BIOMATERIAL FOR SOFT TISSUE REPLACEMENTS

David N. Ku and Jin Wu Fan

Georgia Institute of Technology, Atlanta, GA 30332-0405, USA

Keywords: Biomaterial, mechanics, elasticity, strength, wear, biocompatibility, medical devices.

Abstract: Typical biomaterials are stiff, difficult to manufacture, and not initially developed for medical implants. A

new biomaterial is proposed that is

similar to human soft tissue. The biomaterial provides mechanical

properties similar to soft tissue in its mechanical and physical properties. Characterization is performed for

modulus of elasticity, ultimate strength and wear resistance. The material further exhibits excellent

biocompatibility with little toxicity and low inflammation. The material can be molded into a variety of

anatomic shapes for use as a cartilage replacement, heart valve, and reconstructive implant for trauma

victims. The biomaterial may be suitable for several biodevices of the future aimed at soft-tissue

replacements.

1 BACKGROUND

Most of the existing biomaterial technology is

limited to materials such as silicones, Teflon®,

polyethylene, metal and polyurethanes that do not

exhibit the mechanical and physical properties of

natural tissue. These materials are stiff, difficult to

manufacture, and not initially developed for medical

implants. Artificial tissue substitutes have not been

found to withstand the rigors of repetitive motion

and associated forces of normal life. Cadaver tissue

is limited in supply and due to the risk of infection is

coming under increased scrutiny by FDA and is not

accepted in Europe.

As an example, one of the most successful

med

ical implants is the artificial knee replacement

for the treatment of arthritis. Arthritis and joint pain

as a result of injury are major medical problems

facing the US and patients worldwide. Worldwide,

approximately 190 million people suffer from

osteoarthritis. This condition affects both men and

women, primarily over 40 years of age. The spread

of arthritis is also fueled by the rise of sports

injuries. Activities enjoyed by many can translate

into injury or joint damage that may set up a process

of deterioration that can have devastating effects

decades later. The number of patients that have

arthritis is staggering and growth is expected with

baby boomers entering the prime “arthritis years”

with prolonged life expectancies. Growth of the

world’s elderly is expected to increase three times

faster than that of the overall population.

Current standard treatmen

t is to surgically

implant a total knee replacement (TKR) that is made

of metals such as cobalt chromium or titanium.

These devices are highly rigid, providing no shock

absorption. Further, the metal integrates poorly with

bone and the HMWPE caps often create micro-

particulate debris with strong inflammation. The

invasive surgery not indicated for those under age 60

and usually reserved for end-stage patients. A

revision procedure is technically demanding, and

amputation may be required.

An alternative biodevice may be a soft tissue

replacem

ent. Arthritis stems from damage to

cartilage, the soft tissue between the bones.

Damaged cartilage leads to grinding of bone on bone

and eventual pain and limited joint function.

Biodevices that replace the soft tissue would restore

diarthroidal joint function much better and protect

further damage by a more natural stress distribution.

A similar problem exists for heart valves.

Prosth

etic heart valves are made from metal and

pyrolitic carbon which do not function like native

heart valves. The use of hard materials creates high

shear zones for hemolysis and platelet activation.

Tissue valves are subject to calcification, again a

problem of hard tissues not acting like natural soft

tissue. An alternative would be to design a biodevice

with soft tissue flexibility and endothelial cell

N. Ku D. and Wu Fan J. (2008).

BIOMATERIAL FOR SOFT TISSUE REPLACEMENTS.

In Proceedings of the First International Conference on Biomedical Electronics and Devices, pages 23-29

 SciTePress

covering to provide a wide-open central flow and

low-thrombogenic surface.

Yet another example for the need for soft tissue

replacements is a replacement part for reconstructive

surgery after traumatic accidents or cancer resection.

For many parts of the body, a replacement shape

needs to have smooth contours as well as soft tissue

compliance to yield a natural shape. The base

biomaterial should not have chemical composition

that is non-organic, such as silicone, which can

induce a hyper-immunogenic response.

