Study of the energy system on the manufacturing of carbon fiber prostheses and its environmental impact

1. Introduction and objectives

The purpose and scope of this project are primarily academic, aimed at learning to conduct a comprehensive study on the energy system surrounding a product. This energy study must be calculated and examined in all manufacturing phases of the project to achieve the most realistic approximation possible, allowing a detailed understanding of the environmental impact associated with the use of this technology. These manufacturing phases encompass everything from raw material extraction to the management of waste generated by the carbon fiber prosthesis once its usage phase concludes. In this manner, the report's scope will not only concentrate on the economic system but, by expanding the boundaries of the studied system, will also analyze the social and environmental aspects. In other words, we will scrutinize the processes, environmental impact, and energy usage across all phases of polyacrylonitrile processing. This comprehensive analysis aims to ensure that, from the treatment of carbon fiber to its weaving and subsequent prosthesis manufacturing, the waste is efficiently utilized and managed at the end of its useful life.

Our working group is particularly motivated by the accessibility of data facilitated by one group member's connection with a company dedicated to manufacturing carbon fiber prostheses. The motivation also stems from our concern to investigate whether a material like carbon fiber, seemingly emitting fewer kilograms of emissions than steel or other metal alloys used in prostheses production, offers superior environmental and economic benefits compared to alternative materials. By doing this report we will discover if our thoughts are true.

Our main objectives are:

* Evaluate the environmental impact of the different carbon fiber manufacturing processes, and find the most sustainable solution.

* Analyze the energy consumption and carbon footprint of the complete life cycle of the product.

* Analyze the economic possibility of recycling and reusing components in prosthetic devices.

* Investigate social implications, influencing accessibility and affordability of the product.

About the limitations and constraints of this study, first of all we must consider the limited access to detailed and accurate information about the entire life cycle of carbon fiber prostheses, covering aspects such as raw material extraction, manufacturing processes, and waste management. To address this constraint, it is essential to work closely with the manufacturing company and other relevant stakeholders, seeking collaboration to find as much data as possible.

In the realm of industry-specific challenges, the carbon fiber prosthesis sector may safeguard certain information to not share this information with other competitors. To reach this goal of having real information about the company, it could be important to ask more than one company to increase the probability of having a positive answer about sharing information with our team.

Another constraint involves limitations in time and the number of people working in the project, which may impede the ability of finishing the project with the overall result we wish at the first steps of the project. According to time limitations, we will just do this report about the available data in the last months of 2023. After some time the results of this report could not be valid anymore.

Assumptions and simplifications, often necessary in modeling complex systems, present a constraint as they can introduce uncertainty into the study. To address this, documenting all assumptions made during the study have to be well defined and referenced and cited.

The interconnected nature of carbon fiber manufacturing within larger systems adds complexity, making it challenging to isolate its impact from other processes. To navigate this constraint, it is crucial to clearly communicate the study's boundaries, emphasizing that the analysis specifically focuses on the energy system and environmental impact of carbon fiber prosthesis manufacturing. By doing this isolation of the carbon fiber prosthesis manufacturing system we define our system by this parts:

The main scope of this project is to analyze the environmental impact of creating a carbon fiber foot prosthesis. We're focusing specifically on the stages involved in manufacturing the prosthesis: the slicing of sheets, vacuum compression over a plaster mold, application of resin and hardening powder, and the mechanization process. Our aim is to calculate the environmental cost using both Life Cycle Costing (LCC) and Life Cycle Assessment (LCA) methods based on those manufacturing stages.

In this study, it is considered the raw materials and the energy consumption during manufacturing. The functional unit being analyzed is the foot prosthesis itself, enabling us to assess its environmental impact accurately.

It should also be noted that the functional unit in this project is the production of a single prosthesis for a specific client. Therefore, each prosthesis will undergo a specific analysis, but our project can serve as a reference.

It's important to note that our investigation will solely cover these manufacturing phases and elements. Anything beyond this scope, such as specific mechanisms and machines employed during production, will not be included in our analysis. This ensures a focused and comprehensive evaluation of the environmental cost of creating carbon fiber foot prostheses.

2. Literature review

1. History and Development of Carbon Fiber

The beginnings of carbon fiber began in 1886 with the creation of the National Carbon Company in Cleveland, Ohio. Around the same time, carbon arc lamps illuminated the streets of the US, and National Carbon began its production of carbon electrodes. Thomas Edison, in 1879, may have created the first carbon fiber by carbonizing cotton threads or bamboo fragments for his incandescent light bulb filaments.

In 1956, the modern era of carbon fibers began when Union Carbide opened its Parma Technical Center near Cleveland. In 1958, Roger Bacon demonstrated the first high-performance carbon fibers. Using a high-pressure carbon arc method, he discovered graphene filaments with surprising properties, such as a tensile strength of 20 gigapascals and a Young's modulus of 700 gigapascals, far exceeding the properties of steel. These fibers, graphite polymers, were structures with unique properties. Although Bacon published his findings in 1960, the fibers were laboratory phenomena due to their high production cost. Subsequent research focused on developing more economical and efficient methods for its manufacture.

Union Carbide developed a series of high modulus yarns based on the hot drawing process, beginning in late 1965 with results of approximately 172 GPa in its Young's modulus. The Thornel line continued with increasing levels of modules for over ten years. During this period, the US Air Force supported much of Union Carbide's research in rayon-based fibers in an attempt to develop a new generation of rigid, high-strength composites. Simultaneously, scientists abroad were creating their own carbon fiber industries based on polyacrylonitrile (PAN), which had been discarded by American producers after failed attempts to make high-modulus fibers. In 1961, Japanese researchers, led by Akio Shindo, manufactured a carbon fiber with a modulus of more than 140 GPa, about three times that of rayon-based fibers at the time. This Japanese process was quickly adopted, leading to pilot-scale production in 1964. In the same year, William Watt in England invented an even higher modulus fiber from PAN, which was quickly commercialized.

