Material used”, the researchers determined that there is no correlation between the two factors, since changing the material used does not affect the physical shape of the heel inserts. The symbol “O” was used to represent this lack of correlation.

4.2. Usefulness of products

4.2.1. Design suggestion of “Design a – Reference 1 (Model No.2)”

The key factors identified from the customer’s perspective for this particular heel insert style are presented in Figure 17. In addition, Table 7 and Figure 18 present the parameters that have been assigned for evaluating the overall usefulness of the product.

Figure 17. Customer perception on Design a – Reference 1 (Model No.2).

Figure 18. Assigned parameters of usefulness (U) for Design A

Table 7. Summarized design suggestion of “Design a – Reference 1 (Model No.2)”

Reference 1 (Model No. 2)	Level of importance (L) (Max. = 5)	Rate of popularity of use (R) (Total 168 pp.)	Rate of use (F x D) (Total 24 hr.)
Foam	4.1 (Code A)	33	5

Therefore, applying Eq. 3(3) $\mathit{Usefulness}\mathit{\hspace{0.17em}}\left(\mathit{U}\right)=\mathit{Level}\mathit{of}\mathit{importance}\mathit{\hspace{0.17em}}\left(\mathit{L}\right)\mathit{x}\mathit{Rate}\mathit{of}\mathit{popularity}\mathit{of}\mathit{use}\mathit{\hspace{0.17em}}\left(\mathit{R}\right)\mathit{\hspace{0.17em}}\mathit{x}\mathit{Rate}\mathit{of}\mathit{use}$(3) can determine Usefulness (U) of Design A:

(5) $\begin{array}{rl}& \mathit{Usefulness}\mathit{\hspace{0.17em}}\left(\mathit{U}\right)=\mathit{Level}\mathit{of}\mathit{importance}\mathit{\hspace{0.17em}}\left(\mathit{L}\right)\mathit{\hspace{0.17em}}\mathit{x}\mathit{Rate}\mathit{of}\mathit{popularity}\mathit{of}\mathit{use}\mathit{\hspace{0.17em}}\left(\mathit{R}\right)\mathit{\hspace{0.17em}}\mathit{x}\mathit{Rate}\mathit{of}\mathit{use}\\ & =\left(4.1/5\right)\times \left(33/168\right)\times \left(5/24\right)\\ & =\mathbf{0}.\mathbf{0336}\end{array}$(5)

4.2.2. Design suggestion of “Design B – Reference 2 (Model No.3)”

The key considerations obtained from the customer’s viewpoints of this heel insert style are illustrated in Figure 19. The “U-shaped heel cups” are placed directly and easily under the heel, providing cushioning and support. Heel cups could help treat heel pain caused by plantar fasciitis and prolonged standing. The assigned parameters for calculating the usefulness of the product are shown in -Table 8 and Figure 20.

Figure 19. Customer perception on Design B – Reference 2 (Model No.3).

Figure 20. Assigned parameters of usefulness (U) for Design B

Table 8. Summarized design suggestion of “Design B – Reference 2 (Model No.3)”

Reference 2 (Model No. 3)	Level of importance (L) (Max. = 5)	Rate of popularity of use (R) (Total 168 pp.)	Rate of use (F x D) (Total 24 hr.)
Gel-Silicone	4.1 (Code A)	33	8

Therefore, applying Eq. 1(1) $\mathit{Samplesize},\mathit{n}=\mathit{N}\times \left(\frac{\frac{{\left(\mathit{z}\right)}^2\times \mathit{p}\times \mathit{q}}{{\left(\mathit{e}\right)}^2}}{\left(\mathit{N}-1\right)+\frac{{\left(\mathit{z}\right)}^2\times \mathit{p}\times \mathit{q}}{{\left(\mathit{e}\right)}^2}}\right)$(1) can determine Usefulness (U) of Design B:

(6) $\begin{array}{rl}& \mathit{Usefulness}\mathit{\hspace{0.17em}}\left(\mathit{U}\right)=\mathit{Level}\mathit{of}\mathit{importance}\mathit{\hspace{0.17em}}\left(\mathit{L}\right)\mathit{x}\mathit{\hspace{0.17em}}\mathit{Rate}\mathit{of}\mathit{popularity}\mathit{of}\mathit{use}\mathit{\hspace{0.17em}}\left(\mathit{R}\right)\mathit{x}\mathit{Rate}\mathit{of}\mathit{popularity}\mathit{of}\mathit{use}\\ & =\left(4.1/5\right)\times \left(33/168\right)\times \left(8/24\right)\\ & =\mathbf{0}.\mathbf{0537}\end{array}$(6)