Soft tissue replacements should start with a

biomaterial that has compliance ranges similar to

human soft tissue, be strong and wear resistant,

manufactured to personal shapes, and have long-

term biocompatibility. Cellular in-growth or

preloading of cells can then be performed on this

established scaffold. These features are

demonstrated in a new biomaterial described in this

paper.

2 METHODS

2.1 Biomaterials

Soft tissue-like devices can be made from polymers

such as poly vinyl alcohol as thermoset materials.

As an example, a PVA cryogel can be made

according to the full descriptions in US Patent U.S.

Patent Numbers 5,981,826 and 6,231,605. The

cryogels are made in a two stage process. In the first

stage a mixture of poly (vinyl alcohol) and water is

placed in a mold, and repeatedly frozen and thawed,

in cycles, until a suitable cryogel is obtained.

Poly(vinyl alcohol) having an average

molecular weight of from about 85,000 to 186,000,

degree of polymerization from 2700 to 3500, and

saponified in excess of 99% is preferred for creating

soft tissue-like mechanical properties. High

molecular weight poly (vinyl alcohol) in crystal

form is available from the Aldrich Chemical

Company. The PVA is then solubilized in aqueous

solvent. Isotonic saline (0.9% by weight NaCl,

99.1% water) or an isotonic buffered saline may be

substituted for water to prevent osmotic imbalances

between the material and surrounding tissues if the

cryogel is to be used as a soft tissue replacement.

Once prepared, the mixture can be poured into

pre-sterilized molds. The shape and size of the mold

may be selected to obtain a cryogel of any desired

size and shape. Vascular grafts, for example, can be

produced by pouring the poly (vinyl alcohol)/water

mixture into an annular mold. The size and

dimensions of the mold can be selected based upon

the location for the graft in the body, which can be

matched to physiological conditions using normal

tables incorporating limb girth, activity level, and

history of ischemia.

The new biomaterial, commercially available as

Salubria® from SaluMedica, LLC, Atlanta, GA is

similar to human tissue in its mechanical and

physical properties. The base organic polymer is

known to be highly biocompatible and hydrophilic

(water loving). The hydrogel composition contains

water in similar proportions to human tissue. Unlike

previous hydrogels, Salubria is wear resistant and

strong, withstanding millions of loading cycles; yet

it is compliant enough to match normal biological

tissue. The material can be molded into exact

anatomic configurations and sterilized without

significant deterioration.

2.2 Mechanical Characterization

2.2.1 Tensile Testing

Tensile test specimens were cut from sheets of

Salubria. They were tested in accordance with

ASTM 412 (die size D) in tension to failure using an

Instron Model 5543 electro-mechanical load frame

pulling at a rate of 20 inches per minute.

2.2.2 Stress-Strain Constitutive Relationship

The stress is a function of the load and the cross-

sectional area. However, the cross-sectional area

was difficult to measure. But the stretch ratio relates

the final and initial area due to the assumption of

incompressibility. That means the final area equals

the initial area divided by the stretch ratio.

Therefore, the ultimate stress calculation is a

function of the load at the breaking point of the

sample, the stretch ratio and the initial cross-

sectional area. The initial cross-sectional area is the

product of the initial width of the sample, w

, and the

initial thickness, t

Stretch Ratio:

oCC /

(1)

Initial Cross-Section Area: (2)

* twA

Final Stress:

ult

(3)

In order to get an estimation of the pressure in

an intact tube the following simplified assumptions

were used. It was assumed that a tubular specimen

will burst when the circumferential wall stress is

BIODEVICES 2008 - International Conference on Biomedical Electronics and Devices

equal to the ultimate stress

ult

. However, when an

artery is under physiologic load conditions it is in a

state of plane strain and undergoes finite two

dimensional stretches.

