Article

Behaviour of SS-316L Hydroxyapatite Coated in Simulated Body

Fluids

Sri Helianty¹, Ahmad Fadli¹* , Yunita Magdalena Silalahi¹, Yohana Dwi Nita Barus¹

¹Chemical Engineering Department, Faculty of Engineering, Universitas Riau, 28293 Tampan,

Pekanbaru, Indonesia.

Abstract

Hydroxyapatite (HA) is a calcium phosphate mineral that closely resembles the inorganic component of

natural bone. The incorporation of polycaprolactone (PCL) into HA enhances its mechanical strength,

flexibility, and bioresorbability, producing composites with excellent biocompatibility and bioactivity in

simulated body fluid (SBF). This study investigates the bioactivity and degradation behaviour of HA/PCL

coatings on SS 316L stainless steel substrates. The relationships among coating thickness, shear strength,

crystallinity, and pH variation in SBF were systematically examined. HA/PCL coatings were prepared using

the dip-coating method and immersed in SBF at 37 °C for 7, 14, 21, and 28 days. Crystallinity and

degradation characteristics were analysed using X-ray diffraction (XRD) and weight loss measurements.

The results showed that HA/PCL-coated SS 316L exhibited noticeable weight loss after seven days of

immersion due to Ca²⁺ ion release from the composite. Extended immersion led to increased HA

crystallinity, indicating continued apatite formation and confirming the coating’s bioactive and

biocompatible nature. Overall, the HA/PCL composite coating effectively enhances the bioactivity and

provides controlled degradation of metallic implants, demonstrating strong potential for orthopaedic

and dental biomedical applications.

Keywords: HA/PCL coating, Hydroxyapatite, Polycaprolactone, Simulated Body Fluid, SS 316L

*

Corresponding author

Email addresses: fadliunri@yahoo.com

DOI: https://doi.org/10.22437/chp.v9i2.44633

Received May 30^th2025; Accepted October 13^rd2025; Available online November 25^th2025

(https://creativecommons.org/licenses/by/4.0 )

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Graphical Abstract

Introduction

incorporation of PCL enhances the coating’s

flexibility and adhesion to the metallic

substrate, while also allowing for gradual

degradation and controlled ion release

during the healing process [5,6]. Such hybrid

HA/PCL coatings combine the mechanical

resilience of polymers with the biological

functionality of ceramics, making them

promising for orthopaedic applications.

Hydroxyapatite (HA, Ca₁₀(PO₄)₆(OH)₂) is a

bioceramic material that closely resembles

the mineral component of natural bone in

both chemical composition and crystal

structure.

Owing

to

its

excellent

and

biocompatibility,

bioactivity,

osteoconductivity, HA has been widely

employed as a coating material for metallic

implants to enhance osseointegration and

bone regeneration [1,2]. When deposited

onto metallic substrates such as stainless

steel, titanium, or magnesium alloys, HA

The bioactivity of these coatings is typically

evaluated in vitro using simulated body fluid

(SBF), which replicates the ionic composition

of human plasma [6]. According to Kokubo

and Takadama [7], the ability of a material to

induce the formation of a bone-like apatite

layer on its surface in SBF strongly correlates

with its in vivo bone-bonding potential [8].

During immersion, the process of apatite

establishes

a

strong

chemical

and

mechanical bond at the implant–tissue

interface, thereby improving fixation and

long-term stability [3].

Despite these advantages, pure HA is

nucleation,

crystal

growth,

and

inherently

brittle

and

exhibits

poor

transformation into stable HA phases serves

as an indicator of surface reactivity and

degradation stability [9].

mechanical strength, which can lead to

coating delamination or fracture under

physiological loading conditions [4]. To

overcome these shortcomings, researchers

have developed composite coatings that

integrate HA with biodegradable polymers

Recent studies have shown that several

parameters,

including

SBF

pH,

ion

concentration, and coating layer thickness,

significantly impact the degradation kinetics

such

as

polycaprolactone

(PCL).

The

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and bioactivity of HA/PCL coatings [10-12]. In

particular, coating thickness plays a dual

role: a thicker layer generally limits ion

diffusion and slows degradation, whereas a

porous and thinner layer enhances Ca²⁺ and

PO₄³⁻ ion exchange, promoting faster apatite

formation [6]. Although these findings

highlight the importance of coating design,

the precise correlation between HA/PCL

coating thickness, structural integrity, and

material [11]. All chemicals used were of

analytical grade and utilised without further

purification.

