A Needs-Based Design Method for Product–Service Systems to Enhance Social Sustainability

Abstract

This study proposes a design method for the evaluation and redesign of product–service systems (PSSs) from the perspective of social sustainability, one that applies Max-Neef’s framework of fundamental human needs. The proposed method systematically connects PSS functions and requirements—identified through service blueprints and value graphs—to “satisfiers” and “barriers” extracted via needs-based workshops. This connection enables the identification of functions that either contribute to or hinder the fulfillment of fundamental human needs and guide the generation of redesign proposals aimed at sufficiency-oriented outcomes. A case study involving a smart-cart system in Osaka, Japan, was conducted to demonstrate the applicability of the method. Through an online workshop, satisfiers and barriers related to both physical and online shopping experiences were identified. The analysis revealed that existing functions such as promotional information and automated checkout processes negatively impacted needs such as understanding and affection due to information overload and reduced human interaction. In response, redesign concepts were developed, including filtering options for information, product background storytelling, and optional slower checkout lanes with human assistants. The redesigned functions contribute to the fulfillment of fundamental human needs, indicating that the proposed method can enhance social sustainability in PSS design. This study offers a novel framework that extends beyond traditional customer requirement-based approaches by explicitly incorporating human needs into function-level redesign.

1. Introduction

One of the UN Sustainable Development Goals adopted in 2015 is the realization of sustainable consumption and production (SCP). This is a universal call to action to end poverty, protect the planet, and ensure that by 2030 all people enjoy peace and prosperity [1,2]. The achievement of SCP patterns requires not only improving environmental sustainability by reducing environmental loads and resource consumption, but also improving the sufficiency of SCP for satisfying fundamental human needs and enhancing social sustainability. This means that an approach is needed that takes into account not only the production side but also the consumption side to achieve SCP patterns [3].

On the production side, product and service design has been studied mainly in the areas of circular economy [4,5] and eco-design, with a focus on environmental sustainability [6,7,8]. On the consumption side, there have been several studies on business models and consumer behavior, generally with a focus on environmental sustainability [9,10,11], and social sustainability [12,13]. However, there are few studies on product and service design that consider social sustainability [14,15].

In the field of product and service design, many design methodologies have been proposed for the design of functions based on requirements in product design [16,17,18,19] and service design [20,21]. Similarly, requirements-based design methodologies have been proposed [22,23], but almost no product and service design methodologies have been studied from the viewpoint of sufficiency for consumers.

To achieve SCP, it is important to improve sufficiency for consumers; this can be achieved in the context of fewer products and services, and for this purpose, a design method aiming to provide highly sufficient products and services is necessary. However, in most cases, sufficiency is evaluated at the global level [24,25], or national level [26,27], and there are few methods available for evaluating sufficiency at the product or service level. Moreover, even if sufficiency could be evaluated at the product or service level, it is not clear how to design products and services to improve sufficiency.

Focusing on the regionality of consumer sufficiency, there have been several studies using the human scale development (HSD) approach proposed by Max-Neef [28,29,30]. The HSD framework is widely referenced in the field of sustainable consumption, particularly in discussions of social sustainability, where it is frequently cited alongside Doyal and Gough’s theory of human need [31]. In recent years, the HSD approach has also been applied in studies on the circular economy to evaluate social sustainability and its relationship to the fulfillment of human needs [32,33]. There have also been efforts to apply the HSD approach to product and service design. For example, Kobayashi and Fukushige proposed the living-sphere approach as a specific product design approach for locally oriented sustainable design [34]. The living-sphere approach is characterized by incorporating Max-Neef’s fundamental human needs framework into the evaluation of the sufficiency of products and services, with the goal of designing highly sufficient products and services based on the relationship between fundamental human needs and products/services.

In this study, we aim to contribute to social sustainability in sustainable consumption by designing products and services that help fulfill Max-Neef’s description of fundamental human needs. Specifically, we seek to operationalize the conceptual structure of the living-sphere approach and propose a needs-based design method for product–service systems (PSSs) based on it. To demonstrate the applicability of the proposed method, a case study was conducted on a smart-cart system implemented in Osaka, Japan. The aim of the case study was to examine how the method can be used to identify functions that promote or hinder the fulfillment of fundamental human needs, and to propose redesign ideas that enhance social sustainability in PSS design. The remainder of this paper is organized as follows. In Section 2, we review related works on HSD, and product and service design. Section 3 outlines our proposed needs-based PSS design method, and Section 4 shows a case study of its application to a smart cart. Section 5 discusses the proposed method and the case study, and Section 6 concludes the paper.

2. Related Work

2.1. Human-Scale Development and Its Applications

To help groups or communities engage in the process of change and development, Chilean economist Manfred Max-Neef proposed the HSD approach [28,35]. HSD has three main pillars: (1) the satisfaction of fundamental human needs, (2) the generation of growing levels of self-reliance, and (3) the construction of organic articulations associating people with nature and technology, global processes with local activity, the personal with the social, planning with autonomy, and civil society with the state.

In HSD, fundamental human needs are the same in all cultures and historical periods, changing only at a very slow pace according to our evolution as a species; satisfiers are the values, attitudes, norms, laws, institutional arrangements, organizations, actions, and ways of using spaces, resources, and nature that define needs satisfaction in specific contexts, and which vary across cultures and through time [36,37]. Max-Neef organized fundamental human needs into two categories: axiological and existential. The axiological category includes subsistence, protection, affection, understanding, participation, idleness, creation, identity, and freedom. The existential category includes being, having, doing, and interacting.

Table 1. Matrix of fundamental human needs.

	Being	Having	Doing	Interacting
Subsistence
Protection
Affection
Understanding
Participation
Idleness
Creation
Identity
Freedom

shows a matrix that relates axiological and existential categories, and within which the individual cells represent satisfiers. Max-Neef also proposed a workshop method to construct not only a matrix of positive satisfiers (those that contribute need fulfillment), but also a matrix of destructive elements—referred to as negative satisfiers—that hinder need fulfillment within a society [37,38,39]. For example, Kobayashi et al. introduced the term barriers to refer to these negative satisfiers in order to clearly distinguish them from positive ones [39]. Following this approach, in this paper we refer to positive satisfiers simply as “satisfiers”, and to negative satisfiers as “barriers”. We adopt this terminology throughout the manuscript for consistency and clarity.