The stretch ratios are:

= (4)

(5)

Rewritten to solve for the pressure, P:

(6)

This equation can calculate the pressure if we

know that data of the initial dimensions, the stress

and the stretch ratios. From the equation (6) we can

estimate the corresponding pressures.

2.2.3 Unconfined Compression

Cylindrical unconfined compression samples were

cast in a custom mold. Samples were tested in

unconfined compression on an Instron Model 5543

electro-mechanical load frame and on a DMTA IV

dynamic mechanical analyzer. Rates of 1% strain

per second and 20 inches per minute were tested.

2.2.4 Ultimate Strength

Ring specimens were pulled in tension until they

failed. Ring specimens of Salubria were

preconditioned twenty five cycles. Then the

specimens were distracted at 0.1mm/s, 1m/s,

10mm/s, and 100mm/s using a MTS 858Mini Bionix

Test System. Comparisons are made to normal

coronary arteries using identical protocols. The load

at failure was recorded as the ultimate load, and the

ultimate stress was calculated. Failure of the ring

specimen was defined as a complete tear of the ring

through the entire wall. The stress was derived

based on the assumption of incompressibility and

was defined as the ratio of load and cross-sectional

area. The stretch ratio was defined as the ratio of the

final and initial circumference. The final stress at

failure represented the ultimate strength for the

tension tests. To determine the final stress, an

equation was derived based on the assumption of

incompressibility [3] which means that the initial

volume V

and final volume V are equal. In the

present experiments the stretch ratio is defined as the

ratio of the final and initial circumference, Equation

(1). The ultimate stress,

ult

defined by the load at

the breaking point of the sample divided by the final

cross-sectional area, was calculated using Equation

(3).

2.2.5 Fatigue Resistance

Ring specimens were cycled at different cycles, and

then pulled in tension until failure. The frequency of

the cyclic tests was set at 2 Hz because this value is

close to physiologic frequency of heart beats (~1.2

Hz) and strain rates effects testing showed that there

are no significant difference to do cyclic test under

1HZ to 5HZ. The purpose of the cyclic tests was to

experimentally determine how the fatigue affects the

ultimate strengths of porcine common carotid

arteries.

2.2.6 Cyclic Compression

A cyclic compression study was performed to assess

the response of Salubria biomaterial cylinders to

repetitive compressive loading at physiologic stress

of approximately 1.3 MPa. The specimen is loaded

for 1 million loading cycles at a rate of

approximately 1.5 Hz. Dimensional integrity was

measured using an optical comparator and

mechanical modulus of elasticity was determined at

20% strain.

2.2.7 Wear Testing

An accelerated wear tester was built to test the wear

rate of Salubria biomaterial. Polished stainless steel

rollers with a diameter of 1 5/8 inches (chosen to

approximate the average radius of the femoral

condyles) are rotated so that they slide and roll

across the test sample, creating a peak shear load of

approximately 90N (0.2 MPa). Separate testing has

shown the coefficient of friction for polished

stainless steel against Salubria biomaterial to be

equivalent to that of porcine femoral condyle with

the cartilage surface abraded away to subchondral

bone or roughly 4 times that of porcine femoral

condyle with intact cartilage surface. The rotating

cylinders exert a normal load of approximately 200N

on the sample. The wear tester is operated at a rate

that subjects the sample to 1000 wear cycles per

hour where one cycle is defined as one roller to

sample contact. Data has been collected with the

sample lubricated and hydrated with water (a worst

case scenario since synovial fluid should provide

some surface lubrication). Wear rate is measured by

BIOMATERIAL FOR SOFT TISSUE REPLACEMENTS

weight loss of the sample over a number of cycles.

Salubria biomaterial is tested for >10,000,000 cycles

against polished stainless steel rollers.

2.3 Biocompatibility

Biomaterials for use in humans must pass a full

complement of biocompatibility tests as specified by

ISO and the USFDA. The material was tested for

ability to produce cytotoxicity, intracutaneous

irritation, sensitization by Kligman maximization,

Ames mutagenicity, chromosomal aberration, and

chronic toxicity.