Figure 1. Polyacrylonitrile (PAN) configuration

Japanese and English researchers had access to pure, highly oriented PAN, something very important to have good mechanical properties in this type of material. This technology turned out to be the most used. Leonard Singer and Allen Cherry designed a machine that applied stress to the viscous mesophase to align the molecules, and then heated the material to convert it into a highly oriented carbon fiber. The physical properties of these graphitized mesophase pitch fibers were striking, with ultra-high elastic modulus approaching 1,000 GPa and ultra-high thermal conductivity, making them especially useful for applications where stiffness and heat dissipation were important, such as brakes of airplanes and electronics.

Today, all commercial carbon fibers are based on rayon, PAN or pitch. Pitch-based fibers are unique in their ability to achieve ultra-high Young's modulus and thermal conductivity, finding assured applications in critical military and space sectors. However, its high cost has kept production minimal, with only a few Japanese companies and Cytec currently manufacturing commercial mesophasic fibers.

Over the past 20 years, the cost of producing carbon fibers has decreased dramatically, and researchers continue to reduce it. As they do, many applications previously considered impossible become reality. Although carbon fibers are used to a limited extent in automotive applications, it could reach the point where entire body panels are made from them. Related to that car’s panels, the carbon fiber is currently used in some parts of the high tech industry like the biomedical industry for the prosthesis.

The following graph represents how the demand of carbon fiber has grown along the last years while the usage of this material has grown a lot too. This is extremely related to how the price of this material has decreased significantly.

Figure 2. Growth in demand of carbon fiber over 50 years from 1970-2020

2. Manufacturing of Carbon Fiber and Types Available

1. Type of carbon fiber

Carbon fiber, a composite material, primarily originates from carbonaceous precursors such as polyacrylonitrile (PAN), pitch fiber, or rayon. Its synthetic origin allows this material a wide diversity of available types. This variety arises from strategic precursor choices and transformation processes, enabling the creation of fibers tailored to specific needs. So then, each type of carbon fiber has specific properties that make it suitable for particular applications. The choice relies on the specific requirements of strength, rigidity, lightness, and other mechanical properties for a given application. A list of the main types of carbon fiber and their specific physical properties is presented below:

* Standard Carbon Fibers (PAN-Based): Most commonly used carbon fibers. Produced from heat-treated PAN fibers offering excellent strength and good electrical conductivity.

* High-Strength Carbon Fibers (HM): Specific heat-treated PAN fibers for strength enhancement while maintaining relative lightness.

* Intermediate-Modulus Carbon Fibers (IM): Balanced strength-rigidity carbon fibers.

* High-Modulus Carbon Fibers (HT) and Ultra-High-Modulus Carbon Fibers (UHM): High or ultra-high rigidity carbon fibers (high modulus of elasticity) for structural rigidity applications.

* High-Tenacity Carbon Fibers (HTS): High-tenacity carbon fibers for excellent impact and tensile strength.

* Pyrolytic Carbon Fibers (PyC): Carbon fibers from high-temperature thermal decomposition of hydrocarbon gases, and characterized by an aligned crystalline structure for high-temperature applications.

2. Carbon fiber applications

Thanks to its exceptional properties, carbon fiber has become an indispensable player in various industries. Its lightweight, strength, rigidity, and other unique characteristics make it a preferred choice in sectors ranging from aerospace to medicine. Let us delve into the diverse applications of carbon fiber, shedding light on the major advantages it brings to each sector.

The 3 main uses of carbon fiber are linked to the properties that people know well: lightweight and strength, rigidity and impact resistance, and elasticity and flexibility.

Firstly, the exceptional lightweight nature of carbon fiber, combined with high strength, makes it an ideal choice for applications requiring materials that are both light and robust. In the aerospace industry, for instance, manufacturers extensively use carbon fiber in the construction of aerospace components such as airplane wings. Airbus, an industry giant, heavily incorporates carbon fiber in crafting the wings of the renowned Airbus A350, thereby contributing to the reduction of the aircraft's overall weight and improvement in energy efficiency.

Secondly, its outstanding rigidity, coupled with its impact resistance, positions it as the material of choice for sports equipment demanding both lightness and durability. High-end tennis rackets, such as those produced by the Wilson brand, utilize carbon fiber in their frames. This application allows tennis players to benefit from a lightweight yet robust racket, providing an optimal balance between rigidity for control and impact resistance for enduring performance. Moreover, the material presents not only a good resistance to impact, but also to extreme conditions. In instance, in the field of aerospace engineering, it is used in the fabrication of space structures. SpaceX, led by Elon Musk, incorporates carbon fiber components into its rockets, capitalizing on the necessary lightness and strength to withstand the extreme conditions of space.

Thirdly, its ability to offer elasticity and flexibility makes it valuable for applications such as fishing rods. The G. Loomis company, specializing in high-end fishing gear, incorporates carbon fiber in the production of its fishing rods. This use results in lightweight, responsive, and flexible rods, delivering an optimal fishing experience.

However, its exceptional properties allow it a very wide field of applications, which are less known but just as important. In the medical field, where it is directly linked to our study, carbon fiber is employed for the production of prosthetics and implants due to its biocompatibility and impermeability to bodily fluids. In the maritime industry, its resistance to corrosion contributes to boat hull construction. Finally, applications in the electronics industry are also found, due to its electrical conductivity. So then, carbon fiber remains a versatile material that revolutionizes various sectors, from airplanes to sports equipment, medical prosthetics to luxury yachts, thanks to its exceptional properties.

3. Study of Different Types of General Prosthesis and Mechanical analysis.

In this state-of-the-art analysis, we delve into the diverse landscape of general prosthetics, exploring a spectrum of options ranging from traditional to cutting-edge designs. Our focus extends beyond the prosthetic types themselves to encompass critical factors such as the forces and moments they must withstand, as well as the movement patterns they aim to replicate. By comprehensively addressing these aspects, we aim to provide a holistic understanding of the current state of general prosthetics, offering valuable insights for further development in optimizing strength, functionality, and biomechanical mimicry.