While most of the applications of the HSD framework have aimed to identify community issues and propose solutions, a few cases have applied the HSD framework to evaluate services. Gimelli et al. analyzed how water service arrangements can hinder or contribute to the satisfaction of the whole range of fundamental human needs in an urban informal settlement by conducting semi-structured, in-depth individual interviews and informal individual and group discussions including 29 participants aged 18 years or older (in India) [40]. Jolibert et al. analyzed the causes of conflict over the Sado River in Portugal by extracting the satisfiers of fish farmers, reserve managers, and otters, who are stakeholders in the river, through semi-structured interviews and a literature review [30]. The commonality among them is the application of the HSD framework to the sufficiency assessment of services provided in the community.

The living-sphere approach has also been proposed as an attempt to apply the HSD framework to product design [34] (Figure 1). In this approach, it is assumed that fundamental human needs are fulfilled by activating the satisfiers. Many products are connected to satisfiers in the framework of the living-sphere approach. Therefore, extracting adequate satisfiers is a key task in the living-sphere approach. Examples of the application of this approach include the modeling of the relationship between a product and relevant satisfiers through its functions and requirements, and an evaluation of bathroom design in Vietnam using satisfiers as evaluation criteria [41,42]. However, there are no examples of HSD being applied to the designs of products or services.

2.2. Product and Service Design

Engineering design methods generally produce design details in a top-down process, starting from the product requirements, and then considering the designs of functions, and then structures [16,17]. For example, in function structure mapping [43], structures are designed from functions, and in value graphs and quality function deployment (QFD) [44,45], functions and structures are designed from requirements. In systems engineering, first, the boundary of the target system to be designed is defined; this is followed by a requirements analysis, and then the architecture design [18,19]. In requirements analysis, stakeholders are identified and requirements are detailed. In architectural design, the functional architecture is first designed based on the requirements; this is followed by a design of the physical architecture that assigns functions to hardware and software [19].

Service design has been extensively studied in the marketing and engineering fields. Various service design methods have been proposed; one common process consists of designing services and products based on customer requirements [20,46]. In particular, a marketable set of products and services capable of jointly fulfilling users’ requirements is called a PSS [47], and there has been a shift from service design to PSS design. Various methods have been proposed for service design and PSS design [23,48,49], but one of the most commonly used methods is service blueprints [22,50]. Service blueprints are used to clarify the functions of the services and products needed to fulfill customer requirements, and can model the functions of involved stakeholders separately. Although there are no fixed rules for representation, a typical service blueprint includes five elements, as shown in Figure 2: customer actions, onstage/visible contact employee actions, backstage/invisible contact employee actions, support processes, and physical evidence [50]. In the field of PSS design, extensions of service blueprints to PSS and EcoDesign [51,52], as well as PSS design frameworks incorporating service blueprints, have been proposed [21].

As described above, existing methodologies in product and service design focus primarily on deriving product or service functions from product and customer requirements, whether through top-down engineering approaches or structured service design frameworks. While these methods are effective in designing products and services based on customer requirements that have been identified, they do not explicitly consider the broader and more socially grounded concept of fundamental human needs. Moreover, these methods typically lack mechanisms to evaluate how design elements may contribute positively or negatively to human well-being—what Max-Neef refers to as satisfiers/barriers.

Although some approaches attempt to explain customer requirements by referring to psychological theories such as Maslow’s hierarchy of needs, this model has been widely criticized. Maslow himself later acknowledged that the hierarchical nature of needs was not universally applicable [53]. Furthermore, scholars have argued that the model reflects individualistic and Western cultural assumptions that may not be suitable for societies that prioritize social or collective needs over individual self-actualization [54,55]. In contrast, Max-Neef’s framework proposes a culturally neutral, non-hierarchical view in which all fundamental human needs are equally important and can be fulfilled simultaneously. This framework allows for a more inclusive and socially responsive interpretation of customer requirements.

As a result, conventional design approaches may overlook important aspects related to sufficiency and social sustainability in design outcomes. This gap calls for a new approach that incorporates these overlooked dimensions in the design of products and services.

3. Needs-Based Design Method for Product–Service Systems

3.1. Concept

To address both product and service aspects, this study focuses on PSSs. The living-sphere approach illustrated in Figure 1 shows the relationships between fundamental human needs and satisfiers/barriers, satisfiers/barriers and requirements, requirements and functions, and functions and structures. The relationships between fundamental human needs and satisfiers/barriers have been clarified in the HSD framework, while those between requirements and functions, and between functions and structures, have been clarified in the field of engineering design. However, the relationships between satisfiers/barriers and requirements are not yet clear.

According to Max-Neef, satisfiers and barriers do not refer to available economic goods, such as products or services, but rather to economic goods that empower satisfiers to meet fundamental human needs fully and consistently [28]. Therefore, in this study, satisfiers and barriers are regarded as concepts that are more abstract than the requirements for products and services. Based on this understanding, we attempt to connect these abstract elements to conventional design concepts by associating satisfiers and barriers with requirements, and further linking them to functions using design methods such as value graphs and service blueprints.

In addition, because the structures of a PSS are determined under various constraints, this study covers the functional design of a PSS and does not cover the assignments of functions to structures. A model that connects functions and requirements to satisfiers/barriers is constructed in a bottom-up manner. Functions are first extracted using a service blueprint, and the corresponding requirements are then derived through a value graph by repeatedly asking why each function is needed. These requirements are subsequently connected to the satisfiers/barriers identified through needs-based workshops.