Preparation of Hydroxyapatite and SBF.

Hydroxyapatite powder was synthesized [6],

and simulated body fluid (SBF) was prepared

by sequentially dissolving reagent-grade

salts in distilled water maintained at 36.5 ±

1.5 °C under continuous stirring at 300 rpm.

The pH of the solution was adjusted to 7.40

± 0.01 using hydrochloric acid (HCl) to

replicate the ionic conditions of human

plasma. The final SBF composition was

confirmed to match that reported by Kokubo

and Takadama [7], and the solution was

stored at 20 °C before use in immersion

experiments.

degradation

behaviour

remains

insufficiently understood, especially for

stainless steel 316L (SS 316L) substrates that

are widely used in orthopaedic implants.

Therefore, this study aims to elucidate the

relationship

between

HA/PCL

coating

thickness and degradation behaviour in SBF.

Specifically, it investigates (i) the correlation

between coating thickness, shear strength,

HA/PCL Coating Process

and crystallinity; (ii)

the

influence

of

immersion time on pH variation and

degradation rate; and (iii) the formation and

evolution of apatite layers on HA/PCL-coated

SS 316L surfaces. The outcomes are

expected to provide new insights into the

The HA/PCL coatings were applied onto SS

316L substrates using the dip-coating

technique [6]. The coating thickness was

controlled by adjusting both the immersion

duration and withdrawal speed of the

substrates. Following coating, the samples

were air-dried at room temperature to

minimize potential thermal degradation of

the polymer matrix. The selection of

experimental parameters was guided by the

procedures described in Kokubo’s Bioactive

Glass Ceramics: Properties and Applications

physicochemical

HA/PCL

interactions

in

governing

physiological

coatings

environments and to guide the design of

next-generation bioactive coatings for

durable and functional orthopaedic implants

Materials and Methods

(1991)

investigated in this study included coating

shear strength, layer thickness, and

[12].

The

primary

variables

Materials

The AISI 316L stainless steel was selected as

the substrate due to its high corrosion

immersion duration in simulated body fluid

(SBF). All other experimental conditions-such

as solution composition and temperature-

were maintained constant throughout the

experiments.

resistance,

mechanical

stability,

and

widespread use in biomedical implants.

Hydroxyapatite (HA) was synthesised using

chicken eggshells as a calcium precursor,

providing a sustainable and cost-effective

source of calcium. Polycaprolactone (PCL), a

biodegradable aliphatic polyester known for

its flexibility and bioresorbability, was

employed as a polymeric binder and matrix

HA/PCL Soaking Testing of HA/PCL in SBF.

The degradation and bioactivity of the

HA/PCL-coated samples were assessed

following ISO/FDIS 23317 guidelines and the

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procedure established by Kokubo and

Takadama [7]. The coated samples were

immersed in 80 mL of SBF at 37 °C for 7, 14,

21, and 28 days to simulate physiological

conditions [8]. During the immersion period,

the pH of the SBF solution was periodically

monitored using a calibrated pH meter to

detect

degradation

ionic

exchanges

processes.

and

A

potential

schematic

flowchart summarising the SBF synthesis

and immersion testing procedure is

presented in Figure 1.

Figure 1. Flow diagram of the HA/PCL-coated SS 316L immersion process and simulated body

fluid (SBF) synthesis steps.

XRD and Weight Loss Analysis

reproducibility, and the mean values were

used for analysis.

After each immersion interval, the samples

were removed from SBF, rinsed with distilled

water, and dried at room temperature. X-ray

diffraction (XRD) was performed to identify

crystalline phases and evaluate changes in

the degree of crystallinity of the HA/PCL

coatings. The weight loss of each sample was

recorded before and after immersion to

determine the degradation rate according to

equation 1.

After immersion in SBF for 7, 14, 21, and 28 days,

the samples were dried and analysed using X-ray

diffraction (XRD) to identify the phase structure

and crystallinity of the formed HA layer. The pH

of the SBF solution was measured periodically

using a calibrated pH meter to monitor ion

exchange and potential degradation of the

coating [13].

Result and Discussion

Wo− Wt

X-Ray Diffraction (XRD) Analysis

Weight Loss (%) =

100%

(1)

Wo

The XRD analysis was performed to evaluate

biocompatibility and apatite formation on

HA/PCL-coated SS 316L samples after 28

days of immersion in SBF (Figure 2).

