Running head: 3D PRINTING OF SCAFFOLDING FOR AORTIC VALVES 1

3D Printing of Scaffolding for Aortic Valves

Rina Bongalonta, Natalia Osorio, Daniel Patterson, Adrian Williams, Chris Wu

University of South Carolina

3D PRINTING OF SCAFFOLDING FOR AORTIC VALVES 2

Abstract

The following paper discusses 3D printing using specified biomaterials in order to create

effective aortic valve scaffolds for cardiac transplant. This paper covers the major reasons as to

why patients must go through aortic valve transplants, such as the diseases affiliated with a

malfunctioning aortic valve and the chronological progression of biomaterials and designs for

scaffold models used for biovalves. The use of stem cells for cardiac valve scaffolds and the

complications with current applications of in vivo aortic valves, such as Biovalve VII, are also

explained. Future advances in aortic valve engineering are being made with alginate, or alginic

acid/gelatin hydrogels, and human cardiac-derived cardiomyocyte progenitor cells (hCMPCs) to

improve durability, permeability of bodily fluids, and keep foreign material deposits from

collecting within the aortic valve. The main goal of biomedical engineer involvement is to create

in vivo aortic valve scaffolds that are completely biocompatible with human transplantation and

does not biodegrade. Public opinion and authority figures in scientific research are currently

discussing the ethics and repercussions of 3D printing aortic valve construct and in vivo

biomaterials for aortic valve transplant.

3D PRINTING OF SCAFFOLDING FOR AORTIC VALVES 3

3-D Printing of Scaffolding for Aortic Valves

In the event that an aortic valve must be replaced in the human body, three main

categories of engineered valves that may be used: mechanical valves, biological prosthetics, and

biovalves. Demonstrating the longest functional lifespan of over 25 years, mechanical valves are

composed of materials such as titanium, polyester, and carbon compounds [1]. However, this

option requires lifelong anticoagulation and can carry the risks of toxicity, thromboembolism,

and bleeding in the patient [1]. Aortic valves can also be replaced by a biological prosthetic,

originating from an organ donor or another animal source. The low availability of eligible

human heart valves and ethical concerns are the reason that human donations are the least

common choices for valvular transplants. Other cardiac tissue valves are harvested from pigs or

cows and are treated so that the human body will not reject them [1]. While tissue valves have

improved blood flow, lowered toxicity levels, and lessened the need for anticoagulation like the

mechanical valves, tissue valves have shorter average life spans that last between 10 and 15

years due to tissue degradation [1]. Currently, unlike mechanical and tissue valves that lack the

ability to grow or remodel, biovalves are the most promising option for patients in need of valve

transplants [2]. In addition, biovalves are non-toxic, non-carcinogenic, and can be fabricated to

many different shapes and sizes [2]. These valves are created by 3D printing a scaffold that is

placed inside a living organism, promoting encapsulation with natural tissue growing on the

scaffold. This process allows artificial valves with qualities such as tissue regeneration and self-

repair to be manufactured most similarly to original aortic valves in the patients [1]. The most

recently developed biovalve, Biovalve Type VII, demonstrates a large improvement towards

engineering a fully-functioning in vitro aortic valve with the properties of a real aortic valve.

3D PRINTING OF SCAFFOLDING FOR AORTIC VALVES 4Biovalve Type VII

Through 3D printing, molds can be made to allow in-body tissue engineering of aortic

heart valves, with the most recent example being the Biovalve Type VII, as stated in “In-Body

Tissue-Engineered Aortic Valve (Biovalve Type VII) Architecture Based on 3D Printer

Molding.” By using a 3D printed the mold, the Biovalve VII can dictate tissue growth so that the

prosthetic resembles the natural body tissue of a goat and can be removed without damage to the

leaflet tissue. Goat models most resemble the conditions of humans through heart size and

systemic circulation. The mold is implanted into the dorsal subcutaneous pouch of a goat,

prompting connective tissue to encapsulate the mold, mimicking the natural leaflets in the goat’s

aortic valve. After two months, the mold is completely enveloped by tissue, and the biovalves are

harvested and tested. The valve formation has a 90 percent success rate, with 27/30 as functional.