4.2.3. Design suggestion of “Design C – Reference 3 (Model No.4)”

The key considerations obtained from the customer’s viewpoints of this heel insert style are illustrated in Figure 21. . The primary design of Model No.4 features a thin thickness with no tapered design, a textured gel surface with small circles on top, and an oval-shaped boundary. Compared to other designs, this model has a relatively small size. To address the issue of heel pain after walking or standing, users have resorted to changing their shoes during the day while performing office tasks. The assigned parameters used for calculating the product’s usefulness are shown in Table 9 and Figure 21 and Figure 22.

Figure 21. Customer perception on Design C – Reference 3 (Model No.4).

Figure 22. Assigned parameters of usefulness (U) for Design C

Table 9. Summarized design suggestion of “Design C – Reference 3 (Model No.4)”

Reference 3 (Model No. 4)	Level of importance (L) (Max. = 5)	Rate of popularity of use (R) (Total 168 pp.)	Rate of use (F x D) (Total 24 hr.)
Gel	4.1 (Code A)	40	3

Therefore, applying Eq. 1 can determine Usefulness (U) of Design C:

(7) $\begin{array}{rl}& \mathit{Usefulness}\left(\mathit{U}\right)=\mathit{Level}\mathit{of}\mathit{importance}\mathit{\hspace{0.17em}}\left(\mathit{L}\right)\mathit{x}\mathit{Rate}\mathit{of}\mathit{popularity}\mathit{of}\mathit{\hspace{0.17em}}\mathit{use}\mathit{\hspace{0.17em}}\left(\mathit{R}\right)\mathit{x}\mathit{Rate}\mathit{of}\mathit{use}\\ & \begin{array}{c}=\left(4.1/5\right)\times \left(40/168\right)\times \left(3/24\right)\\ =\mathbf{0}.\mathbf{0244}\end{array}\end{array}$(7)

4.3. Simulating the characteristics of the various shapes and materials of the heel inserts

The simulation involved the application of a vertical force (245 N, half of body weight) on the heel inserts while assuming that the bottom surface was fixed on the floor, in order to simulate a standing position. Table 10 (Ashby, 2005; Suzuki, 2016) presents the material properties, such as yield strength, elastic modulus, mass density, and Poisson’s ratio, which are utilized in the finite element analysis.

Table 10. The absolutely required properties for finite element analysis

Material	Elastic modulus (N/m^2)	Poisson’s ratio	Mass density (Kg/m^3)	Yield strength (N/m^2)
Latex foam	1900000	0.499	960	168000000
Silicone gel	150700	0.05	2330	308000
EVA foam	50000000	0.25	950	480000000
Original gel	28900	0.05	2330	102000

The simulation was conducted using static analysis. After obtaining the results, the most suitable shape and material of heel inserts with the lowest stress could be identified, and these properties would serve as guidelines for the development of a new product. The results of the FEA, including stress results, are presented in Table 11 and Figure 23. The stress from each specimen with variations in material was analyzed to determine the trend for the best type of heel inserts and material to develop and generate optimal engineering designs of heel inserts that meet the customer’s requirements. Figure 24 provides an overview of the general steps involved in conducting FEA.

Figure 23. The stress (kPa) on foot insoles from finite element (stress vs. shape).

Figure 24. The steps required for FEA

Table 11. Stress results from finite element analysis

Material Shape	Stress (kPa)
	Latex Foam	Silicone Gel	EVA foam	Original Gel
	636	1050	876.8	1060
	420	222	331	216.3
	325.5	285.6	260	285.6
	199.6	1034	669.9	1034

4.4. Summarize the obtained answers

Based on the findings presented in Figure 23, it is suggested that the materials “silicone gel” and “original gel” may not be suitable for the heel inserts shaped as “Specimen No.1 and No.4”. This is because the cross-sectional shape of these inserts appears to be oval-like in the 3D model, and the maximum stress occurred around the center of the heels, indicating less durability compared to other materials such as latex foam and EVA foam. In contrast, latex foam produced the lowest stress for “Specimen No.1 and No.4”.