This modeling process enables the designer to understand how each function contributes to or inhibits the fulfillment of fundamental human needs. As a result, the proposed method provides an integrative framework that supports sufficiency-oriented design by incorporating social sustainability considerations into conventional product and service design.

3.2. Procedure

Figure 3 shows the procedure of the needs-based redesign method. To redesign an existing product to improve its sufficiency, this method first analyzes the existing product, and then models its relationship with satisfiers/barriers and redesigns its functions. The following illustration explains each step of the procedure.

3.2.1. Step 1: Extracting Satisfiers and Barriers

Satisfiers and barriers can be extracted through needs-based workshops [37,38], interviews [30,40], and literature reviews [32,56]. In this study, workshops were adopted for two reasons. The first is that interviews and literature reviews require post-processing, which may introduce the designer’s intention into the analysis. The second is that the authors have held needs-based workshops several times, both in person and online [39,57].

The needs-based workshop proposed by Max-Neef is a 2-day workshop with 50 participants, but this has been difficult to organize and implement in recent years owing to the amount of funds and time required for the exercise. For this reason, various shorter workshops with smaller numbers of participants have been proposed [58]. Kobayashi et al. proposed a 5 h workshop with about 20 participants [39]. This method was modified by Murata et al. such that workshops could be conducted online by combining multiple digital tools [57]. In this study, we employed the workshop method proposed by Kobayashi et al. for in-person sessions and the that proposed by Murata et al. for online sessions [39,57].

3.2.2. Step 2: Extracting Functions

The functions of a PSS include both product functions and service functions, and services have a variety of stakeholders. Therefore, in Step 2, a service blueprint is used to analyze the product and service functions separately for each stakeholder.

3.2.3. Step 3: Extracting Requirements

In Step 3, a value graph is used to extract the requirements of the target PSS by asking the designer why the function is required for each function [44].

3.2.4. Step 4: Relating Requirements to Satisfiers and Barriers

Paired comparisons of the requirements and satisfiers/barriers are performed, and if it is judged by the designer that a relationship may exist between them, the requirements and satisfiers/barriers are connected. Although this step is intended to be performed by a designer, verification by multiple persons is recommended, because the connection decisions depend on the designer’s judgement.

3.2.5. Step 5: Redesigning Functions

Figure 4 shows an overview of the model constructed in Steps 1 to 4. In Step 5, the functions are redesigned in order to improve the sufficiency, based on the model.

To fulfill fundamental human needs, it is necessary to realize satisfiers and to prevent the realization of barriers. The following three methods can be suggested for the redesign of functions to improve sufficiency.

(1)

Modify or remove functions that connect only to barriers;

(2)

Separate or modify functions that connect to both satisfiers and barriers;

(3)

Add functions that can realize satisfiers that have not been realized by existing functions.

In the next section, we describe the results achieved when applying the proposed method to smart carts.

4. Case Study: Needs-Based Functional Redesign of Smart Carts

4.1. Design Target

With the progress of digitalization in the retail industry, new services such as cashless payment and e-commerce sites are becoming more widespread. Another such service is smart carts, which are shopping carts equipped with a scanner for reading barcodes and a tablet terminal for displaying information; these have been recently introduced as a new checkout system in some supermarkets. Figure 5 shows the appearance of a smart cart used in Japan. Smart carts are coming into wider use, but what motivates customers to accept smart carts is not clear.

In this case study, we clarified which functions of smart carts contribute to their adoption by analyzing which functions of the smart carts realize the satisfiers of customers or prevent the realization of barriers.

4.2. Application of the Proposed Method to a Smart Cart

4.2.1. Step 1: Extracting Satisfiers and Barriers

The needs-based workshop aimed to extract satisfiers and barriers in the context of a supermarket. The workshop was held online in November 2021. The workshop participants ranged from people in their 20s to people in their 60s, and were grouped according to whether they were frequent users of physical stores or online stores. The reason for creating these two groups was that smart carts have both the functions of physical stores (e.g., users can physically select and purchase products) and those of online stores (e.g., customer purchase data can be managed and users may be presented with various types of information).

Table 2. Members of each group in the needs-based workshops.

			20s	30s	40s	50s	60s
Physical store	Satisfier group	male			1
		female	1		1		1
	Barrier group 1	male		1
		female	1		2		1
	Barrier group 2	male		1
		female				3
Online store	Satisfier group	male		2
		female		1			1

shows the members of each group in the needs-based workshops. There was much input from frequent users of physical stores, so one satisfier group and two barrier groups were created for physical stores. However, there were few responses from frequent users of online stores, so only one satisfier group, and no barrier group, was created for online stores.

Table A1. Satisfiers in physical stores. (N/A = Not Available).

	Being	Having	Doing	Interacting
Subsistence	healthy	N/A	buying foods, going to a shop	easy accessibility, helping each other, store for everyone
Protection	secure	N/A	using an eco-bag, convey my thoughts	rule compliance, information on products bought is not stored, always something you want, electricity in the store even in the case of a disaster
Affection	with someone, accepted	N/A	feeling grateful, feeling the personality, perceiving the producer’s care and dedication	store that makes us feel the seasons, gathering place (without buying), store rooted in the community, special feeling
Understanding	N/A	similar experiences	observing, associating with the product, broadening my horizons, going to the shop	dissemination of various information, dissemination of adequate information, interest in new things, love of new things
Participation	N/A	rewards card	recycling trash	reflecting opinions, ingenuity to facilitate participation, various ways to participate
Idleness	N/A	N/A	N/A	N/A
Creation	N/A	interest in new things, love of new things	becoming a producer, talking to people who don’t use supermarkets, acting and thinking	someone I can talk to, dissemination of various information
Identity	N/A	belief, principles, preferences, self-understanding	going to a shop I’ve never been to	supermarkets only in the local area
Freedom	capable of anything	information, options, means	challenging, having many options	N/A

and

Table A2. Barriers in physical stores.