Nowadays there are different types of prosthetics this analysis aims to analyze more concretely the foot prosthetics. In the actual market the most common are; Prosthetic feet, crafted from materials such as plastic, metal, or carbon fiber, serve as artificial replacements for natural limbs lost due to various circumstances, including birth injuries, diseases, or severe injuries leading to amputation. Prosthetic feet can be categorized into basic/non-articulated, articulated, and dynamic response types, considering factors such as mobility, functionality, and the forces they endure.

Basic prosthetic feet, exemplified by the SACH foot, offer limited mobility and are suitable for patients with low functionality due to specific amputations. Articulated prosthetic feet, featuring single or multiple axes, provide increased mobility, offering stability and adaptability on uneven surfaces. Dynamic response prosthetic feet, the most advanced, store and release energy during walking, contributing to a more natural gait. It is also important to note the existence of high-efficiency prostheses, specifically designed for running, underscoring the significance of comfort, response, and design considerations. These are typically constructed using carbon fiber.

These technologies must meet a diverse range of demands, as highlighted in the literature, particularly in Basic Biomechanics of the Musculoskeletal System. The literature emphasizes the need for an exhaustive analysis of forces over the cross-sectional area of the bone.

Figure 3. Shear stress and normal strain of a bone.

The bone resistance is characterized by the stress-strain diagram and the deformation diagram. These are shown in the following illustrations. Also elasticity and plastic deformations should also be taken into account.

Figure 4. Stress-strain diagram and deformation diagram of a bone analysis.

Normally bone can be replaced by some biomaterials that have more or less the same properties such as Co-cr alloy, Titanium, Bone cement, Alumni… In the following table there is a list of implanting biomaterials that can substitute bone.

Figure 5. Table of the mechanical properties of selected biomaterials.

Finally, one of the most important factors in fabricating a prosthesis is considering fatigue and endurance, especially significant for foot prostheses subjected to very high fatigue cycles. It is also essential to consider the utility of the prosthesis, as the conditions for a professional running foot prosthesis differ from those for an elderly person who only uses it for smooth and slow walking. Fatigue and endurance are characterized by biomechanics and are performed and analyzed on the designed prosthesis.

Figure 6. Example of Dynamic Prosthesis.

It is important to study the properties of the element to be substituted and the components that make up the foot. In the following illustration, you have a schematic characterization of the foot in a medial view and dorsal view, where the components can be differentiated and analyzed.

Figure 7. View of the medial view of the foot.

Figure 8. View of the dorsal view of the foot.

After the characterization, it is important to analyze the foot and simplify our analysis to the maximum extent. The following illustration demonstrates a simplification of the mechanical forces applied to the foot for the study.

To decompose the intricate mechanisms of the foot and ankle, which are inherently complex, simplification becomes essential. However, this complexity can be significantly streamlined, particularly when considering forces, as depicted in the last illustration. Upon simplifying our problem, we can pinpoint the simplest mechanism that can effectively function as a foot.

Figure 9. (from the left to the right) Foot forces diagram simplification and Foot mechanism simplification.

And finally, characterize the different axial movements that our engineered prosthesis has to perform and resist. The foot and the ankle can undergo a total of three rotations, namely dorsiflexion/plantarflexion, eversion/inversion, and abduction/adduction. The following illustration shows the axes of these three motions.

Figure 10. Food axes analysis. Figure 11. Free-body diagram of a food in a stand up situation.

After characterizing all the elements, and in our case, the initial step is to calculate the free body diagram of the lower leg during a demanding activity such as stair climbing. Then, after extracting some results regarding the pressures on the foot when standing, utilizing a methodology based on finite elements. The analysis yields the following conclusions: the regional peak pressures measured in kPa while standing.

Figure 12. Mean regional peak pressures measured in kPa in the standing analysis of a food.

3. Study of Various Types of Carbon Fiber Prosthesis

Although the main role of a prosthesis is to replace a missing limb, there are different types that differ in the application for which they are designed. Each has an internal skeleton mainly made of carbon, but certain materials can be added depending on the constraints to which it will be subjected.

1. Dynamic Prosthesis

The dynamic carbon foot prosthesis (picture opposite) is designed with a focus on replicating the natural movement of the human foot. Composed of lightweight carbon fiber, its key feature is its ability to provide optimal energy return during walking. This type of prosthesis is particularly sought after for its ability to improve gait, reduce fatigue, and adapt effectively to various terrains. Its advantages include a more natural walking experience, but usually results in the need for fine tuning and increased cost. Ongoing research aims to enhance biomechanical integration, exploring technologies like integrated sensors for real-time terrain adaptation.

2. Modular Prosthesis

The modular carbon foot prosthesis (picture opposite) stands out for its customizable and interchangeable components. Customization is specific to each user: specific color, addition of decorative elements, etc. Constructed from durable materials such as aluminum and titanium alloys, its modular design allows for flexibility in adjustments and ease of component replacement. This type of prosthesis finds application in catering to varying activity levels and evolving user needs. One of its main qualities is adaptability coupled with component longevity. On the contrary, its possibilities come with an added weight due to modularity and the necessity for periodic adjustments, but research trends tend to develop lighter and more durable modular connections.

3. Sport Prosthesis

Engineered for athletic pursuits, the sport carbon foot prosthesis (picture opposite) utilizes advanced materials, often carbon composites, to strike a balance between lightness and strength. Tailored to specific sports requirements, these prostheses often integrate customizable adjustment systems for optimal performance. Their applications span a wide range of sports, including running and hiking. The advantages lie in enhanced biomechanical response and shock resistance, at the cost of higher costs and the need for expert fitting.

4. Aesthetic Prosthesis

The aesthetic carbon foot prosthesis (picture opposite) places emphasis on replicating the natural appearance of a human foot. While its internal structure may incorporate carbon for lightness, the external covering often features materials that mimic skin texture, such as silicone. Primarily used for cosmetic reasons, these prostheses contribute to improved self-esteem.