For the flat-top surface of heel inserts in “Specimen No.2 and No.3”, “Silicone gel, EVA foam, and Original gel” are recommended. These two shapes have significantly lower maximum stress than “Specimen No.1 and No.4” for almost all materials. For “Specimen No.2”, materials such as “silicone gel” and “gel” appear to provide proper support for the heel inserts, which contain a tapered thick shape with a big and flat area around the top surface.

For “Specimen No.3”, it is suggested that this shape can support all four types of materials: latex foam, silicone gel, EVA foam, and original gel, as weight can be distributed evenly over the highest contact area along the heel inserts area since the “heel-cups style” shape is applied to reduce fatigue.

Besides considering shock absorption issues, the manufacturer should also take into account other factors such as comfort, odor, water absorption, durability, and price when developing and improving the product to meet customer satisfaction and requirements. It is recommended that these factors be carefully examined in further steps.

4.5. Concept development

The previous section contains six key ideas that provide important insights into the design of heel inserts. In the next section, the design team will use these key ideas to create a platform for usefulness calculation and conceptual design activity. To aid this process, existing products of heel inserts will be examined and discussed as design references. These references will help the design team to develop new and improved heel inserts that meet the needs and requirements of customers.

Part 1: Usefulness concept – the key point for applying the usefulness concept in this study is to identify the direction of the physical shape of heel inserts. The study focuses on two key concepts—“tapering down concept” and “heel cups” - to design a classic style of heel inserts that require flexibility and elasticity properties. Elasticity is crucial for the object to regain its original form once the deforming force is removed, as seen in rubber, gel, or silicone products. On the other hand, flexibility refers to an object that is easily deformed and will remain that way. The study also emphasizes the importance of selecting the appropriate material for making the heel inserts. This material should be analyzed and discussed to ensure that it meets the required properties for flexibility and elasticity. By applying the usefulness concept, the study aims to design heel inserts that are useful and practical for users.

Part 2: Customer perceptions – there are three main shapes that are suitable for making heel inserts: gel, gel-silicone, and foam with fabric on top. The shape of the heel inserts is essential and taken into consideration by the target users to match their shoes’ shape with the inserts. This is because, after adding heel inserts, a pair of shoes should look almost the same as the original or as good as the original. Moreover, the choice of material is also important in creating useful heel inserts. The study considers different materials, such as gel, gel-silicone, and foam with fabric on top. Each material has its own unique properties that make it suitable for certain types of shoes or activities. For example, gel inserts may provide more cushioning and shock absorption, while foam with fabric may offer more breathability and comfort.

For the key consideration, the study aims to identify the most suitable shape and material for making heel inserts that are useful and practical for users. By considering these factors, the study can create heel inserts that not only provide support and comfort but also match the users’ shoe shape and style preferences.

Part 3: Collecting data − Users perceive that heel inserts can support their body weight, reduce the risk of slipping, and prevent heel pain while walking. This highlights the practical usefulness of heel inserts for users. The study uses data obtained from questionnaires to guide the design team in creating a conceptual model of heel inserts, where the physical shape of the heel insert is considered a key issue. Additionally, the study identifies popular materials for heel inserts, such as gel, gel-silicone, and foam. These materials have unique properties that make them suitable for different types of shoes and activities. By understanding the users’ preferences and needs, the study can create heel inserts that are not only practical but also aesthetically pleasing to the users.

For the key consideration, the study aims to create useful and practical heel inserts that meet the users’ needs and preferences. By considering the physical shape, material, and users’ perceptions, the study can design heel inserts that provide support, comfort, and reduce the risk of foot pain and injury.

Part 4: Key concept obtained − the design of the heel inserts should have a slim design with tapering shape, which narrows at one end. The size and shape of the heel inserts should be suitable for improving posture and other medical issues. The inserts should assist users with posture and ankle stability, relieve pressure on the hips, knees, and back, and shift weight to the heel. To ensure that the heel inserts do not slip out of place, the entire body of the inserts should be permanently attached to the shoe around the heel area using double-sided adhesive tape. The inserts should be permanently attached to each pair of shoes. The size of the heel inserts should come in various sizes to fit different shoe sizes. Users can also cut the inserts to fit their heel’s area, which is suitable for any shoes with a heel of less than 3.5 cm or lower, as mentioned in Table 1. The design of the heel inserts should not be a “one-size-fits-all solution” since different users have different shoe sizes and heel shapes. By providing heel inserts in various sizes and allowing users to cut the inserts to fit their heels, the design can provide a customized and comfortable fit for each user.

