**5. Concluding remarks**

**Stimuli Material system**

Ion Ion-crosslinked GC hydrogel

Light Hydrogels based

on PAM crosslinked by photosensitive agents

Methacrylated HA hydrogel

Photo-crosslinked methacrylated Gellan Gum hydrogel

Methacrylated HA hydrogel with photo-crosslinker

PEG based hydrogel with photosensitive crosslinker

DNA DNA crosslinked PAM system

> Piezo-controlled substrates and AFM stiffness clamp

HA hydrogel Stiffening &

PEG hydrogel (PEG vinyl sulfone

Hydro-lysis Photo-crosslinked alginate hydrogel

AFM/ stiffness clamp

**Nature of change**

98 Advances in Biomaterials Science and Biomedical Applications

Stiffening Irreversible change

Stiffness; Irreversible change

Stiffness↓ Adhesive property Irreversible change

Stiffening & softening, potentially coupled with strain/stress Reversible change

Instantaneous change in stiffness Unidirectional

Softening due to degradation

structure change

Softening due to degradation

Stiffness; Swelling Hydrolytic degradation

Stiffness ~22 to ~17 kPa (with

**Range of changeRate of**

chemical crosslinking)

Softening Stiffness: 5.5~7.2 kPa Approximately

Stiffness: ~3 to ~30

Stiffness: a few kPa to 22 kPa (by physical crosslinking)

Stiffness: 1.6 to 3.8 kPa; 3-12 kPa

Stiffness: ~5.9 to 22.9

Stiffness: 3.6 to 90 nN/µm

Stiffness: ~25 to ~180

Stiffness: e.g., ~5 to 30 kPa for one case

Stiffness: from ~1 kPa-3 kPa to very low

kPa

kPa Stress > 0.5 Pa

kPa

**change**

0.5~0.6 kPa/ min

Approximately 9 kPa/hr (short term); 2 kPa/day (long-term)

Approximately 20 kPa/ min

Approximately 0.1 or 0.3 kPa/min (during gelation)

Up to 8.5 kPa/ day

Step change (instantaneously)

From ∼900 Pa/day to 500 Pa/day

**Invasiveness of stimulus and**

UV radiation with low energy density Depth of penetration and limit on dose **Ref.**

[88]

[76]

[79]

[88]

[80]

[20, 81, 83]

[84]

[92]

[78]

[91]

**potential issues**

N/A Under physiological conditions, divalent ions exchanged by mono-valent ones

UV exposure for 3 min

UV exposure for a few min Potential toxicity of photoinitiator Depth of penetration and limit on dose

UV exposure for one min

UV exposure for a few min Potential toxicity of photo-initiator Depth of penetration and limit on dose

N/A N/A Depth of penetration and limit on dose [77]

type

morphology

7-8 kPa/ day In sample preparation (with cells), UV exposure for 10 mins

0.7 kPa/ day Dense crosslinking may impede cellular

Good cell viability

Hydrogel degraded in 16 hours

radicals

Depth of penetration and limit on dose

No differentiation in cellular responses between forces, stress, and stiffness Potentially interference from DNA with bioactivity (e.g., as anti-sense DNAs), and potential issue with DNase

BLAST search against target specie & tissue

growth limited by diffusion& concentration of

Applicable only to cells with dynamic

There is an increasing recognition of the discrepancy between static nature of the current cell culture substrates or scaffolds and the dynamics in ECM in natural or diseased tissue, dur‐ ing development and aging, or at tissue-scaffold interfaces. This has motivated the develop‐ ment of materials with controlled changing properties that mimic those of ECM. An array of stimuli, including environmental factors (temperature, pH, light, electrical potential) and non-environmental cues including enzyme and DNA, have been implemented to trigger dy‐ namics in a number of material platform such as SAMs, polymeric hydrogels and other sub‐ strates with surface chemistry and modifications.

To date, most of the effort along this line has been devoted to *in vitro* models, and *in vivo* studies of the effect of dynamic tissue properties on cellular behavior are still rather limited, which awaits further development in cell biology and proper tools such as imaging techni‐ ques [12, 14, 29].

Understanding the interplay between cells and the extracellular matrix (ECM) including its dynamic aspect is fundamental to biology, development, aging and pathology, and can aid in the design of biomaterials. Ultimately, the system enabling both spatial and temporal control [96] of cells would be most relevant in terms of bio-mimicry and tissue engineering applications. Some of the potential directions include creating dynamic adhe‐ sive gradient to guide cell migration or neurite outgrowth at desired time point, con‐ structing scaffolds with suitable mechanical rigidity to inhibit glia cell growth (thus hinder scar formation) while promoting nerve regeneration with compliance gradient, and developing dynamic platform for stem cell harvesting and differentiation for cellbased therapies.

### **Acknowledgments**

The helpful discussions and advice from Langrana group at Rutgers University, New Jersey, USA as well as previous collaborators are greatly appreciated.