3. Methodology

1. Presentation

The methodology applied on this project will be based on a basic LCA (Life-cycle assessment) and LCC (Life Cycle Costing) to provide a complete description of the environmental impact of a carbon fiber foot prosthesis. The data to provide the complete study will be provided by an enterprise called Ottobock SE & Co. KGaA, which fabricates the prosthesis in Duderstadt, Denmark.

2. Life cycle inventory

To carry out a Life Cycle Assessment (LCA), in-depth knowledge of the different stages of product manufacturing and the materials used is necessary. It is more interesting in our case to consider a finalized product rather than a sample in terms of weight. The study focuses on the finished product in terms of quantity, i.e. a foot prosthesis.

1. Processes and flows

Firstly, each raw material transformation process involves a specialized machine, i.e. consumption of electrical energy.

The first step is sheet slicing from Aramid and Kevlar fabrics and carbon fiber sheets. From the products obtained, a vacuum compression over plaster mold is carried out in order to improve the physical capacity of materials and give the desired shape. Then the compacted sheets are assimilated together from resin and hardening powder to form the different solid parts of the prosthesis. Finally, if it requires it, the different articulations and degrees of freedom that it must offer are added by a mechanization process.

From the identification of the different processes, a deeper study can be carried out. Thus, Fiber Cutting and Laminate Manufacturing procedures begin with the cutting of fiber into precise dimensions, followed by placement onto the plaster mold facilitated by a vacuum system. Specifically, 16 laminates are being fashioned—12 composed of Aramid 1414 and Kevlar 49 Woven Fabrics, while the remaining 4 utilize Carbon Fiber Fabrics. Manual application of resin, using a brush to evenly distribute the matrix, ensues. The vacuum process is aided by a PVA bag, eliminating excess material. Subsequently, the laminates are left to rest at room temperature for 48 hours to achieve the necessary composite sheets.

Laminate Cutting to Required Dimensions involves the use of a CNC machine to precisely cut the obtained laminates into the required dimensions suitable for the prosthesis. From this detailed study, a flowchart of the operation of the system can be realized:

Figure 17. Flowchart of the system

2. Materials description

The materials used to produce a foot prosthesis are: Aramid 1414 and Kevlar 49 Fabrics totaling 41.364 grams are being employed, along with 16.763 grams of Carbon Fiber Fabrics. The Orthocryl Lamination Resin 80:20 Pro, weighing 58.12 grams, is being used in a quantity equivalent to that of the fabric, which is a typical ratio. Additionally, Hardening Powder (Ottobock Health Care 617P37) is being incorporated at 1.16 grams, constituting 2% of the resin's total weight. Polyvinyl Alcohol (PVA) is being used in a quantity of 11.6235 grams. Lastly, a Rectangular Plaster Mold with a weight of 30 kg is being utilized in the process.

The calculation of the fibers' quantities is based on the understanding of the entire volume of a prosthesis, approximately 38.3 cm^3. Assuming that 12 out of 16 layers comprise Kevlar and 4 out of 6 sheets are crafted from carbon, computations indicate that the volume occupied by Kevlar fibers is 28.28 cm^3, while carbon fibers encompass 9.575 cm^3. By applying their respective densities (1.44 g/cm^3 for Kevlar and 1.75 g/cm^3 for carbon), the weight of Kevlar fibers tallies to 41.36 grams and carbon fibers amount to 16.76 grams.

In resume :

Material

Quantity

Aramid 1414 and Kevlar 49 Woven Fabrics

41.364 g

Carbon Fiber Fabrics

16.763 g

Orthocryl Lamination Resin 80:20 Pro

58.12 g

Hardening Powder (Ottobock Health Care 617P37)

1.16 g (2% of the resin)

Polyvinyl Alcohol (PVA)

11.6235 g

Rectangular Plaster Mold

30 kg

Table 1. Quantities for each raw material

3. Assumptions and limitations

After the complete analysis of the product studied, the materials used and the manufacturing processes, the data collected must be implemented on the OpenLCA software. As it has a limited database, assumptions and simplifications must be made.

A first simplification of the system studied is that the machines used during the design are reduced to an energy flow. Thus, we will only take into account their energy consumption, and the impacts linked to manufacturing, transport, purchasing and maintenance are therefore not taken into account.

Then, a second hypothesis on all flows is made. These are provided from the market, meaning that the impact of the component, its transport, its manufacturing/operation are directly provided from the software database. In addition, some materials were not available, so we selected the most similar flow. Thus, we selected components for the simulation are gathered in the table below:

Flow(s) selected

Initial flow(s) from the system

Aramid, Kevlar

Glass fiber reinforced plastic, polyamide, injection molded

carbon fiber

Carbon fiber reinforced plastic, injection molded

Plaster

Base plaster

PVA

Polyvinyl fluoride, film

Resin

Orthophthalic acid based unsaturated polyester resin

Powder

Coating powder

Electricity

Electricity, medium voltage

Table 2. Most similar flow selection

4. Software details

The information about the software and the impact assessment method are gathered below:

Concretely the software used is OpenLCA on the version 1.10.3.OpenLCA is a free, open-source Life Cycle Assessment (LCA) and Footprint software developed by GreenDelta. It offers a comprehensive range of features for environmental, social, and economic sustainability assessments. The software supports large systems and databases, GIS integration, and detailed analysis capabilities. OpenLCA is available for Windows, Mac, and Linux, and can interact with web-based databases. It's notable for its transparency, continuous updates, and lack of license costs, although some databases might require purchase. OpenLCA is suitable for various applications, including industry, consultancy, education, and research.

More deeply we have used the database: ecoinvent_371_apos_unit_20201221 and for the impact assessment, the method used, to calculate the LCA and its derivatives, is ReCiPe Midpoint (H).

5. Description of Scenarios

We will provide a comparison of two scenarios: a base scenario in which the classical methodology is employed to mold a carbon fiber prosthesis using a plaster mold, and a second scenario aimed at reducing environmental impact. In the second scenario, we will use a wooden mold. At first glance, the porosity of the wooden mold may appear to create imperfections in the carbon fiber prosthesis, but the use of PVA will help correct any irregularities in the wood.