For the key consideration, the design of the heel inserts should prioritize comfort, stability, and practicality. By incorporating a slim design with tapering shape, size variability, and permanent attachment to the shoe, the design can create useful and practical heel inserts for users.

Part 5: Suggestion for conceptual design − the alternative designs of the heel inserts illustrated in “Reference 1 to 3” should be considered and compared based on the concept of “usefulness (U).” This implies that the designs of the heel inserts should be evaluated based on their practical usefulness to the users. Reference 2, which illustrates heel cups with gel-silicone heel pads and a U-shaped ergonomic design, is preferred by target users based on their comments. This design may offer better comfort, support, and stability to the users compared to other designs. However, it is also important to apply other techniques to check whether the suggested design platforms are suitable for developing heel inserts. These techniques could include testing the designs on a small group of users, conducting biomechanical analysis, and evaluating the durability and safety of the materials used. By applying these techniques, the study can ensure that the suggested design platforms meet the users’ needs, provide practical usefulness, and are safe and durable for long-term use.

Part 6: Findings − based on the results obtained from the questionnaires, the users’ preferred material for the heel inserts is “Gel” with a maximum percentage of 23.81% compared to other materials. Therefore, the heel inserts with “Gel” material are selected as the master model for calculating the ratio of usefulness. Assessing the ratio of usefulness of the three different designs presented in Reference 1, 2, and 3, the usefulness of “Design A—Reference 1 (Model No.2)” is 0.0336, the usefulness of “Design B—Reference 2 (Model No.3)” is 0.0537, and the usefulness of “Design C—Reference 3 (Model No.4)” is 0.0244, or in ratio form, 1.38:2.20:1. Based on these results, it is recommended to apply the concept of “Reference 2 (Model No.3)—Tapered gel-silicone heel cups with U-shaped ergonomic design” in the suggested design of heel inserts for flat shoes (as shown in Figure 25). In the design stage, the key points obtained from the customers’ requirements presented in Table 3, as well as the engineering viewpoints from FEA, should be taken into consideration to ensure that the final design meets the practical usefulness requirements and offers improved comfort, support, and stability to the users, while reducing the risk of foot pain and injury.

Figure 25. The guidelines for developing a new design of heel inserts.

4.6. Analyzing material selection and cost

After obtaining results from previous activities, further analysis is required to reveal hidden issues regarding heel inserts for flat shoes. The analysis process is divided into two main sections: Analyze Part 1, which focuses on the relationship between heel inserts and cost, and Analyze Part 2, which assesses the satisfaction level of user

ANALYZE Part 1: focuses on the relationship between the material used in heel inserts and their cost, in order to help users make informed purchasing decisions.

To determine the relationship between heel inserts’ material and cost, a cost analysis was conducted for five different materials of heel inserts: gel, gel-silicone, foam, silicone, and rubber. The popularity of “Gel” as an insert material among the 168 target customers was 40%, but the cost of gel inserts was a key consideration in the purchasing decision, as shown in Table 12. The results of the cost analysis showed that foam inserts were the cheapest, followed by rubber, silicone, gel-silicone, and gel. However, it is important to note that the cost of the heel inserts does not necessarily reflect their effectiveness or comfort level. Therefore, when making a purchasing decision, customers should consider not only the cost of the inserts but also their individual needs and preferences. It is also important for designers to consider the balance between cost and effectiveness when selecting the material for the heel inserts.

Table 12. The relationship between heel inserts’ material and cost

Rank	Type of material	No. of interviewees (Total = 118)	Cost
			THB	USD
1	Silicone	55	52–358	1.47–10.11
2	Gel	30	52–430	1.47–12.14
3	Foam	22	52–133	1.47–3.75
4	Rubber	11	42–113	1.19–3.19

Note: * 35.426 THB per 1 USD (Date: 15/Nov/22) (Bank of Thailand (BOT), 2022

The summarized data revealed that the price of the heel inserts’ product, from the customers’ viewpoints, could vary from less than one dollar to hundreds of dollars over the shoes’ price depending on the quality and the material used. The maximum price that the target users could afford was around $150. The target users mentioned that the “mix-material heel inserts as gel-silicone” might be more expensive than the other single material types; therefore, “gel-silicone” heel insert was ignored in the studied.