Where Wo and W_trepresent the sample

weights before and after immersion,

measurements

times to

respectively.

repeated

All

three

were

ensure

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Figure 2. XRD patterns of HA/PCL-coated SS 316L specimens after immersion in SBF, corresponding to

shear strength values of (a) 1.45 MPa, (b) 1.95 MPa, (c) 1.99 MPa, (d) 2.22 MPa, (e) 6.74 MPa, (f) 23.22 MPa,

(g) 24.17 MPa, and (h) 25.79 MPa.

The highest peak intensity appeared in

sample (f) at 750.98 a.u. with a shear

strength of 23.22 MPa, while the lowest was

in sample (c) at 362.79 a.u. with 1.99 MPa.

The peaks corresponded to ICDD card No.

01-072-1243 for hydroxyapatite, confirming

the formation of a biocompatible HA phase

release facilitated apatite nucleation and

crystal growth in PCL/HA composites [16]. In

the present study, the crystallinity increased

up to 86.8%, which falls within the typical

range for hydroxyapatite coatings (60–90%)

[14]. The increase in crystallinity correlated

positively with adhesion strength, indicating

with

a

well-ordered

crystal

structure.

improved

coating

stability

and

Polycaprolactone (PCL) peaks were not

detected, as PCL primarily acts as an

adhesive polymer [14]. During immersion,

an ion-exchange process occurred between

the coating and the SBF medium, in which

Ca²⁺ ions were released from the coating into

the solution, increasing the local calcium

concentration [15]. These Ca²⁺ ions, derived

from Ca(OH)₂, reacted with PO₄³⁻ to form

apatite, while OH⁻ ions from hydroxyapatite

biocompatibility.

Apatite Formation Within Simulated Boy

Fluids.

The formation of bone-like apatite on the

coating

surface

followed

a

two-stage

mechanism: nucleation and crystal growth.

In the early stage, the hydroxyl (OH⁻)

functional groups on the coating surface

served as nucleation sites, attracting Ca²⁺

and PO₄³⁻ ions from the SBF. As immersion

continued, these nuclei grew into a dense

promoted

further

apatite

bonding

throughout the immersion period. Sharma

et al. (2019) also reported that Ca²⁺ ion

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and continuous apatite layer [16]. Hydroxyl

(OH⁻) groups at the surface attracted Ca²⁺

and PO₄³⁻ ions. Their bonding formed

apatite, which thickened as immersion

proceeded. The progressive thickening of

elaborated by Dridi et al. (2021), who

described

the

transformation

from

amorphous calcium phosphate (ACP) to

crystalline hydroxyapatite with increasing

immersion

time

[17].

Therefore,

the

apatite

layer

indicates

enhanced

observed evolution of crystallinity and

surface mineralisation behaviour confirms

the coating’s ability to support in vitro apatite

formation, a key indicator of bioactivity and

osteointegration potential [16].

bioactivity of the HA/PCL-coated SS 316L.

This process is consistent with the general

model of apatite formation proposed by

Kokubo and Takadama [7] and further

Figure 3. XRD values of HA/PCL-coated samples with different shear strength values: (a) 1.45 MPa, (b)

1.95 MPa, (c) 1.99 MPa, (d) 2.22 MPa, (e) 6.74 MPa, (f) 23.22 MPa, (g) 24.17 MPa, and (h) 25.79 MPa, after

immersion in simulated body fluid (SBF).

The

Immersion.

Effect

of

pH

Variation

During

processes occur simultaneously, promoting

the formation of a stable apatite layer on the

coating surface. The initial decrease in pH

reflects partial dissolution of the HA/PCL

The pH of the SBF solution serves as an

indirect indicator of the ionic exchange

between the coating and the surrounding

medium. As shown in Figure 3, for coating

thicknesses of 123.6 µm and 102.6 µm, a

slight decrease in pH was observed during

the first seven days of immersion, attributed

to the release of Ca²⁺ and PO₄³⁻ ions into the

solution. After approximately 14 days, the

composite,

while

the

subsequent

stabilization demonstrates the coating’s

chemical balance and capacity to maintain

physiological pH—an essential factor for

biocompatibility [16,17].