2.3.1 Rabbit Osteochondral Defect Model

In addition to the standard biocompatibility testing,

the ability of Salubria biomaterial to withstand load

or cause local inflammatory responses in a widely

used rabbit osteochondral defect model was assessed

(e.g., Hanff et al., 1990). A cylindrical plug (3.3

mm diameter, 3 mm depth) was implanted in the

right knee of each of sixteen New Zealand white

rabbits. An unfilled drill hole was made in the left

knee of each animal to serve as an operative control.

After three months of implantation in the

patellofemoral groove, eight rabbits were sacrificed

for histologic analysis of the implant site and

surrounding synovium. The remaining eight animals

were sacrificed and the implant assessed for any

change in physical properties. In addition, the

distant organ sites that are known targets for PVA

injected intravenously were assessed histologically

for any sign of toxicity due to implantation. Tissues

assessed included liver, spleen, kidney, and lymph

node.

2.3.2 Particulate Inflammation

Salubria biomaterial was tested for particulate

toxicity or inflammatory reaction in the joint. The

study design was based on a study conducted by Oka

et al. (2000) on a similar PVA-based biomaterial in

comparison to UHMWPE. Particulate sizes for the

study varied from approximately 1 micron to 1000

microns. The total volume of particulate injected

over the 2 divided doses was designed to represent

full-thickness wear of 10 x 10 mm diameter cartilage

implants.

2.3.3 Ovine Knee Inflammation Model

In vivo testing was performed using a meniscus

shaped device made of Salubria and implanted into

the sheep knee joint. Devices were removed at 2

week, 3 week, 2 month, 4 month and 1 yr. intervals.

Animals were fully load-bearing on the day of

operation and after. Full range of motion with no

disability was observed. Gross examination of

surrounding tissues and histology of target end

organs (liver, kidney, spleen, and lymph nodes) were

evaluated for acute or chronic toxicity.

3 RESULTS

3.1 Tensile Testing

Tensile Stress vs. Strain of Salubria

Sample 480-G-1

0 50 100 150 200 250 300 350

Strain (%)

Stress (MPa)

Figure 1: Representative Tension Curve.

Plots of stress versus strain in tension (figure 1)

show a non-linear response. Due to the non-linearity

of the loading curve, tangent modulus values at a

defined percent strain are used to characterize the

material stiffness. Tangent modulus ranges from

1.2-1.6 MPa. Ultimate tensile strength is 8-10 MPa.

The stress-strain curve exhibits a non-linear elastic

behavior similar to natural soft tissue.

3.2 Unconfined Compression

Figure 2 is a representative curve of stress versus

strain in compression. Compression loading curves

show a non-linearity suggesting that Salubria is a

viscoelastic material similar to cartilage. Tangent

modulus values in compression range from 0.1 to 7

MPa. Plastic compressive failure occurs at or above

65% strain.

3.3 Creep and Creep Recovery

Creep and creep recovery experiments were

performed to assess the performance of Salubria

biomaterial under long-term static loading at

physiologic loads of up to 480 N-force. High loads

were applied for 24 hours creating deformation of

BIODEVICES 2008 - International Conference on Biomedical Electronics and Devices

3.8 Animal Testing

50% of the initial height. Initial loading

demonstrates a biphasic visco-elastic behavior.

After 24 hours of recovery in saline, sample height

had returned to within 5% of the original height.

The compressive modulus of the material before the

test and after creep recovery was unchanged.

After three months of implantation in the

patellofemoral groove, eight rabbits were sacrificed

for histologic analysis of the implant site and

surrounding synovium. Tissues assessed included

liver, spleen, kidney, and lymph node. The Salubria

biomaterial was well-tolerated with subchondral

bone formation surrounding the implant, no fibrous

tissue layer or inflammatory response, no implant

failures or evidence of wear debris formation, no

osteolysis, and no toxic effects on the implant site or

distant organ tissues.