“Silicone material (53.39% of the 168 women experienced on wearing flat shoes)” was shown the most popular material type for making heel inserts since the users thought (based on the reviews, the advertisement, or the personal experiences) that its property was more flexible than gel (the second rank). When comparing the slip property, gel was more slippery than silicone and stable foam material. If gel was chosen for making the heel inserts, it might cause discomfort for standing long time or walking.

For “rubber (the fourth rank)”, most of the users thought that the sole of shoes was made from rubber where it could not help them to reduce the pain on their foot directly. Wearing rubber sole shoes would make their feet smell bad and horrible after a long day of activity.

From the target users’ opinions, “silicone” was recommended to be designed as the heel inserts, whereas “rubber” must be the last choice for this case. All answers obtained from the target users were influenced by the reviews, close friends, sale assistants, fashion advisors, or media advertisements.

Therefore, the customers believe that the “Silicone Material” is the best choice for making heel inserts. However, in order to check the data obtained from those perceptions are matched to the applicable mechanical or physical characteristics of the heel inserts where the materials and geometric shapes have been played as important issues.

ANALYZE Part 2: focuses on the satisfaction level and the expectation after using the heel inserts with different materials.

The participants were asked to rate the scale of comfortness level of each material for foot inserts. The rate scale is started from 1 to 5 (1 = very low, 2 = low, 3 = moderate, 4 = high and 5 = very high). Then data collected were gathered and analyzed to find that which one of material that the customers would prefer the most (Table 13).

Table 13. The satisfaction level and cost per unit

Material	Satisfaction Level (percentage)			Cost
	Average	S.D.	Satisfaction Level of comfortness perception	THB	USD
Silicone	4.686	0.47	Very high	52–358	1.47–10.11
Gel	4.271	0.78	High	52–430	1.47–12.14
Foam	3.780	1.04	Moderate - High	52–133	1.47–3.75
Rubber	3.136	3.14	Moderate	42–113	1.19–3.19

Note: * 35.426 THB per 1 USD (Date: 15/Nov/22) (Bank of Thailand (BOT), 2022)

Based on the customer feedback, it seems that “silicone material” has several benefits when it comes to foot comfort and health. It is particularly popular among athletes, disciplined forces, and consumers who need to stand and walk for long periods. The following are some of the benefits of using “silicone material” for shoe insoles:

1. Soft and comfortable: Silicone material is soft and comfortable, which makes it an ideal material for making shoe insoles. It provides cushioning and support to the feet, reducing fatigue and discomfort.

2. Shock absorption: Silicone material has a shock-absorbing effect, which helps to reduce the impact on the feet when walking or running. This makes it an excellent material for shoe inserts, particularly for athletes.

3. Relieves heel fatigue and pain: Silicone material can relieve heel fatigue and effectively eliminate or reduce heel pain. This is particularly beneficial for people who suffer from plantar fasciitis or other foot problems.

4. Prevents and improves symptoms: Silicone material can effectively prevent and improve symptoms such as knee pain, low back pain, and spinal lesions caused by foot problems. This makes it an excellent material for people who need to stand or walk for long periods.

5. Tear-resistant and extensible: High-quality silicone insoles for shoes are tear-resistant and have certain extensibility, making them durable and long-lasting.

It seems that silicone material is an excellent choice for shoe insoles, particularly for people who need to stand or walk for long periods or suffer from foot discomfort. Its soft and comfortable properties, shock-absorbing effect, and ability to relieve heel fatigue and pain make it a popular choice among athletes, disciplined forces, and consumers.

5. Results and discussion

5.1. Material selection versus cost

Despite being the most popular material choice, gel was found to be relatively expensive compared to foam and rubber. Therefore, the cost of gel heel inserts may affect the purchasing decision of some customers. However, it is important to note that the cost of the heel inserts should be weighed against the potential benefits of improved comfort and reduced pain.

Based on these findings, it may be worthwhile for the design team to consider offering heel inserts in multiple materials to cater to different customer preferences and budget constraints. Alternatively, they could explore ways to reduce the cost of gel heel inserts without compromising on quality or effectiveness.

5.2. Satisfaction level and expectation

To assess the satisfaction level and expectations of the users after using heel inserts with different materials, a survey was conducted. The survey was sent to 168 women who had previously experienced foot pain while wearing flat shoes. The survey asked about their level of satisfaction and expectations after using heel inserts made of five different materials: gel, gel-silicone, foam, silicone, and rubber.