4. Results and Discussion

IMPLEMENTED DATA

Before getting into the results, the implemented data for each of the processes involved in the production of a carbon fibre prosthesis will be shown, including the different inputs and outputs flows, their amount and measurement unit as well as their respective providers chosen.

Sheets slicing process

Vacuum compression over plaster mold process

Resin and hardening powder application process

Mechanisation process

Raw materials involved

Since the different components of the prosthesis are complex materials and do not express the nature of their composition, a look into the raw materials involved in the production of a carbon fibre prosthesis is needed. As it can be seen from Annexes [], the main raw materials involved and their respective quantities are the following:

Calcite

8.48408 kg

Iron

0.04381 kg

Carbon dioxide

0.47833 kg

Laterite

0.05140 kg

Clay

1.52642 kg

Natural gas

0.48920 m3

Coal

1.91696 kg

Nitrogen

0.04372 kg

Crude oil

0.68227 kg

Oxygen

0.11299 kg

Fluospar

0.01576 kg

Sand

21.62604 kg

Gangue

0.4386 kg

Shale

0.47159 kg

Granite

0.02902 kg

Sodium chloride

0.03768 kg

Gravel

2.29051 kg

Water

25.95994 m3

Gypsum

0.29245 kg

It can be deduced that some of the raw materials involved may have an important direct environmental impact, such as carbon dioxide, coal, natural gas and crude oil, while some others may have an indirect effect by being used in certain processes, such as calcite, gravel or sand.

LCA RESULTS

Once all the processes and data have been introduced in the software, the ReCiPe Midpoint (H) assessment method is carried out. The results for all the impact categories covered by this method can be consulted in Annexes [], however, the most relevant ones considered for the analysis are the following:

Impact category

Result

Main pollutants

Climate change

10.52678 kg CO2-Eq

CO2, CH4

Fossil depletion

2.00892 kg oil-Eq

Oil, Coal, Natural Gas

Freshwater ecotoxicity

0.10145 kg 1,4-DCB-Eq

Cu, Ni, Mn, Zn

Human toxicity

2.05455 kg 1,4-DCB-Eq

Mn, Ba, Pb

Ionising radiation

0.73593 kg U235-Eq

Rn-222, C-14

Metal depletion

0.17252 kg Fe-Eq

Cu, Fe, Zn

Water depletion

0.15212 m3

Considering that a single prosthesis weighs slightly over 100 grams, the results for these categories can be considered as remarkably high, specially in the case of climate change (10.52678 kg CO2-Eq), fossil depletion (2.00892 kg oil-Eq) and human toxicity (2.05455 kg 1,4-DCB-Eq), where several kilograms of the equivalent unit of measurement are produced.

With the goal of understanding where these emissions come from within the processes, the contribution trees for each impact category are unfolded. From Annexes [], it can be concluded that the main contributor is always the plaster mold followed by the carbon fibre, while the remaining components (Kevlar fabrics, PVA, resin, powder and electricity) stay in a secondary plane with a significantly lower contribution. This can be proven by the three main impact categories mentioned before, where the contribution of the plaster mold and carbon fibre are the following

Impact category

Plaster mold contribution

Carbon fibre contribution

Climate change

74.97 %

13.13 %

Fossil depletion

58.60 %

19.12 %

Human toxicity

62.67 %

22.18 %

This is a clear indicator that the use of a plaster mold is the main responsible for most of the environmental impact of the production of a carbon fibre prosthesis, however, this can be reassured by taking a look at the top 5 contributors for each of the impact categories in Annexes []. Some of these are clinker production, sand quarry operation, quicklime production, heat production, hard coal mine operation and petroleum production, which are all processes related to the production of plaster.

With all of this, it stays clear that if one component were to be changed with the goal of reducing environmental impact regarding the mentioned impact categories that would be the plaster mold, since it would not make sense to replace the carbon fibre. To achieve this, the 30 kg plaster mold has been replaced by a wooden mold in the vacuum compression process, as it can be seen in the following image:

Since the density of the plaster is 2300 kg/m3, its volume in wood will be 13.0435 liters.

Followingly, the same assessment method has been carried out and in the next section a comparison of the two scenarios will be made

2| SYSTEM COMPARISON

Comparison of the 2 scenarios and the impact on climate change and pollution (toxicity at different scales)

A comparison of environmental impacts is made between the base case and the alternative case, these climate impacts could be diverse and various based on LCA analysis. The impacts include agricultural and land comparison, climate change, fossil depletion, freshwater ecotoxicity, freshwater eutrophication, human toxicity, ionizing radiation, marine ecotoxicity, marine eutrophication, metal depletion, natural land transformation, ozone depletion, particulate matter formation, photochemical oxidant formation, terrestrial acidification, terrestrial ecotoxicity, urban land occupation, and water depletion. However we will choose only 5 of these based on their significance to our assessment, which are highlighted in red.

In the first place, climate change is measured in kilograms of CO2, which is the main greenhouse gas contributing to the cause. In the base case (plaster mold), a total of 10.53kg of CO2 are emitted to the atmosphere, being the three top contributors clinker production, heat production and quicklime production. It is notable that clinker production is one of the main contributors, emitting almost thirty percent of the total emissions.

If we switch to the alternative case with wooden mold, the situation improves as the CO2 emitted is down to only 3.66kg of CO2, which is only a third part of the original emission. Top 3 contributors are heat production, electricity production and nylon 6-6 production.

Freshwater ecotoxicity assesses the potential toxic effects of substances on aquatic systems, particularly in freshwater environments, this measure is parametrized in kilograms of 1,4-DCB (dichlorobenzene). In the first case, with a total of 0.101kg of 1,4-DCB, is almost double of the second case, with only 0.063kg of 1,4-DCB. On the other hand, the top 3 contributors remain the same for both cases.

Nevertheless, for the following second case, observe how emissions resulting from scrap copper remains almost the same, which means that most of the reductions have happened due to the reason of mining operations.