	Being	Having	Doing	Interacting
Subsistence	in poor health, lack of products that seem safe	additive, items with high environmental impact, over-packaging, allergy, toxicant, virus (COVID-19)	buying overpriced items, choosing only high-calorie foods, telling a lie	no barrier-free paths, poor security around the store, traffic conditions, no stores nearby, disaster, only accessible by car, no allergy-friendly products
Protection	lost without knowing where to buy, easy for elderly cars to crash into, restricted who can enter, crowded	shopping bag, unsanitary foods	bringing children, losing track of the child	climate change, many imported items, spacious store, far from police station, excessive labor, many bad people, no windows kids running around, large parking space, mass production and consumption
Affection	tired, dirty, unsanitary, too low price, high price	no kindness, processed goods, items placed in a messy manner, little communication, sales of side dishes, food loss in stores	complaining in a harsh tone, taking from products with longer expiration dates, bad responding from the clerk, not carrying the products the family wants, not knowing the manufacturer	decline of local stores, no producer label, dirty shopping basket, cold waitresses, clerks with a bad attitude
Understanding	unsure how to use, counterfeited place of origin	unknown instructions or recipes, difficult items, misleading information, vague product information, excessive information, no price indication, deceptive advertising	shopping in less time, forgetting my glasses, going to a noisy place, not providing accurate information, greenwashing	advertisements that create preconceived notions, confusing price display, clerks lacking knowledge, shortened operating hours, masks and partitions
Participation	required to be a member, not a member, no discounts available due to non-membership	many items, many tasks, a small child, no notice of sale	forgetting to wear a mask, eating out, forgetting my rewards card, registering as a member	shortened operating hours, distant distance to stores, rainy day handling, crowded parking, lack of knowledge by employees, sold all kinds of things, inappropriate responding, no barrier-free access
Idleness	difficult to just look at items, spoken to	shopping apps, discount flyers, rewards card	going shopping for daily necessities, seeing a discount on a product I want, sacking	nobody but me can go shopping, music and advertising in the store, dirty toilets, lack of rest area, excessive handling, excessive advertising
Creation	coerciveness, tired, sold as a product kit	seasoning for single use, special detergent, no time to think, stereotype, goods at home, shopping list, cheap items, obvious answer, everything I want, unprocessed fish, fewer options	buying pre-cooked products, buying a cooking kit, not making one myself, choosing items in easy-to-reach positions	confusing display of items, advertisements in magazines and on TV, lack of variation, fewer items on display, fixed items, bundle selling
Identity	not a price I’m willing to pay, no size that I want, satisfied with what I have	recommendation of a friend, confidence in the brand, sense of discrimination, prejudice, limited personal choice	selecting based on low price, buying recommended products, buying brand name goods, buying as recommended, using the same ones as everyone else, using discount coupons, choosing products with someone else	different products for sale in different regions, TV advertising, products setting by the store, no place for requests to be heard, lack of variation, intrusive behavior
Freedom	in closing time, not a member, not available to hold in hands, interrupted	products difficult to dispose of, bundle, rewards card, sense of discrimination, high price	shopping in a fraction of the time, selecting by price, considering family preferences, going shopping with children, not matching the business hours with the rhythm of my life	working environment, excessive customer service, price, uncomfortable accounting due to lack of employees, small variety of items, short business hours, no refills, fewer employees

show the extracted satisfiers and barriers, respectively, for physical stores, and

Table A3. Satisfiers in online stores. (N/A = Not Available).

	Being	Having	Doing	Interacting
Subsistence	healthy	means of communication, money, willingness to buy, food availability	choosing safe ingredients, buying what’s good for health, shopping, staying healthy	online supermarket, proper temperature control and transportation, food quality control and preservation
Protection	protected personal information, honest	security, relief, safety, no need to go out	setting a password, searching for reviews, making sure the company is safe	secure membership site, inquiry support, check for broken eggs, prevention of intrusion of suspicious persons, safe driving, separated by refrigerated, frozen, and room temperature
Affection	N/A	family, producer information, love for family, desire to cook for you	thinking of my family, knowing the story of products, asking what you want to eat, serving your favorite homemade food, receiving words of kindness, knowing food preferences in the family	wide variety of products, shopping list, project that tries to make users’ lives more enjoyable, never out of stock
Understanding	intuitive, easy to search for products	production and processing information, information on how to use the ingredients, payment method such as credit card, information on how to use an online supermarket, ingredient allergies	learning about the product development story, finding out what you need to know, knowing the attitude of the supermarkets and their approach to society, knowing the rationale for price, understanding how online supermarkets work, comparing and contrasting, enlarging product image	inquiry support, information about producers and products, careful product description, clear product image
Participation	available when I want	reason not to use real stores, expectation, online supermarket I want to use, PC/smartphone, internet literacy, interests, concerns, awareness, benefits of participation, first time use discount	gathering information from reviews, registering as a member, learning about the unique benefits of online supermarkets, earning points, matching delivery time with the time I want to pick up	reviews that show advantages, ease of member registration, delivery area, demonstration
Idleness	easy to buy	package receiving options, plan, time, subscription, nothing to think about, automatic selection of what to buy	setting up a pickup box, deciding on a menu, deciding what to buy, taking advantage of gaps in time, buying anytime, anywhere, not buying anything extra	regular delivery, recommendation, subscription delivery, past history purchase, automatic selection
Creation	N/A	social media, good memories of the past, request from family, number of recipes, knowledge and wisdom about shopping and cooking, recipes for popular dishes, good combination, ability to search	deciding on a menu, thinking about what is missing, asking what others are making, finding out how to use the ingredients, knowing the season, having a recipe in mind	recommendation, recipe suggestions tailored to the ingredients
Identity	in fashion, told that it tastes good, praised by family members, recognized for the hard work, praised for buying good stuff	information on trends, shopping deals, information on seasonal ingredients, family smiles, information on sales campaign, recipes for popular dishes, good arrangements	buying my favorite foods, making the best use of my spare time, finding out on social media, arranging to my liking, buying quickly, getting a good deal, completing of work as a homemaker	team spirit, suggestions for dishes I don’t know
Freedom	available for order 24 h, free to cook	easy operation and routine, money, options of pick-up time, no need to go to the supermarket, no need to move, no need to wander around	buying in the gap time, shopping light and easy, shopping without worrying about time, deciding on my own, shopping without worrying about weight, shopping while rummaging around, buying whatever I like, taking my time in choosing, making what you want to make	language-independent, free shipping, anytime, anywhere

shows the satisfiers for online stores. The satisfiers and barriers shown in these tables met the following two conditions: the majority of the group participants agreed with their assignment, and the participants’ assumptions are correct.