The Weight Loss and Degradation

The degradation behaviour of the HA/PCL

coatings was further evaluated through

weight loss measurements after immersion

in SBF for 7, 14, 21, and 28 days. As

pH

stabilized,

indicating

a

dynamic

equilibrium between ion release and apatite

precipitation. This behavior suggests that

the

degradation

and

reprecipitation

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presented in Figure 4, significant weight loss

was recorded after seven days, with the

thicker coating (123.6 µm). It was exhibited

a weight loss of 0.71%, while the thinner

coating (102.6 µm) showed 0.43%. The

degradation rate gradually decreased with

prolonged immersion, reaching 0.43% and

0.32%, respectively, after 28 days. This trend

indicates that degradation slows as a stable

apatite layer forms on the surface, reducing

further ion exchange between the coating

and the SBF. The morphology suggests that

the porosity of the PCL matrix influences the

degradation

diffusion of ions. Limited ion exchange

results in slower weight reduction,

rate

by

modulating

the

confirming that layer thickness and coating

density are critical parameters controlling

the effective degradation rate [6,18]. These

results

align

with

observations

by

Mehdizade et al., who reported that thicker

bioceramic layers in Mg–HA composites

exhibited lower degradation rates and

enhanced mechanical stability [19]

Figure 4. Weight variation of HA/PCL-coated SS 316L samples with 123.6 mM and 102.6 mM ion

concentrations after immersion in SBF

to long-term coating stability. A notable

increase

in

hydroxyapatite

crystallinity,

Conclusions

reaching 86.8% confirms enhanced apatite

nucleation and surface bioactivity.

This study confirms that the structural and

mechanical

characteristics

of

HA/PCL

Acknowledgement

coatings on SS 316L significantly influence

their degradation behavior and bioactivity in

simulated body fluid (SBF). The coating shear

strength (6.74–23.22 MPa) and thickness

(102.6–123.6 µm) were found to have a

direct influence on pH stability and ionic

exchange, leading to a controlled release of

Ca2+ ions, as evidenced by the gradual

weight loss over the 28 days. The reduction

in degradation rate indicates the formation

of a protective apatite layer that contributes

This research is supported by the University

of Riau, Chemical Engineering Department.

We extend our sincere appreciation to the

University of Riau, Chemical Engineering

Department

for

providing

facilities,

resources, and technical assistance essential

to the completion of this study. We would

like to express our special thanks are also

extended to Muhammad Ghazy Fernandes

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Chempublish Journal, 9(2) 2025, 221-229

based biocomposites for bone tissue

for preparing and providing the coating

samples used in this study. This research

would not have been successfully completed

without the collective effort of all involved.

regeneration

International

Sciences.

in

Journal

orthopedics.

Molecular

2022;23(17):9721.

of

https://doi.org/10.3390/ijms23179721

[4]. Bohner M, Lemaitre J. Can bioactivity be

tested in vitro with SBF solution?

Author Contributions

Conceptualization,

YMS

and

YDNBB.;

Biomaterials.

2009;30(12):2175–2179.

Methodology, YMS and YDNBB; Software,

YMS and YDNBB.; Validation: AF and SH.;

https://doi.org/10.1016/j.biomaterials.2

008.12.068

Formal

Analysis,

YMS

and

YDNBB.;

[5]. Fadli A, Hariz A, Helianty S, Rifaldi M.

Hydroxyapatite-polycaprolactone

coating on 316L stainless steel surface

using dip coating method. Chempublish

Investigation, YMS and YDNBB.; Resources,

YMS and YDNBB.; Data Curation, YMS and

YDNBB.; Writing – Original Draft Preparation,

YMS and YDNBB, ; Writing – Review & Editing,

YMS and YDNBB, AF, SH; Visualization: YMS

and YDNBB.; Supervision, AF and SH; Project

Administration, AF.

Journal.

2024;8(2):109–118.

https://doi.org/10.22437/chp.v8i2.3084

2

[6]. Suchý T, Bartoš M, Sedláček R, Šupová M,

Žaloudková M, Martynková GS, Foltán R.

Various simulated body fluids lead to

significant differences in collagen tissue

Conflict of Interest

The authors declare that there are no

conflicts of interest.

engineering

scaffolds.

Materials.

2021;14(16):4388.

https://doi.org/10.3390/ma14164388

[7]. Kokubo T, Takadama H. How useful is

Ethical Standards

This article does not contain any studies

involving human or animal subjects.

SBF

bioactivity?

2006;27(15):2907–2915.

in

predicting

in

vivo

Biomaterials.

bone

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