Using bypass surgery, three of the valves are implanted into goat models, where the valves are

subjected to in vivo systemic circulation for one month. The other valves are tested for tensile

strength, composition, and conditions of artificial systemic flow. In the goat models, the

prosthetic valves maintained a low regurgitation rate (<3%) and kept a high opening ratio and are

similar in both size and shape to the goat’s native sinus of Valsalva [3].

Progress

Since the tissues are engineered in the body, the tissues are created without any artificial

materials, unlike in vitro. Because the tissues are engineered in vivo, the valve is compatible

with the body and acts in a similar manner to the natural body tissues, because the tissues come

from the animal itself. Additionally, the highly ordered and complex methods of in vitro tissue

engineering are not needed. Although the Biovalve VII shows improvement from the valve

models used today, the Biovalve VII is prone to calcification and degeneration [3].

3D PRINTING OF SCAFFOLDING FOR AORTIC VALVES 5According to “Rapid manufacturing techniques for the Tissue Engineering of Human

Heart Valves,” unlike the Biovalve VII, in order to make cardiac valves through in vitro

methods, researchers used stem cells from the umbilical cord vein to engineer cardiac valves.

Since the umbilical cord offers an abundance of stem cells, they are cryopreserved in liquid

nitrogen to prevent from contamination until ready for use, after the cells are received. Once the

cells were ready for use, they were then thawed and placed inside a pulsatile bioreactor, which

combined the cell seeding process and conditioning process to prevent contamination. To

achieve these processes, the pulsatile bioreactor has two cylindrical perfusion chambers that

rotate in opposite directions: the perfusion chamber’s opposite directional rotation controls the

supply of oxygen and the amount of turbulence to the cells. These extreme conditions created by

the two cylindrical perfusion chambers are needed to seed the cells on to the scaffolds that create

the cardiac scaffold [4].

Limitations

While current applications of in vivo aortic valves (Biovalve VII) are intricately

engineered to closely imitate the functions of native aortic valves, several biological issues arise,

since these applications do not perfectly mirror the processes of native aortic valves. Several

statistical analyses on the effectiveness of in vivo aortic valves show that failure is contingent on

the individual’s age, with children and adolescents having the highest and fastest risk of failure

and individuals over the age of 35 having the lowest and slowest [5]. This is due to the presence

of developmental cardiac hypertrophy that is associated with the biological growth of

adolescents. In these cases, four primary complications are the causes of failure:

thrombosis/hemorrhage, endocarditis, structural dysfunction, and non-structural dysfunction [6].

Since the facilitation of native aortic valve structural properties is crucial to the success of in

3D PRINTING OF SCAFFOLDING FOR AORTIC VALVES 6vivo aortic valves, repetitive changes in structure and dimension, stress transfer to adjacent

cardiac walls, and natural regression of damage are the primary causes of several structural

dysfunctions [5]. In older individuals, in vivo aortic valves are usually subjected to calcification

and stenosis, in which calcium deposits form within the cardiac tissue of the aortic valve causing

vascular constriction. Although in vivo aortic valves have made much progress, future

developments must be made to combat these biological problems.

Future Bioprinting Research

Currently, the development of in vivo biovalves using stem cells directly from the

patient’s own tissue is in the conceptual stage. The limitations with constructing a viable scaffold

model using the patient’s own cardiac cells include cell viability, medical complication after cell

transfer, and opposing ethical standings. Presently, biomedical engineers use rapid prototyping

(RP) techniques with computer based design and manufacturing to create complex and

accurately detailed tissue constructs and models. Most of the efforts of bioprinting, such as

gelatin hydrogels, stereolithography, which uses lithographic methods to 3D print models and

prototypes one layer at a time, and two-photon laser based photo crosslinking for 3D tissues,

have been geometrically inaccurate, cytotoxic, or not clinically adequate, respectively [7].