The results showed that the highest level of satisfaction was reported by users who had used heel inserts made of silicone (67.86%), followed by gel (47.62%), foam (40.48%), gel-silicone (33.33%), and rubber (19.05%). Users who had used silicone heel inserts reported that they felt more comfortable, had less foot pain, and felt more stable while wearing flat shoes. They also reported that the silicone material was more durable and long-lasting compared to other materials.

In terms of expectations, most users expected heel inserts to provide comfort and support, reduce foot pain, and improve stability while wearing flat shoes. They also expected the heel inserts to be durable and long-lasting. Interestingly, users who had used gel heel inserts reported having high expectations but lower levels of satisfaction compared to users who had used silicone heel inserts. This could be due to the slipperiness of the gel material, which may have caused discomfort or instability while wearing flat shoes.

The survey results suggest that silicone is the most effective material for heel inserts in terms of user satisfaction and expectations. The results also highlight the importance of considering user preferences and expectations when designing heel inserts for flat shoes.

6. Conclusion and contribution

The ultimate goal of the study is to support the design team in creating heel inserts that offer practical usefulness, improve comfort, support, and stability for the users, and reduce the risk of foot pain and injury. The design team can achieve this goal by evaluating the shape, size, and material of the heel inserts based on the feedback from the target users, and applying techniques such as biomechanical analysis, testing, and evaluation of the durability and safety of the materials used. By creating heel inserts that meet the needs and preferences of the users, the study can contribute to the development of more effective and practical solutions for foot pain and injury. The guidelines for creating heel inserts for flat shoes have been developed based on various methodologies, including quality function deployment, house of quality, key factor consideration, usefulness calculation, and finite element analysis. The recommended shape for the heel inserts is similar to Model No. 3, which provides proper support to reduce fatigue, stress, or pain around the heels. This shape is tapered with heel cups to distribute weight evenly over the highest contact area of the inserts. To meet customer requirements, the key characteristics of the heel inserts should be to reduce pain, lightweight, and odorless. Therefore, the recommended shape for the heel inserts is a combination of a rectangle with a rounded end, a U-shaped ergonomic design, and heel cups. The materials that are suitable for this design are flexible and soft, such as gel-silicone and gel. These materials are easy to clean and do not leave any stains or odors on the inserts. Additionally, materials such as latex foam and EVA foam can be used in combination with soft fabric to create the inserts, but they may generate an unpleasant smell. The topmost surface of the soft fabric may also tear apart easily after using the inserts for a while.

To support manufacturers in creating high-quality heel inserts products, close attention should be paid to the factors mentioned above, including the selection of the right materials and structural design. For the benefit of target users, a platform for selecting the appropriate design of heel inserts that matches the materials used has been generated. It is important to note that the physical shape and size of a heel insert alone cannot tell the whole story of its comfort and accuracy after using the product. By selecting the right design with the right material, trial-and-error activity and random repurchasing of new products can be minimized.

Furthermore, the proposed approach for creating new product designs has been modified and constructed based on the classic module of product development processes (PDPs) presented by Darian Unger and Steven Eppinger (Figure 26). This approach can be used as a guideline for future product development (Figure 27). The created diagram illustrates the key concept of a modularized design platform proposed by this research that emphasizes flexibility. In the “Create prototype” stage, the design team can choose to change or replace the materials used for a part or component if the original materials are unavailable or fail virtual simulation testing. Similar quality materials can be used as substitutes. Once the material selection for a part or component is finalized, only that module is reconsidered in the “Construct design iterations” stage, rather than redesigning the entire machine or product. If a part is redesigned using new materials, the process of “Classify the required components” is applied to reclassify main components and sub-components. The sequence of steps is then followed in a straight line from left to right in the diagram. Compared to the previous diagram (Figure 26), the loop is only present in the “Detailed design” stage, where the specific details of each component, including material selection and physical properties, are carefully considered without necessarily affecting the overall design or material platform.

Figure 26. Module of the PDPs (Darian Unger & Steven Eppinger) for general staged process.

Figure 27. Module of the PDPs of actual process proposed by this research.

Author contributions

Conceptualization, literature review, methodology, formal analysis, investigation, data validation, supervision, software, writing—review & editing S.R.; literature review, methodology, writing—original draft preparation, collected data, performed the first data analyses, software J.T.; and literature review, collected data, methodology, writing—original draft preparation T.N. All authors have read and agreed to the published version of the manuscript.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This research received no external funding.