One example of an incorrect satisfier was the inclusion of “online supermarket” in the satisfier group for physical stores—this should instead be considered as an online-store satisfier. One example of an incorrect barrier was the inclusion of “not wearing a suit” in the barrier group for physical stores—in this case, a participant stated that “not wearing a suit makes it difficult to enter a store that would have a dress code”; it was determined that the participant did not assume the context of a supermarket.

4.2.2. Step 2: Extracting Functions

Smart carts allow customers to perform several actions using the cart itself, the tablet, and the scanner, and these actions are realized by the functionality of the smart-cart equipment as well as a server on the backend. Figure 6 shows the service blueprint of the smart cart. The service blueprint was created with reference to the shopping experience, using the smart cart and information published by companies that have developed or introduced smart carts. Because the smart cart is used from the point of entry to the point of payment, the customer actions are also described as well.

Table 3. List of smart-cart and server functions.

(a) List of Smart-Cart Functions
Service provider	Action	Function
Smart cart	Recognize ID card	Recognizing ID card
	Recognize information selected	Recognizing information selected
	Display information	Displaying special offer
		Displaying map information
		Displaying purchase history
		Displaying trivia
		Displaying recipe information
	Display recommendations	Displaying recommendations
	Recognize the item by barcode	Recognizing the item by barcode
	Recognize the item selected	Recognizing the item selected
	Display item information	Displaying item information
	Display related item information	Displaying related item information
	Recognize the age-restricted item by barcode	Recognizing the age-restricted item by barcode
	Recognize the age-restricted item selected	Recognizing the age-restricted item selected
	Display age verification	Displaying age verification
	Check the weight	Checking the weight
	Display a confirmation of whether the item has been placed in the shopping cart	Displaying a confirmation of whether the item has been placed in the shopping cart
	Recognize the cancelled item by barcode	Recognizing the cancelled item by barcode
	Recognize the cancelled item selected	Recognizing the cancelled item selected
	Recognize the shopping bag by barcode	Recognizing the shopping bag by barcode
	Display shopping bag information	Displaying shopping bag information
	Display the total amount	Displaying the total amount
(b) List of Server Functions
Service provider	Action	Function
Server	Register customer information	Registering customer information
	Update ID card balance	Updating ID card balance
	Link the customer’s ID to the shopping cart	Linking the customer’s ID to the shopping cart
	Send information	Sending special offer
		Sending map information
		Sending purchase history
		Sending trivia
		Sending recipe information
	Get the shopping cart location	Getting the shopping cart location
	Send recommendations based on location	Send recommendations based on location
	Register the item as purchased	Registering the item as purchased
	Send item information	Sending item information
	Send related item information	Sending related item information
	Judge as underweight	Judging as underweight
	Cancel the purchase of the item	Canceling the purchase of the item
	Register a shopping bag for purchase	Registering a shopping bag for purchase
	Send shopping bag information	Sending shopping bag information
	Calculate the total amount	Calculating the total amount
	Perform payment from the ID card	Performing payment from the ID card

lists the functions of the smart cart. Although Figure 6 describes the function of the sales assistant, the sales assistant is not mentioned in Table 3 because the sales assistant is considered to belong to the supermarket, not to the smart cart. Specifically, the sales assistant performs store-level maintenance tasks, such as shelf restocking, bag replenishment, and cart disinfection, which support overall operations but do not directly influence the customer’s purchasing experience. Therefore, this role is excluded from the functional analysis in this study, which focuses on the functions that constitute the smart cart-based service system from the customer’s perspective.

“Send information”, the action of the server in Figure 6, is separated into the functions “sending special offer”, “sending map information”, “sending purchase history”, “sending trivia”, and “sending recipe information” in Table 3. Similarly, “display information”, the action of the smart cart, is separated into the corresponding functions “displaying special offer”, “displaying map information”, “displaying purchase history”, “displaying trivia”, and “displaying recipe information” in Table 3.

4.2.3. Step 3: Extracting Requirements

The requirements of the smart cart were extracted by repeatedly questioning why the function would be required for the customer and why the function would be required for the store for each function. The reason for targeting both customers and stores is that the smart cart realizes not only customer requirements but also the requirements of the store, such as the management of customer and product information.

4.2.4. Step 4: Relating Requirements to Satisfiers and Barriers

A paired comparison was made between the requirements extracted in Step 3 and the satisfiers and barriers in physical stores, in addition to the satisfiers in online stores described in Table A1, Table A2 and Table A3. Figure 7 shows the connections between the functions and requirements and the satisfiers/barriers in the smart-cart model.

In Figure 7, requirements are grouped not by the nature of the functions themselves, but as based on the types of satisfiers or barriers they are associated with. This classification was applied post hoc, after the paired comparison, to clarify how different functions promoted or hindered the fulfillment of fundamental human needs. The grouping does not reflect functional categories but rather highlights the relationships between customer requirements and satisfiers/barriers, as identified in the Max-Neef framework.