3.4 Wear Testing

Results for one formulation of Salubria biomaterial

tested for >2,000,000 cycles against polished

stainless steel rollers demonstrate minimal wear.

3.5 Cyclic Compression

At time of explantation, the samples were

essentially unchanged (see Fig. 3). Based on

histologic examination in comparison to the

operative control, there was no evidence of

inflammatory reaction in either the surrounding

cartilage/bone (see Fig. 4) or in the synovium. In

fact, a layer of normal hyaline cartilage partially

covered the implant surface. The cartilage surface

of the patella also showed no changes in the area

articulating against the Salubria implant. There was

no sign of toxicity on histologic examination of the

distant organ sites.

Repetitive compressive loading at physiologic stress of

approximately 1.3 MPa was imposed. After 1 million

loading cycles at a rate of approximately 1.5 Hz, there

was minimal change (<5%) in sample dimensions and

no change in modulus of elasticity at 20% strain.

3.6 Ultimate Strength

Sixty-four specimens were pulled at four different

stain rates. Ultimate stress increased as a weak

function of increasing strain rates. The ultimate

stress at 100 mm/s was 4.54 MPa, greater than the

3.26 MPa at 0.1 mm/s. The differences between

0.1mm/s and 100 mm/s was highly significant with

p<0.001. The differences between 0.1 mm/s and 10

mm/s gave p=0.013; and 1 mm/s to 100 mm/s was

p=0.018. The difference between 1 mm/s and 10

mm/s was not statistical significance. Strain rates

between 1 and 100 mm/s correspond to a cyclic

frequency of 0.5 Hz to 5 Hz for fatigue testing.

Compressive Stress vs. Strain of Salubria

Sample 482-G-1

0 20406080

Strain (%)

Stress (MPa)

Figure 3: The left-hand knee shows a Salubria implant in

the patellofemoral joint of a rabbit knee after 3 months

implantation. The right-hand knee is an operative control.

On excision for mechanical testing, the sample

edges firmly adhere to the surrounding bone. The

indentation force (i.e., the force required to cause a

certain amount of sample deformation) of the

implant is unchanged from a non-implanted control.

Comparison material characterization testing showed

that the implanted samples were not different from

non-implanted controls.

Figure 2: Representative Compression Curve.

3.7 Biocompatibility

The following table outlines the results of standard

biocompatibility testing performed on Salubria

biomaterial, in accordance with ISO 10993-1 and

FDA Blue Book Memorandum #G95-1.

BIOMATERIAL FOR SOFT TISSUE REPLACEMENTS

Table 1: Biocompatibility Testing.

ISO 10993-1 Recommended Testing

Requirement

Test Performed Test Results

ISO MEM Elution L929 cells,

GLP.

Pass

Cytotoxicity, ISO 10993-5

Direct Contact Neurotoxicity

Pass

Kligman Maximization Method

Pass

Sensitization and Irritation,

ISO 10993-10

Primary Vaginal Test: Repeat

Exposure

Pass

Sub-acute and Sub-Chronic Toxicity,

ISO 10993-6

Sub-acute and sub-chronic

toxicity

Pass

Ames Mutagenicity: Dimethyl

Sulfoxide Extract, 0.9% Sodium

Chloride Extract

Pass

Genotoxicity, ISO 10993-3

Chromosomal Abberation

Pass

Subacute or site Specific

Implantation with chronic

Toxicity

Pass

Biocompatibility study in

Rabbits following Intra-articular

injections.

Pass

Implantation,

ISO 10993-6 and 10993-11

Rabbit Trochlear Osteochondral

Defect

Pass

(b)

(a)

Figure 4: (a) This digital scan of a paraffin tissue block containing a Salubria implant demonstrates that the implant remains

in place over 3 months of implantation in the rabbit patellofemoral groove. (b) Hematoxylin and eosin stain of a section

from the tissue block in (a) showing the implant site – the implant has been removed during the staining process. There is

no evidence of inflammatory reaction; the surrounding cartilage and bone are normal in histologic appearance.