Metal depletion measures the depletion of metal resources and evaluates the environmental impact associated with their extraction and use, and it is measured in kilograms of iron (Fe). In the case of plaster mold the emissions rise to 0.173kg of Fe, whilst in the second case of wood mold it has been reduced to only 0.105kg of Fe. Top three contributors also remain the same, but notably, the emissions of manganese concentrate and iron ore mine operations are almost identical, which means that, for the wooden mold, improvements have occurred to other emitting sources.

As it is depicted below, for the second case, the first and second bars are clearly higher than others, which means that they did not experience any enhancement in terms of emission to the atmosphere.

Natural land transformation assesses the impact of human activities on natural land cover, including factors such as deforestation, urbanization, and other changes that alter the natural landscape, therefore it is measured in meter square of area (m2). Within this context, the plaster mold needs an area which is 3.5 times larger than the one needed by the wooden mold. For the first case, even with land recultivation, the space required is still much bigger, as they are mainly related to quarry operations.

For the second case, quarry operations are non-existent, therefore all-natural land occupied are linked to production, as shows:

Finally, photochemical oxidant formation approaches the potential for the formation of harmful oxidants in the atmosphere that are often related to air pollution and smog, measured in kilograms of NMVOC (non-methane volatile organic compounds). Again, plaster mold emits almost the double (0.03kg of NMVOC) of the emissions of wooden mold ( 0.018kg of NMVOC)

As seen in the following image, the main contributor for plaster mold is again clinker production, hence we can conclude how clinker is one of the main reasons to the different negative effects resulting from the plaster mold case:

For the wooden case, clinker production is not necessary, saving significantly harmful emissions, and switching attention to other sources:

Analyzing these 5 factors, without going beyond the environmental context, we can conclude how harmful emissions can be reduced in a great extent by just choosing the right materials. However, other parameters such as phyco-chemical properties should be considered when choosing materials for specific purposes of production.

The reasons behind the improvement of these parameters can be possibly explained in the following way:

The wood comes from trees that can be replanted, meaning that it is a renewable source that can absorb the carbon dioxide emitted to the atmosphere during the process, and therefore mitigating the overall carbon footprint associated with wood products. Another reason is the fact that the production of wood products requires less energy compared to the plastic, leading to a lower CO2 emission. Wood is biodegradable so it can decompose naturally unlike the plastic, resulting in an enhanced performance in terms of freshwater ecotoxicity. The metal depletion is reduced as wood does not require mining and refining processes associated with certain metals, also its production involves fewer harsh chemicals compared to the plastic. In addition to this, its manufacturing process generally emits fewer pollutants that contribute to the formation of photochemical oxidants.

However, we should consider how the chosen material can impact positively or negatively in the ease of use, precision and overall functionality. Whilst assessing this matter, material characteristics is a significant aspect, in which wood is generally durable and can withstand multiple uses for a longer-term production, whereas plastic can be broken with more easiness. Moreover, wood tends to be more stable dimensionally and more resistant to temperature and humidity variations, which can contribute to the precision and stability of the mold over time. However, plaster have their own advantages, for instance, plaster is porous, leading to a better absorbing of the moisture during the molding process.

On the other hand, wooden molds can be easily customized to fit individual patient requirements, as they are easy to carve and shape, whilst the plaster offer not only the simplicity to personalize, but also to be done in a relatively quick way.

Finally, in terms of functionality, wooden molds may be heavier compared to plaster molds, which is a disadvantage when manipulating and manufacturing during the fabrication process. In addition to this, plaster molds can adapt well to intricate shapes by capturing effectively fine details during the molding process.

In summary, the choice between these two materials must be made carefully by examining various factors such as the needs of the patient and carbon footprints, the best result will be a balance of every variable considered but keeping in mind environmental impacts all the time.

Comparing our emission results with the analysis by Prenzel et al. where they used polyacrylonitrile (PAN) to make the fiber carbon prosthesis (90% of the carbon fiber prosthesis are made of PAN nowadays), resulted in emissions of 29.1kg of equivalent CO2 for 1kg of carbon fiber, which is much more than our hypothetical cases, even for the plaster mold. Interestingly, they were able to reduce these emissions by 43% without changing the initial material considering just the energy source precursors, energy source for carbon fiber production changes and optimizing technologically the process. And if optimized with ongoing research activities to reduce energy consumption in stabilization and carbonization, this reduction could rise up to 53%. With this realistic example we can see how optimizing the process and using clean energies can contribute significantly to an improved environmental performance [5].

3| ENERGY COMPARISON

Comparison of the consumption of the different scenarios and which energy source is involved

The energy employed in both scenarios is another crucial factor when a LCA comparison is performed. In this case, various energy sources have been considered, including geothermal, kinetic (wind), potential (hydro) and solar. The workflow of the production of a carbon fiber prosthesis is linear, therefore the energy required is added linearly step after step, hence, to calculate how much energy is demanded by each step, a sum of the mentioned consumed energy must be made, following with an extraction of the consumed energy in the previous stage. Graphs are displayed below for both scenarios:

Observe how there are few behaviors that are interesting. To start with, hydro is definitely the most used source, with little utilization in resin and mechanization fase for both scenarios, but in the vacuum stage for the wood mold, they experience a significant reduction. The relative use of wind energy is similar in both cases, being the second most exploited source of energy. Geothermal and solar are almost unutilized, although the presence of geothermal in the plaster mold case is greater than in the wood case, where viceversa occurs. Finally, the majority of the energy is still used in the sheets and vacuum fase, although the absolute quantity of energy required is reduced significantly.

4| LIFESPAN EMISSIONS COMPARISON

Comparison of the weight of kgCO2-eq for each technology regarding its lifespan + conclusion

As seen in the previous section, the difference in equivalent CO2 emissions per kg of prosthesis is significant. Additionally, it is important to remember that the difference originates only in the change of mold. In the base case, a plaster mold is used, a material that must be produced through artificial processes and using different materials to obtain it. In its internal process (out of the scope of this report) there are phases such as the extraction of natural plaster, the creation of calcium sulfate from sulfuric acid and limestone, the creation of the gypsum paste, its cooking and its drying. On the other hand, wood, of purely natural origin, eliminates these artificial parts of the production of the material, obtaining much lower equivalent CO2 emissions because its process only mainly involves the cutting, transportation and treatment of the wood.