The model reveals several functions that contribute to the realization of satisfiers. Figure 7b shows the functions connected only to satisfiers. For example, the smart-cart function, “recognizing information selected” is connected via the customer requirement “information selection by customers” to multiple satisfiers associated with both physical and online stores, such as “option” and “easy to search for products”, and can be judged to be a function that is important for customer sufficiency. Other smart-cart functions related to displaying information, such as “displaying trivia”, “displaying recipe information”, and “displaying purchase history”, are also connected only to satisfiers, indicating that these smart-cart-specific functions contribute to the sufficiency for customers. This suggests that being able to access relevant information and purchase history while shopping with the smart cart significantly contributes to human-need fulfillment, which may be a factor in the adoption of smart carts. Meanwhile, Figure 7c shows that the server functions “sending trivia” and “sending recipe information” are connected to multiple barriers via the store requirement “item promotion.” It is likely that customers will experience decreased sufficiency if they perceive promotional intent in the trivia or recipe information displayed. In other aspects, payment functions in the smart cart, such as “recognizing items by barcode” and “recognizing cancelled items by barcode” as described in Figure 7d, are connected to the satisfiers associated with both physical and online stores, while “reducing the number of sales assistants”, the store requirement connected to them, is connected to multiple barriers. This suggests that even though the smart cart is able to accept payment, the resulting reduction in sales assistants would reduce sufficiency for customers. This indicates that the smart cart not only has functions that improve sufficiency, but also has functions that may reduce sufficiency, and this may be why smart carts are being gradually adopted, rather than experiencing explosive growth.

In the next step, we describe the results of redesigning the functions of the smart cart to improve sufficiency for customers.

4.2.5. Step 5: Redesigning Functions

The functions were redesigned using the three methods given in Step 5 of Section 3.2. The results for each method are described below.

(1) Modify or remove functions that connect only to barriers.

In Figure 7c, the store requirement “item promotion” is connected to the barriers “advertisements that create preconceived notions” and “lots of advertising”, and “item promotion” is also connected to the server functions “sending recommendations based on location”, “sending special offers”, “retrieving shopping cart location”, “sending trivia”, “sending recipe information”, and “sending related item information.” Among these functions, “send recommendations based on location” is connected to the satisfier “recommendations” via the store requirement “providing recommendations”, while “sending special offers”, “retrieving shopping cart location”, “sending trivia”, “sending recipe information”, and “sending related item information” are connected only to barriers. Therefore, it is recommended that these five server functions be modified or removed.

Meanwhile, the service blueprint shows that the server action “send information” realizes the smart-cart action “display information.” This means that the server functions “sending special offers”, “sending trivia”, and “sending recipe information” are necessary to realize the smart-cart function “displaying special offers” in Figure 7e and the smart-cart functions “displaying trivia” and “displaying recipe information” in Figure 7b. Because these functions are connected to satisfiers, they should be retained. Similarly, the server function “sending related item information” is necessary to realize the smart-cart function “displaying related item information” in Figure 7c, and the server function “getting the shopping cart location” is necessary to realize the server function “sending recommendations based on location” in Figure 7c. In these cases, it is recommended that the function be modified so that the barriers are not realized. For example, it is suggested that the types and numbers of advertisements be adjusted so that preconceived notions are not created.

(2) Separate or modify functions that connect to both satisfiers and barriers.

Figure 7c–e shows the functions that are connected to both satisfiers and barriers. These functions should be separated or modified to remove requirements that lead to the realization of barriers. For example, the smart-cart function “displaying related item information” is connected to the satisfiers “dissemination of various information”, “dissemination of adequate information”, “interest in new things”, and “love of new things” via the customer requirements, while it is connected to the barriers “buying as recommended” and “excessive information” in Figure 7c. This can be interpreted as a case in which the related-item information can be perceived either favorably or negatively. To address this problem, a button should be implemented which would allow customers to choose whether to display the related-item information. Alternatively, if the type of related-item information provided is interpreted to be a problem, buttons that allow customers to choose whether to display the related-item information for each information category might be proposed.

In Figure 7d, smart-cart functions such as “recognizing items by barcode”, which are connected to the customer requirement “registering items by customers”, are connected to the satisfier “time” and the barriers “little communication” and “fewer employees”, because the smart cart can recognize items by their barcodes, which reduces the number of sales assistants at the checkout counter. In this case, it is suggested that the payment function be divided into two types: one for those who want to save time and the other for those who want to communicate with others. For example, a combination of both smart-cart payment and chat lanes, as has been introduced in the Netherlands and some areas of Japan, could be used. Chat lanes are traditional checkout lanes but with a slower checkout process, designed for the purpose of talking with the checkout sales assistants.

(3) Add functions that can realize satisfiers that have not been realized by existing functions.

Additionally, functions need to be added to smart carts that realize satisfiers not currently connected to existing functions. For example, the physical-store satisfier “feeling the love of the producers” and the online-store satisfiers “producer information” and “knowing the story of products” can be realized by adding a function to display some background information relating to the items. Also, the online-store satisfiers “food preferences in the family” and “ingredient allergies” could be realized by adding functions to register the likes and dislikes of customers and allergy information and to display them when purchasing foods.

To clarify the relationship between the identified issues and the proposed redesign,

Table 4. Functional redesign summary with corresponding barriers, satisfiers, and fundamental human needs.

Redesign Function	Barrier Removed	Satisfier Enhanced	Need Fulfilled
Adjusting the type and quantity of advertisements to prevent the formation of preconceived notions	advertisements that create preconceived notions, excessive advertising		idleness, understanding
Allowing users to filter or opt out of certain types of information	buying as recommended, excessive information	dissemination of various information, dissemination of adequate information, interest in new things, love of new things	understanding, creation, identity
Introducing optional slower checkout lanes with human sales assistants (“chat lanes”)	little communication, fewer employees	time	idelness, affection, freedom
Providing producer narratives and detailed product background information		perceiving the producer’s care and dedication, producer information, knowing the story of products	affection
Allowing users to register allergy information and dietary preferences		knowing food preferences in the family, ingredient allergies	affection, understanding

summarizes how each redesign function addresses specific barriers or enhances satisfiers, ultimately contributing to the fulfillment of fundamental human needs.

These redesign elements are derived from the structured analysis of satisfiers and barriers in the previous steps and reflect a deliberate effort to improve the fulfillment of fundamental human needs. By aligning functional design with fundamental human needs, the smart cart can be better positioned to support users, not only in practical terms but also in ways that resonate with broader social sustainability.