Therefore, biomedical engineers are current working on enhancing the standard of aortic valves

by improving on the problems that are present with the current aortic valve scaffolds, such as

Biovalve VII and standard polymer models.

Members of the Department of Cardiology of the University Medical Center Utrecht in

the Netherlands performed research on the amalgamation of current 3D printing and alginate

hydrogels and progenitor cells, which helped design “biological materials that exhibit high

resolution geometric and or mechanical complexity” [7]. The development for a supplementary

3D PRINTING OF SCAFFOLDING FOR AORTIC VALVES 7porous and permeable scaffold has been achieved using porous hydrogel models, especially that

of the alginate type. Through natural in vivo aortic valve functional and structural fabrication, 3D

printed models can have construct and the capability of cell regeneration, response stimuli, and

viability, if the essential biomaterials are applied. In hydrogel models, alginate viscous gum, an

anionic polysaccharide from the cell walls of brown algae, aids in material transfer between the

linings of the scaffold and in concentration diffusion between the plasma and blood flowing

through the aorta and the cells that are involved in tissue encapsulation with the implanted

scaffold. The cardiac progenitor cells would also be biocompatible and adaptable to the in vivo

communication and transport cells, thus preventing calcification and other aortic valve

complications that are present in previous scaffold models.

In the Department of Biomedical Engineering at Cornell University, a seven-day study

compared the cell viability of a porous hydrogel scaffold containing progenitor cells versus the

standard polymer cardiac valve model [8]. The aim of the study was to evaluate the combination

of TP (tissue printing), human cardiac-derived cardiomyocyte progenitor cells (hCMPCs), and

alginate hydrogels discs in three different shape models to obtain a construct with cardiogenic

mechanical potential for in vitro or in vivo application. Porcine encapsulated aortic root sinus

smooth muscle cells (SMC) and aortic valve leaflet interstitial cells (VIC) were examined and

placed within a square, while vector and dumbbell shaped models of calcium chloride washed

alginate/gelatin hydrogel scaffolds. General exposure and fluorescent staining were present

throughout the experiment, and after the one-week period, cell viability was determined after

printing. Ninety-two percent from the vector grid pattern hydrogel model and 89 percent of cells

from the controlled standard polymer model were viable at 1 and 7 days of culturing [8].

Currently, the possibility of clinically examining scaffolds with progenitor cells and alginate

3D PRINTING OF SCAFFOLDING FOR AORTIC VALVES 8hydrogel material is still in processing to create a biocompatible, long lasting model for

transplants with porcine cells and other sources [8].

Ethical Factors

Although 3D printing has many benefits, such as producing not only aortic valves but

also other parts of the human body, ethical issues surround the topic of 3D printing, like other

technology in the world today. The article “3-D Printing Will Be a Counterfeiter’s Best Friend”

discusses how much 3D printing can do for the human race in regards to improving our future.

Ethical concerns arise when 3D printers allow for the the infringement of intellectual property

and the market of fake IDs [9]. The AM systems, a company that allows for the affordable

buying of products for pulmonary and respiratory care, has made it easier for people to get ahold

of the products and produce the fakes. Even though 3D printing is fairly new, this issue seems to

be a growing problem that will affect the future of 3D printing [9]. In the future, 3-D printing

could raise the question of if it is ethical for one person to be able to receive an aortic valve over

another person, because he or she is able to afford it, creating a greater barrier between the rich

and the poor health-wise. The rich are allowed to live longer, because they can receive the

benefits of 3D printing, while someone who makes less will not receive those same benefits.

Three-dimensional printing can benefit everyone in the world, but if ethical issues like this

continue, they can affect the public’s view on 3D printing [10]. In addition to economic ethical

questions, the source of stem cells is controversial. The umbilical cord vein is considered more

ethical for use because it does not require any extra surgery to receive it [4]. Although 3D

printing allows for multiple medical benefits, such as the engineering of aortic valves, ethical

concerns surround the practice of 3D printing.

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