The importance of the mold material is such because it is the heaviest component used in the LCA calculation. This already gives us an idea of why this material is so decisive since its weight of 30kg is almost 1000 times greater than that of the second heaviest component, Orthocryl Lamination Resin 80:20 Pro with 0.05812 kg of weight.

PLASTER MOLD

The total emissions of the plaster mold case per prosthesis unit are 10,528 kg of CO2 equivalent. It is noticed that 3 out of 5 top contributions of CO2 equivalent emissions are due to the clinker production, being the main contributor to the RoW category of the clinker production, responsible for 28,36% of the total CO2 equivalent emissions, meaning that it exist few specific factors that could significantly reduce the general CO2 emissions. The second main contributor is related to the necessary heat production during the process, that is not completely related to the plaster production, but is proportionally significant. Finally it is found that the third contributor is due to the quicklime production, a basic element for producing a plaster.

WOOD MOLD

The total emissions of the wood mold case per prosthesis unit are 3,660 kg of CO2 equivalent. It is much lower than the previous plaster case, notice that it represents 34,76% of the previous emissions, meaning that it approximately emits 3 times less.

Because plaster is not used in this case, all the main contributors analyzed previously do not appear at the wood mold case analysis. Could be interesting to highlight that the first and second main contributors are related to the necessary power to run the manufacturing process, being the first one a 9,56% of the total CO2 emissions contribution, meaning that in this case the CO2 emissions are much more distributed among different factors and it would be much difficult to reduce them in this scenario.

It is also important to consider not only the environmental impact but also the functionality of the wooden mold. At first glance, the porosity of the wooden mold may seem to create imperfections in the carbon fiber prosthesis, but the use of PVA can help correct any irregularities in the wood. Additionally, the functional advantages of the wooden mold are attributed to its durability, flexibility, potential sustainability compared to plaster, cost-efficiency profile, and the ability to achieve a suitable surface finish for optimal prosthesis results. Furthermore, it is easier to mechanically process and shape the wooden mold to the required form compared to the plaster mold, while offering the same capabilities.

There is just one missing question regarding the comparison between both cases. Will the lifespan of the product be the same in both cases? Imagine that the carbon fiber prosthesis made by a wood mold is replaced faster than the one made by a plaster mold. If the time of replacement of the wood mold case prosthesis would be 3 times lower than the plaster mold case, then it would produce approximately the same CO2 emissions. Due to this difference not in replacing an inner material of the prosthesis (it consists in replacing the mold material), the difference in the lifespan would be in the confirmation process. A lower surface quality and lower mechanical properties could be the result of that change in the confirmation process. In any case, that will not happen because of the LVA layer between the mold and the prosthesis. In conclusion, the lifespan will be between 3 and 5 years for both analyzed cases, independently of the mold used, so the wood mold option is much better in CO2 emissions terms.

Sensitivity analysis and comparison with literature

Furthermore, a sensitivity analysis should be performed to ensure the robustness of our Life Cycle Assessment (LCA) and verify that the main difference between the two scenarios is indeed based on the use of the two different molds. Specifically, we conducted a sensitivity comparison by testing how the LCA results change when varying one or more input parameters or assumptions. As a result, we observed that the variations in our system ranged between 1% and 7% in the final result comparison

When talking about the comparison with literature it is difficult to compare it as a one entity as there are not carbon fiber LCA’s on the internet to compare but, it can be compared with the most basic elements. Also it can be ensured with literature that manufacturing of carbon fiber prostheses involves energy-intensive and has a significant environmental impact due to high greenhouse gas emissions.Which verifies our study. The environmental impact difference resulting from substituting a plastic mold with a wooden mold can also be considered. Literature indicates that the CO2 emission for wood is around 0.1 kg of CO2 per kg, compared to a more varied and generally higher CO2 equivalent for plaster. This difference supports our findings that a wooden mold is less environmentally harmful than a plaster mold. Additionally, there have been advances in manufacturing technologies in this area, such as 3D printing and the use of bio-based or recycled materials, aimed at reducing environmental impact.

5| LCC ANALYSIS

LCC analysis carried out to evaluate the cost of the technology, regarding the market and other technologies

Prosthetics, particularly those crafted from advanced materials like carbon fiber, represent a significant advancement in modern healthcare technology. Understanding the economic implications within the lifecycle of such prosthetics is crucial for informed decision-making. This study delves into the specific costs associated solely with the materials and energy required to manufacture a single unit of a carbon fiber prosthetic.

It is essential to note that this analysis deliberately excludes certain expenses to maintain a focused scope. Notably, expenses related to facility costs, machinery procurement, labor, engineering, and additional personnel are omitted from this assessment. The objective is to isolate and assess the direct expenditures associated purely with materials and energy in the manufacturing process, providing a narrowed perspective on these specific cost components.

Furthermore, while recognizing the importance of social and environmental externalities in any comprehensive evaluation, this study prioritizes a granular examination of the direct economic implications. Previous sections have thoroughly addressed social and environmental considerations, and this segment aims to elucidate the isolated monetary costs associated solely with materials and energy in the production of carbon fiber prosthetics.

The subsequent analysis aims to provide a clear and concise delineation of the economic impact related exclusively to material resources, energy consumption and cost of CO2 emissions, acknowledging the deliberately confined focus of this segment within the larger context of the prosthetic's life cycle.