5. Discussion

In this section, we discuss the proposed model in which functions, requirements and satisfiers/barriers are connected, the method of redesigning functions based on the model, and the needs-based workshop in the context of a supermarket.

5.1. Effectiveness and Applicability of the Proposed Method

The proposed redesign solutions, as described in Section 4.2, Step 5, demonstrate how the integration of satisfiers and barriers into the design process can lead to more meaningful changes and needs-based improvements in PSSs. Unlike conventional customer-requirement-based approaches—such as QFD or the Kano model—that primarily focus on satisfying customer requirements, our method centers on the fulfillment of fundamental human needs, as conceptualized by Max-Neef.

By explicitly mapping functions to satisfiers and barriers, the proposed framework enables designers to identify not only what should be added to fulfill unmet needs, but also what should be removed or modified to mitigate negative impacts. This dual perspective allows for redesign proposals that are not only functionally effective but also socially responsible. In this sense, the method contributes to the discussion on social sustainability, specifically by addressing the aspect of fundamental human-need fulfillment. Rather than focusing solely on the satisfaction of customer requirements, the proposed framework enables designers to reason about how specific functions might relate to satisfiers and barriers. This structured linkage allows for a more deliberate consideration of the manner in which design decisions may contribute to or hinder fundamental human needs such as protection, freedom, understanding, and affection.

This study primarily contributes to the literature of the field by proposing a novel needs-based design method, in addition to describing a smart-cart case study as a demonstration of its applicability. Through this case study, we confirmed that redesign functions can be systematically derived from the analysis of satisfiers and barriers. This demonstrates the method’s potential to support a sufficiency-oriented and socially responsible design processes.

While it would be ideal to evaluate the effectiveness of the resulting redesign solutions, we consider such empirical assessment to be outside the scope of this study and a valuable direction for future research. For instance, satisfiers and barriers could be incorporated into a framework like QFD to assess how well each function supports fundamental human needs. Additionally, qualitative assessments involving experts or users could offer insights into improvements in sufficiency.

Importantly, to the best of our knowledge, this is the first design method that explicitly supports function-level redesign based on fundamental human needs. While the smart cart was used as a case example, the proposed method is not limited to this context and can be applied to other PSSs, including sharing services. We believe this generalizability strengthens the method’s contribution to socially sustainable design practices.

5.2. Model of Connections Between the Functions of a Product–Service System and Satisfiers/Barriers

In this study, we combined service blueprints and value graph analysis to extract the functions and requirements of the target PSS. We then used paired comparisons to connect these requirements to satisfiers/barriers, as defined by Max-Neef’s framework.

As a result, nine out of fifty-two satisfiers (17%) and five out of seventy-four barriers (7%) for physical stores, and eighteen out of seventy-one satisfiers (25%) for online stores were successfully linked to functions. Notably, the proportion of barriers connected was less than half that of that associated with satisfiers. This imbalance can be attributed in part to the typical tendency in design processes to express requirements and functions in positive terms. For example, when describing a stove, a designer may specify the function “heat water”, which fulfills a requirement such as “producing hot water.” However, negative aspects such as “overheating and causing burns” are rarely represented explicitly in requirement formulations.

In the case of smart carts, the requirement “information selection by customers” was connected to the satisfier “options”, reflecting the customer’s ability to make autonomous choices. However, the same requirement could also be interpreted as being connected to the barrier “vague product information”, since customers may miss relevant information if it is not easily visible or structured. This example illustrates the difficulty in directly linking requirements to satisfiers or barriers due to the differences in levels of abstraction.

Unlike the connection between requirements and functions—which can often be derived by repeatedly asking “why” a function is needed—the connections between requirements and satisfiers/barriers are more interpretive and context-dependent. These connections tend to rely heavily on the manners in which satisfiers and barriers are defined or understood by designers. As such, further refinement of the conceptual structure, such as rephrasing requirements at a higher level of abstraction, may help facilitate more consistent and meaningful mappings in future applications.

5.3. Method Used to Redesign Functions

In this study, we proposed three methods for functional redesign. New functions were proposed using one or more of these methods, indicating that they were effective for functional redesign. In particular, methods (1) and (2) are considered to be helpful for clarifying the direction of ideas because of the relationships not only between requirements and functions, but also between requirements and satisfiers/barriers.

A fourth method, (4) the removal of functions that do not connect to both satisfiers and barriers, was not included in this study because it targeted the improvement of sufficiency; this method may be considered when aiming to be effective in reducing environmental impact. Modern products have become too multifunctional and therefore contain many functions that do not contribute to sufficiency. Reductions in unnecessary functions lead to reductions in product complexity and the number of parts, which in turn leads to reductions in environmental loads during manufacturing and disposal, due to improved ease of disassembly [59,60]. This fourth method may be effective for identifying such functions.

In this study, the number of functions and requirements was relatively small, so the results could be presented in a diagram, as shown in Figure 7, but as the numbers of functions and requirements increase, it becomes more difficult to represent them. In such cases, it will be effective to model and analyze the results in a matrix format instead of a diagram format [61]. Also, while the analysis of whether these functions realize satisfiers/barriers was performed based on the binary presence or absence of a connection, it may be effective to analyze the strength of the connection.

5.4. Needs-Based Workshop for a Supermarket

Generally, needs-based workshops gather residents in a target location, such as a region or environment, and extract satisfiers and barriers for that area. In this study, because the design target was smart carts used in supermarkets, supermarkets were selected as the target place and satisfiers/barriers were extracted from the customers.

Table A4. Satisfiers in Osaka, Japan.