Salubria biomaterial was tested for particulate

toxicity or inflammatory reaction in the joint.

Salubria particulates were biologically well-

tolerated. The biomaterial particulate was deposited

on the superficial synovium with minimal

inflammatory reaction. There was no evidence of

migration from the joint space or toxicity in the knee

or at distant organ sites. There was no evidence of

third body wear or osteolysis.

For the goat study, the native articular cartilage

surfaces were protected in the test group compared

to extensive damage from the control meniscectomy

group. No local inflammation was noted on MRI or

histology. No distant organ inflammation was seen

BIODEVICES 2008 - International Conference on Biomedical Electronics and Devices

in these large animals, confirming the observations

in the rabbits.

4 DISCUSSION

The biomaterial described here exhibits the requisite

characteristics for soft tissue replacements. For

knee cartilage, the material has non-linear

viscoelastic properties similar to native tissue. The

strength and fatigue properties exceed the

requirements for a fully loaded knee articular joint

(Stammen, 2001). For heart valves, the material

must be moldable to complex anatomies and exhibit

low thrombogenicity. For reconstructive anatomic

parts, the biomaterial should be easily molded to

custom shapes and have low inflammation potential.

The biomaterial presented here exhibits these

properties and opens the potential for soft tissue

replacements that more closely match the anatomic

and physiologic requirements.

Although ring specimens and dumbbell shape

specimens are both one-dimensional tests, ring

specimens were used because they provide a good

gripping connection. Ring samples can relieve the

experimental error comes from the inappropriate

clamping dumbbell specimens which can cause the

specimens to slide or break in the neighborhood of

the clamp. There may be damage from preparing

uniaxial dumbbell shaped strips. Dumbbell strips

would also be difficult to obtain because of the small

diameter of the tubular samples.

The biocompatibility testing for Salubria

reflects previous carcinogenicity testing on other

PVA-based biomaterials. PVA hydrogels in the

literature are non-carcinogenic with rates of

tumorigenicity similar to the well-accepted medical-

grade materials, silicone and polyethylene.

Nakamura (2000) reports on a 2-year carcinogenicity

study conducted on a PVA-based biomaterial

subcutaneously implanted in rats. Pre-clinical

investigation of other PVA-based hydrogels and

Salubria biomaterial demonstrates that these

materials are biocompatible in the joint space (Oka

et al., 1990). The rabbit is the most commonly

published cartilage repair model with study lengths

varying from 3 months to 1 year, with little

difference in results at 3 months from those at 1

year. These studies indicate that 3 months is

sufficient to assess biocompatibility and early

treatment failure in the rabbit model. These results

are further confirmed by clinical results on

SaluCartilage

Based on this study, Salubria soft tissue

biomaterial has been shown to be biocompatible

with long-term implantation. There is no evidence

of inflammatory response or local or distant toxicity.

Furthermore, the biomaterial has stable, durable

physical properties over the period of implantation

in joints and would be suitable for use as structure

deceives such as a cartilage replacement. The

biomaterial presented here opens the potential for

soft tissue replacements that more closely match the

anatomic and physiologic requirements for human

biodevices.

REFERENCES

Hanff, G., Sollerman, C., Abrahamsson, S. O., and G.

Lundborg. 1990. Repair of osteochondral defects in

the rabbit knee with Gore-TEXTM (expanded

polytetrafluoroethylene). Scandinavian Journal of

Plastic and Reconstructive Hand Surgery. 24: 217-

223.

Oka, M., et al., Development of an artificial articular

cartilage. Clin Mater, 1990. 6(4): p. 361-81.

Stammen, J.A., Williams, S., Ku, D. N., and R. E.

Guldberg. 2001. Mechanical properties of a novel

PVA hydrogel in shear and unconfined compression.

Biomaterials 22(8): 799-806.

BIOMATERIAL FOR SOFT TISSUE REPLACEMENTS