PLASTER MOLD CASE = 53,83 €/prosthesis

Material

Price per kg

Quantity

Total price per prosthesis unit

Aramid 1414 and Kevlar 49 Woven Fabrics

29,3-37,5 €/kg

41.364 g

1,38€

Carbon Fiber Fabrics

1062-1272,5 €/kg

16.763 g

19,57€

Orthocryl Lamination Resin 80:20 Pro

33,69 €/kg

58.12 g

1,96€

Hardening Powder (Ottobock Health Care 617P37)

44 €/kg

1.16 g (2% of the resin)

0,05€

Polyvinyl Alcohol (PVA)

5,45 € /kg

11.6235 g

0,06€

Plaster for moldeling

0,99€/kg

30 kg

29,70€

CNC mechanization

60,5 €/h

1h

60,5€/h

Electricity sheets slicing

0,1823 €/kWh

0,5 kWh

0,09€

Electricity vacuum compression over mold

0,1823 €/kWh

0,25 kWh

0,05€

Electricity in Mechanisation

0,1823 €/kWh

0,5 kWh

0,09€

CO2 emissions

0,08371 €/kg

10,528 kg

0,881€

WOOD MOLD CASE = 110,26€/prosthesis

Material

Price per kg

Quantity

Total price per prosthesis unit

Aramid 1414 and Kevlar 49 Woven Fabrics

29,3-37,5 €/kg

41.364 g

1,38€

Carbon Fiber Fabrics

1062-1272,5 €/kg

16.763 g

19,57€

Orthocryl Lamination Resin 80:20 Pro

33,69 €/kg

58.12 g

1,96€

Hardening Powder (Ottobock Health Care 617P37)

44 €/kg

1.16 g (2% of the resin)

0,05€

Polyvinyl Alcohol (PVA)

5,45 € /kg

11.6235 g

0,063€

Piece of wood

2,89 €/kg

30 kg

86,7 €

CNC mechanization

60,5 €/h

1h

60,5€/h

Electricity sheets slicing

0,1823 €/kWh

0,5 kWh

0,09€

Electricity vacuum compression over mold

0,1823 €/kWh

0,25 kWh

0,05€

Electricity in Mechanisation

0,1823 €/kWh

0,5 kWh

0,09€

CO2 emissions

0,08371 €/kg

3,660 kg

0,307 €

The wood mold option is more expensive due to the material cost. In conclusion, the cost of the wood mold prosthesis is double than the plaster mold prosthesis, but it emits three times more.

In this LCC, there is not the possibility of calculating a cycle over the years due to the fabrication being one single use until it is replaced between 3 or 5 years later.

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8. Conclusions

This study concentrated on examining the energy system and the environmental repercussions of manufacturing carbon fiber prostheses. It encompassed a range of aspects, from the extraction of raw materials to waste management, with a particular focus on energy consumption and carbon footprint throughout the product life cycle.

Regarding the production phase, the study provided a detailed overview of the processes and materials involved in the production of carbon fiber prostheses, underscoring their environmental impact. Notably, the difference between the two cases analyzed hinges on the production of the mold.

To conclude about the economic implications, taking into account both material and energy costs in the manufacturing process. It reveals that while plaster is a less expensive option compared to a wood mold, considering the Life Cycle Assessment (LCA) results, the use of a wooden mold is more advantageous, not just economically but also socially. Initially, it might be assumed that a wooden mold could lead to a poorer surface finish of the prosthesis, but the application of Polyvinyl Alcohol (PVA) ensures an excellent result. This is assured too by the LVA layer that exists between the mold and the wooden mold, because the porosic surface of the wood will be not directly in contact with the carbon fiber.

When talking about energy consumption and the efficiency of production processes, comparing different manufacturing scenarios. It highlights that hydro energy is the primary energy source used in carbon fiber prosthesis production, followed by wind energy. Geothermal and solar energies play a minor role. The majority of energy is consumed during the sheet creation and vacuum phases, with the overall energy requirement being significantly reduced in the production process. It is also noteworthy that the wooden mold consumes about 30% less energy compared to a plaster mold.

The research concludes that although carbon fiber prostheses have certain environmental impacts, strategic choices in materials and processes can substantially lessen these effects.

An innovative proposition is the development of a 3D printing machine using a reusable mold, such as PLA. This approach could be more environmentally friendly due to the low-temperature requirements of 3D printing with carbon fiber, and the potential for curing the resin with ultraviolet light, which would effectively eliminate bubbles. The PLA mold could be melted and reused, which would substantially lower the environmental impact and reduce costs, as the mold cost would be negligible.

However, this proposed method is slated for future research, as it is not currently available in the market. It promises not only cost reductions but also improvements in the LCA metrics, increased automation, and the feasibility of mass-producing personalized prostheses at reduced costs. It's important to note that the low costs are primarily due to the streamlined process of personalized prosthesis design.

Also it should be explained the limitations and simplifications in the study, such as constrained data availability and the necessity for assumptions in modeling. As we encountered during the study, such as accessing relevant information and ensuring its validity, were addressed by reviewing over 24 papers related to prosthesis construction and adapting the prosthesis design accordingly.

The study paves the way for exploring alternative materials, advanced manufacturing techniques, and a deeper investigation into the environmental impacts of prosthetic devices, particularly focusing on the mentioned 3D system using a reusable PLA mold and a curing method that reduces energy consumption.

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9. Citations and references

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Singh, M. (2014). Growth in demand for carbon fiber over 50 years from 1970-2020.

ResearchGate. https://www.researchgate.net/figure/Growth-in-demand-of-carbon-fibre-over-50-years-from-1970-2020-2_fig1_303943769

Peijs, T., Kirschbaum, R., & Lemstra, P. J. (2022). Advanced Industrial and Engineering Polymer Research.

Nordin, M. (2020). Basic biomechanics of the musculoskeletal system. Lippincott Williams & Wilkins.

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Clayworks. (n.d.). Sustainability. Retrieved from https://clay-works.com/sustainability/.

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Cellura, M., Longo, S., & Mistretta, M. (2011). Sensitivity analysis to quantify uncertainty in life cycle assessment: the case study of an Italian tile. Renewable and Sustainable Energy Reviews, 15(9), 4697-4705.

Fraunhofer Institute for Systems and Innovation Research. (2009). Methodology for the free allocation of emission allowances in the EU ETS post 2012. Retrieved from https://climate.ec.europa.eu/system/files/2016-11/bm_study-gypsum_en.pdf

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