	Being	Having	Doing	Interacting
Subsistence	healthy, satisfied with necessities of life, free from illness and injury, able to sleep, able to eat, breathing	willpower, necessities of life, goal, balanced schedule of work and play, collaborator	breathing, drinking water, eating, thinking, working, creating, eating delicious food, being recognized, helping others, making food	space to interact with others, enough food, clothing, and shelter, not too cold
Protection	safe, satisfied with oneself, relieved, comfortable with one’s mind, financially secure	status, money, child, family, affection, social security system, law, authority, helpful neighbors	asking for help, loving, expressing through words, asking for advice, feeling affection, satisfying the needs of others, having pets	filled with resources, safety space, full of kind people, law, friendly relations with other countries, internet security
Affection	kind to others, together with girlfriend, mentally healthy, safe, loved, loved by others, comfortable, not alone	lover, experience of being loved, self-esteem, affection, sense of education, something very cute, free time	having a special connection, hugging, spending time together, confessing my love, kissing, spending time with my family, acknowledging	good family environment, full of love
Understanding	curious, familiar with cultural background, able to talk	relationship, interest, motivation, free time, knowledge of many different fields, sympathy, intelligence, acceptance, experience in life, willingness to understand, literacy, ability to use a computer	discussing, searching, recognizing, experiencing the same thing, getting necessary information, reading books, observing, asking, speaking the same language, listening to others	school, freedom of speech, active discussion, sharing social context, place with teacher, sharing information, place to talk with others, wealth of resources
Participation	curious, fine, lonely, capable of growth through participation	sense of purpose, money, free time, interest, transportation, PC, courage, friend, IT literacy, benefit	recognizing things as important, having an interest, meeting with someone, making friends, acquiring knowledge, belonging to a group, search for interesting events, finding out about events, getting a chance, networking, taking over knowledge and experience, seeking	free event, suitable participation requirements, attracting public attention, means of communications, transportation system reward, everyone participating, comfortable place, reflecting on my opinions, communicating easily, participatory event, places to go to
Idleness	tired, sleepy, cheerless, overwhelmed, accessible to necessities	free time, assistant, calm mind, money, house, peaceful mind, no work	making time, completing all tasks, tidying up	nothing to do, my mother taking care of me, communication failure, closed store, silence, warm room, safe place, nature, full of food and water
Creation	curious, creative, satisfied, dreamy	person of action, money, idea, knowledge, experience, ideas from others, free time, material, patience, purpose	crafting, looking for ideas, brainstorming, making time, sharing, discussing, thinking about what I can do, walking, taking a bath, playing with a smartphone, participating in events	worked on by many people, sufficient resources, desired by many people, free discussion, having all the essential knowledge, surrounded by materials, relaxed mood, place with teacher, existence of people willing to pay for it, existence of friends who can create together, convenience required
Identity	responsible for oneself, self-expressive	stranger, belonging, social status, strong belief, talent, name, friend, social media account, identified affiliation	working diligently on something, belonging, recording, posting on social media, doing a presentation, making others recognize me, talking, stating my opinions	free to do, all backgrounds respected, lack of identity-based conflict, respecting individuality, accepting me, offering information, existence of people who praise me, well-educated, place with others, no denial when I communicate my intentions
Freedom	responsible for oneself, limited by law, restricted in activity, time-limited, unrestricted, sleepless, decisive, satisfied with a certain amount of freedom, able to go where I want to go	authority, ability, free time, knowledge, ideal, money, technological capability, brainwashing, adaptability to society, mental strength, necessities, target to aim at	recognizing my limitations, living an independent life, setting limits for myself, joining the demonstration, leaving home, living a self-sufficient life, prioritizing, stopping comparing myself to others	less restrictive regulations, certain restrictions, support for ambition, feeling free, no oppression, no information control, information unobtainable

shows some results from a needs-based workshop held in Osaka, Japan in 2020 by the authors [57]. Comparing Table A1, Table A2 and Table A3 with Table A4 shows that more specific expressions of satisfiers and barriers were extracted in the workshop targeting supermarkets. At the same time, many satisfiers and barriers associated with requirements were also extracted. For example, “fewer employees”, which is a barrier to freedom in physical stores, is related to the requirement for stores to have more sales assistants, and it is unlikely that “fewer employees” inhibits freedom. Perhaps freedom is inhibited because there are fewer sales assistants, so people cannot understand how to use tools such as tablets and scanners, and their options are narrowed. Conversely, some barriers, such as “little communication”, which is a barrier to affection in physical stores, can be understood as clearly inhibiting fundamental human needs. This variation suggests that the level of abstraction in satisfiers and barriers is influenced by the focus and facilitation of the needs-based workshop.

Therefore, in applying the proposed method, it is important to consider how the design and scope of the workshop affect the granularity and usefulness of the extracted satisfiers and barriers. In this study, the number of participants per group was comparable to previous needs-based workshops, and thus the group size is unlikely to have significantly affected the results. However, differences in participants’ ages and genders may have influenced the satisfiers and barriers extracted. In particular, the participants were not grouped by age, and gender balance was not strictly maintained in each group. These factors may have contributed to variations in the abstraction level and content of the extracted elements. Future research should therefore examine not only how to optimize workshop facilitation methods to balance contextual relevance with the appropriate level of abstraction, but also how participant demographics and group composition may influence the outcomes of needs-based workshops in sufficiency-oriented design.

6. Conclusions

In this study, which aimed to improve the sufficiency of products and services for the realization of SCP, we proposed a needs-based redesign method that redesigns product and service functions to improve their sufficiency by combining the HSD framework proposed by Max-Neef, service blueprints used in the design of PSSs, and value graphs used in engineering design. The contribution of the proposed method is its ability to model connections from the product and service functions to the satisfiers and barriers, and use this model to redesign functions.

We applied the proposed method to a supermarket smart cart, and the results confirmed that the proposed method can propose functions to improve the sufficiency for customers. However, some limitations of the proposed method were also identified: first, there is the question of how to connect functions and barriers, and another limitation is in the question of how to define satisfiers and barriers during system design. These may require new concepts that have not yet been addressed in engineering design.

Product and service design using satisfiers and barriers is one possibility for sufficiency design, and in the future, it will be necessary to increase the number of cases, while at the same time developing methods to improve the structures that were not targeted in this study.